gwr observatory superconducting gravimeter_2007!10!02

8/6/2019 GWR Observatory Superconducting Gravimeter_2007!10!02

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GWR Observatory Superconducting Gravimeter and Support Systems 1

GWR Observatory Superconducting Gravimeter

and Support Systems

Descriptions and SpecificationsOctober 02, 2007

Overview GWR Instruments, Inc. is the exclusive manufacturer of theSuperconducting Gravimeter (SG). In the SG sensor, levitation of a spherical test mass in

an ultra-stable magnetic field replaces the mechanical springs found in previous gravitymeters. The field is generated by persistent currents in two niobium coils that are

superconducting below a temperature of 9.3 K. The stability is derived from the zero

resistance property of superconductors after the currents are trapped no resistive

(ohmic) losses exist that could cause them to ever decay in time. In addition, adjustingthe ratio of currents in the magnet coils makes the magnetic force gradient (spring

constant) very weak. As a result, small changes in gravity produce large displacements

of the test mass that are easily detected in the capacitive displacement transducer thatsurrounds the mass. The ultra stable magnetic field, weak gradient and operation at

cryogenic temperatures eliminate the sources of noise and drift commonly found in

mechanical spring gravity meters. As a result, the SG is the worlds most sensitive andstable gravimeter.

Figure 1: GWR OSG Superconducting Gravimeter and Integrated Electronics


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To maintain the superconducting state, the Gravimeter Sensing Unit (GSU) is operated at

4 K inside a dewar filled with liquid helium. In 2003, GWR introduced the Observatory

Superconducting gravimeter (OSG) which uses a 4 K refrigeration system and 35 Liter

dewar. In this system, the refrigeration system operates below the boiling point of liquidhelium thereby preventing any loss of the liquid during normal operation. Therefore, the

OSG can operate indefinitely without the need for refilling with liquid helium. The

refrigerator requires less power than previous models, consuming approximately 1.3 kW;however, it has enough excess capacity to liquefy helium gas when it is added to the

dewar from a pressurized gas cylinder. This technique is used to replace any helium lost

during maintenance or power failures thereby eliminating the requirement to transportand transfer liquid helium during normal operation. Liquid helium however is required

when the SG is first setup. The OSG dewar is smaller and lighter than previous SGs. It

weighs only 60 kg and is easily installed on any concrete pad 80 cm x 80 cm.

The data acquisition system and control electronics, used for operating and monitoringthe SG and for recording data are fully integrated in the OSG. The proof mass levitation,

centering, leveling, and system monitoring are all accomplished through computer

interface. This allows the operator to control and monitor the SG from his home oroffice. In addition to gravity and pressure, the data acquisition system (DDAS) records

30 status variables. Alarm thresholds can be set for all channels, to automatically

generate warnings and alert the operator by email to initiate investigation and repair.

After the operator enters the calibration factor, tidal parameters and barometric pressureadmittance into the DDAS, it will calculate a theoretical tide and display the gravity

residual signal in real time. This allows immediate visual examination of the gravity

noise at sub-Gal levels. Observations of small signals and changes in noise level areimmediately observable and with some experience can be identified as of geophysical

origin (atmosphere, ocean, or earthquakes) or due to equipment problems. In the lattercase, GWR can examine the system on-line to analyze the problem with the user to

provide a rapid solution. Remote access reduces the frequency of data gaps and ensures

high quality of overall long-term data.

As demonstrated by results from the Global Geodynamics Project (GGP)1, the SG

provides a continuous record of changing gravity and provides data over wide period

range from seconds (ocean noise) to several years (secular changes). It is common forthe SG to measure small periodic tidal signals and long period seismic signals with a

sensitivity of 1 nano-Gal and better. Therefore, one nano-Gal is generally referred to as

the nominal precision, or sensitivity, of the SG. At quiet sites, typical noise levels at longperiod seismic frequencies are from 0.1 to 0.3 Gal Hz-1/2. For temporal studies, SG data

averaged to 1 minute intervals achieves a precision of better the 0.04 Gal. Forfrequencies less than 1 mHz, SGs have achieved lower noise levels than attained by mostlong-period seismometers and are now being used to study low frequency normal modes.

Reference:1. Global Geodynamics Project (http://www.eas.slu.edu/GGP/ggphome.html)
http://www.eas.slu.edu/GGP/ggphome.htmlhttp://www.eas.slu.edu/GGP/ggphome.html


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GW

Applications

R Observatory Superconducting Gravimeter and Support Systems 3

High precision continuous gravity monitoring for study of geophysicalphenomena such as normal modes, mantle rheology, tides, solid Earth-oceans-

atmosphere interactions, hydrology, and Earth rotation.

Resolving mass density changes associated with elevation changes measured by

GPS, VLBI, SLR, LLR, DORIS and GLONASS Hydrological, geothermal and non-invasive ground water monitoring

Volcano monitoring

Measurement of subsidence caused by oil, gas, or water extraction

Long term crustal motion and sea level monitoring

Correlation & validation with satellite gravity including GRACE, CHAMP,GOCE

Aquifer monitoring and management, measuring depletion and recharging ofmunicipal water supplies

High accuracy gravity reference stations when combined with periodic AbsoluteGravimeter observations

Figure 2: Observatory Superconducting Gravimeter (OSG) , Refrigeration System,

Integrated Electronics with Digital Data Acquisition System and Paroscientific

Meteorological Measurement System


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GWR Observatory Superconducting Gravimeter and Support Systems

Figure 3: Naming conventions of OSG and system components.

GWR Observatory Superconducting Gravimeter and Support Systems


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I. Observatory Superconducting Gravity Sensor and

Refrigeration System

The GWR Observatory Superconducting Gravity Sensor and Refrigeration System

combine the same SG sensor that has been proven by decades of precision gravimetric

measurements with the newest in 4 Kelvin refrigeration technology. The system is

completely integrated, providing the user a system that can be operated and maintained

more easily than any previous SG.

A. GWR Observatory Superconducting Gravimeter (OSG)

The system consists of all the necessary equipment to setup the sensor, automaticallymaintain the systems alignment with the gravity vector, and maintain the SG at

genic temperatures. The analog controller combined with the Digital Data

uisition System provides all necessary gravity and sub-system data. Files are

tinuously logged to a PC supplied with the system and available through a TCP/IPnection. Details of the sensor and subsystems are given below.

rators are advised that supplies such liquid helium, and helium gas are consideredendables and must be provided by the operator at time of installation. Details on

ired expendables are outlined at the end of this document.

SU-4A Gravity Sensing Unit with Single Sphere

shown in Figure 4, the Gravimeter Sensing Unit (GSU) contains a 2.5 cm diameter

erical proof mass. The sphere is levitated by the forces produced by magnetic fieldserated from a pair of superconducting coils. Since the sphere is superconducting, it

aves as a perfect diamagnet so that surface currents are generated which exactly

cel and exclude any applied magnetic field from its interior. It is the interaction

een the sphere's surface currents and the applied magnetic field that produce thetation force. Both the position of the sphere and the vertical force gradient (spring

stant) are optimized by adjusting the ratio of the currents in the two coils.erconducting/normal heat switches are used to "trap" the supercurrents in the

the external power supply to be disconnected from the

en adjusted to their final values. The usetrapped persistent supercurrents to produce an ultra stable levitation force is

onsible for the unprecedented long term stability of the superconducting gravimeter

in comparison to mechanical spring type gravimeters.

A capacitance bridge network, consisting of three spherical capacitor plates positioned

around the sphere, is used to sense the position of the sphere. The upper and lower platesare driven by precisely matched AC signals that are 180 degrees out of phase. Thesphere capacitively couples these excitation signals to the center plate of the bridge.

When the sphere is equidistant from the upper and lower plates, the drive signals cancel

and produce a null signal on the center plate. As changes in gravity cause the sphere tomove from its null position, an error signal is produced. During operation, the position of

the sphere is held close to its null position by a feedback circuit which applies a magnetic

force through a separate feedback coil. Since the force from feedback coil is linear with

cryo

Acq

concon

Opeexp

requ

1. G

As

sphgen

beh

can

betwlevi

conSup

magnetic coils. This allows

magnet coils after the trapped currents have beof

resp


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ck and signal leads to insure that heat leaks into the liquid heliumeservoir are minimized. High quality environmental connectors allow interfacing the

ravity sensor and its subsystems to an electronics package that resides external to the

dewar.

2. GSU-4D - Gravity Sensing Unit with two spheres

The Dual Sphere GSU operates on the same principal as the single sphere sensor.However, this sensor has two superconducting spheres and capacitance detection bridges

separated by a spacing of 20 cm. The spheres are levitated by two separate sets of

superconducting coils wound on the same copper magnet form. By adjusting the currentsin each set of coils, each sphere is independently levitated and centered in its own

magnetic field. After levitation and adjustment of the force gradient, additional

superconducting side coils are used to apply small horizontal forces on one of thespheres. This allows the tilt minimum positions of the two sensors to be precisely aligned.

The Dual Sphere SG operates in a dewar that is 4 taller than the single sensor SG.

With the difference of the signals from the individual sensors of a dual sphere gravimeter,one can detect any offsets produced by magnetic flux 'jumps" that are larger than 0.04

Gal. Offset detection is no longer limited by the ambiguity of removing "real" gravitysignals nor by sources of geophysical noise that may overlap and hide an offset.

neck tube and terminated at the head of the instrument. Great care is taken in design and

manufacturing of the ner

g

Table 1: OSG Single Sphere and Dual Sphere Gravimeter Sensor Specifications

Precision:

0.1 to 0.3 Gal/(Hz)1/2

0.012 to 0.040 Gal for a one-minute averaging time

0.002 to 0.005 Gal for a one-hour averaging time

Drift:Typically less than 6 Gal/year after 6 to 12 month stabilization period

Calibration:

The GSU sensor is not provided with a calibration from GWR. Users normally perform a

rough calibration by fitting the observed gravity signal to a theoretical model of the Earthstides. The OSG calibration can be improved by comparison to the signal from an absolute

gravity meter operating next to the OSG for 5 to 10 days. Many users have reported that

they can calibrate the scale factor to a precision of better than 0.1%. However, theprecis e operator.

A relativ rm, or

by applying a gravitational force with a known mass. Apparatuses for these calibration

the

tor to be constant to better than 0.01% over several years.

ion will depend on the quality of the absolute gravimeter and the skill of th

e calibration can be performed by placing the SG on an acceleration platfo

methods are not currently commercially available.

Scale factor (calibration) stability:By comparing the OSG signal to models of the Earth Tides, several users have reported

scale fac


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ment of high efficiency dewars at GWR Instruments. It is optimized to operate inO ich eliminates the need for re-

cooled down and initialized. Un-

using

een the

outer shell and the inner liquid helium reservoir. Layers of aluminized Mylar

(s g

th rextensive testing, the GSU is installed into the OGD-35L through a large port in theb e port is then sealed and the dewar's vacuum space is evacuated.

T to be customized to achieve the

m rator when operating together as

a em, radial stiffening spokes are

incorporated into the design. These allow the use of an extremely thin neck tubea brations.

T eration system is a separate and isolated system with respect to the

li

02

b

B. OGD-35L-S/D Super Insulated Dewar and OGD-35-REF 4 Kelvin

Refrigeration System and Vibration Isolation System

The OGD-35L liter super-insulated dewar is the result of more than 25 years of

evelopdconjunction with a 4 K cryogenic refrigeration system, wh

filling the dewar with liquid helium after the system is

refrigerated, the dewar has a loss rate of less than 5%/day which corresponds to a 20 day

holdtime. This gives plenty of helium reserves so that power failures and refrigerationmaintenance do not interfere with the continuous gravimeter operation.

The OGD measures 114 cm in height, 42 cm in diameter, and weighs approximately 60

kg with the GSU installed (see Figure 4). This small size makes it ideal for applications

where the gravimeter will be operated in tunnels or vaults at existing geophysical

observatories. The dewar is supported at three points attached to a reinforced flangewelded around the outside of the dewar. The heights of two of the support points are

djustable using precision micrometers and can be precisely controlled bya

thermally controlled levelers which are an integral part of the tilt compensation system.The height of the support points coincide with the height of the sphere. This minimizes

the effects of ground vibrations on the gravimeter.

The dewar is constructed mostly of aluminum with radiation shields situated betw

uperinsulation) are wrapped inside the dewar to minimize radiative heat from enterin

e system. During manufacturing, the GSU is first operated in a test dewar. Afte

ottom of the dewar. Th

his "sealed in" design allows the neck of the dewar

aximum obtainable efficiency of the dewar and refrige

ystem. In order to strengthen the mechanical systs

ssembly while maintaining a rigid system that is not excited by ground vi

he helium refrig

quid helium dewar. The helium gas in the dewar operates at pressures between 0 and

.5 PSI relative pressure. The Refrigeration system operates at approximately 350 PSI orMPa. These two gas circuits do not come in contact with each other. The relationship

etween the two systems is shown in Figure 5 below.


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Figure 5: Dewar and refrigeration system relationship

The OGD-35L REF refrigeration system consists of a Sumitomo CNA-11 heliumcompressor, Sumitomo RDK-101E Coldhead (expander), flexible interconnects hoses,

vibration isolation diaphragm with sealing flange, and a vibration isolation coldhead

support frame. Use of the 4 Kelvin refrigeration system allows collection ofuninterrupted data over much longer periods than previous design which used 11 Kelvin,

or un-refrigerated dewars. Noise related to maintaining the liquid helium is small and

there are only infrequent disturbances related to periodic servicing of the coldhead.

Disturbances associated with scheduled refilling of the dewar have been eliminated.

Liquid Helium replenishment from the high pressure Helium gas cylinder is explained

below.



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Figure 6: Cutaway view of dewar showing sensor mounted inside



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Figure 7: OSG dewar and refrigeration system shown with the Coldhead lifted outof the dewar. The compressor is connected to the coldhead with flexible metal hoses

The refrigeration unit is a closed cycle system consisting of a compressor attached to the

coldhead via flexible hoses. The hoses are supplied in three standard lengths allowing

the compressor to be situated at a convenient location. When placing an order three (3),six (6) or fifteen (15) meter hose lengths should be specified.

The coldhead is mounted inside the neck of the OGD-35L dewar. The coldhead uses amodified Gifford McMahon cycle for cooling and is supplied high pressure helium gas

by the compressor. The gas entering the coldhead is first pre-cooled by exhaust gas and

then cools further when it is allowed to expand. The RDK-101E coldhead has two

cooling s elow theallowing the system to operate without loss of liquid

elium.

n order to prevent mechanical vibrations from the coldhead from disturbing the gravity

easurement, the coldhead is mechanically de-coupled from the dewar. At the two

ooling stages heat transfer is accomplished through gas coupling only. At the head of

tages. The second stage has a minimum operating temperature bboiling point of liquid helium

h

I

m

c



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the SG a thin diaphragm is used to isolate the coldhead from the dewar while maintaining

a gas-tight seal.

The helium gas volume inside the dewar reduces as it cools and condenses. If the cooling

capacity is not controlled, the dewar pressure will drop below atmospheric pressure.Continued operation in this mode can cause failure of the thin vibration isolation

diaphragm. To prevent this condition a small heater placed near the tip of the coldhead is

used to add heat to the system. By precisely controlling the pressure across the vibration

isolation diaphragm, the diaphragm is operated in a state where vibration transmission isminimized.

During power failures of less than 1 hour duration liquid helium that vaporizes ismaintained inside the dewar. After approximately one hour a 0.5 PSI safety relief valve

begins venting helium to the atmosphere as it boils from the bath. During the subsequent

23 hour period without refrigeration, the boiloff rate raises to a maximum loss rate of5%/day or about 1.75 liter/day.

The dewar may be refilled with liquid helium by adding helium gas to the system while

allowing the refrigeration system to liquefy the gas. Gas is supplied from a compressedgas cylinder supplied by the operator. When gas is introduced, the dewar pressure rises

causing the dewar pressure controller automatically to turn the dewar heater off. This

allows the liquefaction process to start. A pair of precision high purity regulatorssupplied by GWR allows the addition of gas with minimal disturbance to the gravity

measurement. During the liquefaction process, room temperature compressed gas is

converted to liquid at rate of between 1 and 2 liters/day when operating form 50 Hzpower, and 1.5 to 2.5 liter/day when operating from 60 Hz power. A standard 200 cu ft

helium gas cylinder converts to approximately 10 liters of liquid Helium. This is aconvenient and simple way to refill the dewar after small helium losses occur from either

power failures or removal and insertion of the coldhead during maintenance.

Figure 8: Gas regulator supplied by GWR Instruments connected to He gascylinder and to the flexible metal hose carrying He gas for liquefaction to SG dewar.

The liquefaction process can be monitored with the LHe screen provided as part of

the data acquisition system user interface.

The coldhead is supported external to the dewar on frame which stands on rubber feet.

This vibration isolation frame incorporates a spring activated slide-mount that lifts thecoldhead vertically out of the dewar. The apparatus allows the coldhead to be removed

and inserted into the dewar without mechanically contacting the dewar neck, thereby

minimizing disturbances to the gravity measurement during this procedure. When done


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e performed at one to two year intervals to service the coldhead.

properly, the slide mount prevents offsets from occurring in the gravimeter record. This

procedure must b

Table 2: OGD Dewar Specifications

OGD-35L-S Single Sensor Dewar Specifications:

Capacity: 35 liters (with GSU installed)

Hold time between refills (un-refrigerated): 20 days minimum

Dimensions: 42 cm diameter x 114 cm highTotal height installed on support feet: Dewar only - 116cm

Dewar with coldhead - 130cm

Minimum height required to transfer liquid helium (with standard equipment): 1Weight of Dewar with GSU installed: 60 kg

Concrete pier required:

80 cm

80 cm x 80 cm

OGD-35L-S/D Dual Sensor Dewar Specifications:

Capacity: 38 liters (with GSU installed)

Hold time between refills (un-refrigerated): 21 days minimum

Dimensions: 42 cm diameter x 124 cm highTotal height installed on support feet: Dewar only - 126cm

Dewar with coldhead - 140cm

Minimum height required to transfer liquid helium (with standard equipment): 19Weight of Dewar with GSU installed: 70 kg

Concrete pier required: 80 cm x 80 cm

0 cm


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Table 3: OGD-35-REF Specifications

Interconnecting Hoses: 6m, 9m or 15m lengths available. Customer must specify the length needed at

the time of ordering. Contact GWR instruments for information on availability of lengths other than

those specified.

Helium loss rate: No loss of liquid helium during normal ope tion as long a power is supplied to the

refrigeration system.

Liquefac

m SRDK 01 Cry

ra

tion rate: greater than 1 liter/day at 50 Hz; greater than 1.5 liter/day at 60 Hz.

Sumito o -1 o-Cooler UnitRefriger

Hz V

*

TART)

ation System power requirement summary:

olt Phase Comp. Comp. Comp. Cold-head

(FLA)

System

LA)

System

(LRA)(FLA) (LRA) Fan

(FLA)

(F

(RUN) (S

50 10 4 0.5 .90 1 13.5 56 0. 14.4 56

60 10 1 14.8 52 0.3 0.5 15.6 52.80

* Factory supplied st input

ple, when

usyst

wa

urrents scale with input voltage.

)

RDK-10

First stage: 3.0/5.0 W at 60 OK (50/60 Hz)

Second stage: 0.1 W at 4.2 OK (50/60 Hz)

Ambient operating temp.: 5 to 28 OC. 28 to 35 OC with 10% capacity loss

Dimensions: Width 103 mm; Length 226 mm; Height 442 mm

Weight: 7.2 kg

Coldhead Service: Factory reconditioning necessary at 10,000 hour interval

CNA-11 Compressor specifications:

Operating Temperature: 4 to 28 OC; 28 to 38 OC with 10% capacity loss

Helium gas pressure (3-6m hoses): Static - 195 to 200 MPa at 20 OC

Operating 2.2 to 2.3 MPa

Dimensions: Width 390 mm; Length 450 mm; Height 610 mm

Weight: 75 kg

AC Power: Single phase

Operating Voltages: ACV 100, 120, 220-230, 240

Power Line Frequency: 50, 60 Hz

Current (@100VAC): Max. 13.9 A / Steady State 12.4 A at 50 Hz,

Max. 15.1 A/ Steady State 13.3 A at 60 Hz

Compressor Service: Mandatory adsorber replacement, 30,000 hour interval.Clean Air Cooler at least one time per year

(Or more often if required by unclean environment)

ep-down transformer provided with 100, 120, 220/230, 240 VAC

taps.

Input voltage MUST be within -5% to +10% of voltage tap rating. For exam

2 0/230VAC tap input volta es shooperating on the 2 g ld not exceed 209-253VAC limits.y cause damageVoltages exceeding these limits can cause the em to stop operating or ma

due to overheating. These conditions void all rranties!

C

Minimum circuit amp capacity: 20 Amps (@ 100 VAC)

Maximum breaker size: 20 Amps (@ 100 VAC

1E Coldhead specifications:


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C. TM-7B GWR Cryogenic T Tilt Compensation

System

1

W beprecisely aligned with the gravity vector. For continuous measurements this alignment

m easured

s ing.

When an ideal gravimeter is cal vertical it simply

m long its axis. Thus to the lowest order in D,the ideal response is a -1/2 gcos D2 2. The ideal gravimeter

will read a ma t

F h er nduct ravi ter, th agn e and q atic nse are close toth f ide gravi r. H ver, ign he resp to ti opposite to that

of th id eld

supporting the sphere. Because of

to me a tionsfrom ver

In o r the SG incorporates two tilt meters mounted

ortho n ted as close as possible to the

gr vi s d externally introduce tilt artifacts dueto int at which the tilt meter is mounted and the position of

th uced by changing temperaturepro k of th

The TM-7B tilt m a thin metal

foi re used tosense the paddle position and its motion in respons

v tes, located on either side of the hanging paddle. When

the e hanging paddle is zero.Wh ignal is produced.

2. TCS-6 - Automatic Tilt Co

The tilt compensation system is necessary to counteract changes in local tilts that are

common even at seemingly stable locations. Tilts can be introduced from settling of the

underlying substrate, varying substrate density, heating and cooling of the support

platform, and changes in humidity or water table.

ilt meters and TCS-6 - Automatic

. TM-7B - GWR Cryogenic Tilt meters

hen making high precision gravity measurements it is crucial that the sensor

ust be maintained for the course of the measurement or must be continuously m

o that deviations from the vertical can be correc ed for during post processt

tilted by an angle, D, from the lo

easures the component of gravity, gcos D, aor -4.9 x 10-4 mgal/ m

ith the ver

radian

ximum value when it is aligned w ical.

or t e sup co ing g me e m itud uadr respoat o the al mete owe the s of t onse lts is

e eal gravimeter. This is a consequence of the geometry of the magnetic fi

the quadratic response to changes in tilt, it is desirable

ch nically maintain the SG alignment with vertical rather than correct for deviatical in post processing.

rde to sense changes in tilt,

go ally inside the GSU. These sensors are moun

a ty ensor (see Figure 3). Tilt meters mountetilt differences between the po

e gravity sensor. These effects include stresses indfiles in the nec e dewar.

eter uses a rectangular pendulum paddle which hangs from

nsing technil. Capacitive se ques similar to that used in the gravity meter ae to tilts. In this case the excitation

oltage is applied to two side pla

paddle is perfectly centered, the resulting signal sensed on then tilts cause the paddle to move from its null position, an error s

mpensation System

Table 4: TM-7B Tilt meter specifications:

Sensitivity: 0.1

m

radians

Dyna ic range: greater than 60 mradians


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l

. The levelers are controlled by electronics in the GEP-3 which heat thermallements inside the levelers causing them to change height. As shown in Figure 3, the

35L dewar, and are part of the structure that

upports the dewar. By varying the lengths of the two actuators relative to the third fixed

withoutansmitting noise or vibrations to the gravimeter. The lower car rests on a hardened steel

ith the plumb line (along

e local gravity vector). The manual adjustment is accomplished by turning precision

s have a range of about 1 mm, providing ampleompensation for most sites.

op operation of the system also improves the precision

f tilt sensing. Since the tilt meters are operated in closed feedback loop at their null

. HTK-4 Liquid Helium Transfer Kit

sary to easily transfer liquid heliumrom a supply dewar into the OGD-35 dewar. It includes the following components:

cover for the dewar and exhaust tube

used to direct cold gas away from critical components of the gravimeter sensor;

4. Helium dip stick (flutter stick) used for measuring the contents of storage dewars;

Tilts at the SG pier are compensated for by two actuators called Automatic Therma

Levelerse

Thermal Levelers bolt directly to the OGD-

spoint, the dewar is maintained at a constant tilt orientation.

Inside the case of each Automatic Thermal Leveler, two sliding cars travel on precision

crossed roller bearings. The bearings allow extremely smooth movementtr

ball which mates to a receptacle on top of the dewar Support Pillar. The support Pillars

are cemented and bolted in place on top of the SG pier.

When the SG is first installed, it is manually aligned precisely w

thmicrometers mounted on top the levelers. This "tilt minimization" procedure is

performed by observing the gravity signal while tilting the instrument in pre-determined

increments. After installation is complete, the levelers are placed in automatic mode

where their length is controlled by a signal derived from the output of the tilt meters. In

this configuration the gravimeter is held to within 0.1 radians of its tilt minimum

position. The thermal levelerc

There are several advantages to maintaining the system at its tilt insensitive position,rather than measuring tilts and calculating their effects. Since the response of the gravity

meter is proportional to the square of the tilt error, small tilt changes that cannot be

compensated for have minimal effect when the gravimeter is held close to the tiltinsensitive position. The closed lo

oposition, electronic gain changes that result from temperature and aging do not affect the

measurement. Actively controlling the tilt also eliminates problems associated with

calibration of tilt effects on the gravity signal, and removing these effects in postprocessing.

Table 5: TCS-6 Tilt Compensation System specifications:

ControlledD

alignment with set vertical: 0.1 radiansynamic range: 2.5 mradians

DThe Helium transfer kit contains all supplies necesf

1. Flexible transfer tube constructed with a superinsulated vacuum sheath;2. Pressure gauge scaled with recommended pressures used during different stages

of the transfer process;

3. Helium Transfer Lid including an insulted


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consider procuring a 30, 60,

r 100 liter supply dewar to transfer LHe from the supplier to the SG operating site.

5. Miscellaneous hoses and fittings needed during the transfer process.The supply dewar is NOT provided as part of the transfer kit. Normally the supply dewaris provided by the same company which has been contracted to provide liquid helium

(LHe) to the operator. However in some cases the LHe provider has access only to large

helium dewars (e.g. 250 liters or more) which are impractical for the small volumerequired to initialize the SG. In this case the operator should

o

Figure 9: Transferring liquid helium from storage dewar to SG dewar using theliquid helium transfer kit supplied by GWR Instruments. The LHe screen in theGWR UIPC software is used to monitor the transfer process.

quipment (PLP) can also be provided for installations where poor powerent

is described in

The IEDP is ny sources of offsets,

drift, and gap prove reliability. When combined

ith the appropriate UPS/Power Conditioning package, it allows the SG to be

anufactured and tested in its final configuration thereby eliminating surprises fromThese include: ground loops, DVM

riations, and variations in ac power. It

tionsite

form

proand

II. Integrated Electronics and Data Acquisition Package for

OSG Gravimeter

The Integrated Electronics and Data Acquisition Package (IEDP) is used to control, and

monitor the SG as well as log SG data. It includes the Gravimeter Electronics (GEP-3),

Data Acquisition System (DDAS-3), Temperature Controlled Electronics Enclosure

(TREE), and all necessary hardware for a turnkey solution for operation of thegravimeter. Uninterruptible Power Supplies (UPSs), Power conditioning and Lightning

rotection ePquality is expected or protection from severe lighting storms is required. This equipm

the following section.

designed to improve performance by eliminating ma

s in the gravity data record, as well as im

w

munanticipated interactions between components.

ging, sensitivity to temperature and humidity vaaallows the operator remote access to all gravimeter subsystems allowing the operator to

quickly and easily verify optimal performance on a daily basis. If problems arise, the

operato stallar can rapidly determine the cause of failure without traveling to the in. A GWR engineer in San Diego can also retrieve data from the system in a standard

at that enables rapid analysis and consultation with the operator. In this way,

blems can be rapidly diagnosed and repaired. This can improve long term data qualityreduce the manpower required to operate an SG.


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Figure 10: Integrated Electronics, Data acquisition System (GPS and Met-3 not

Shown)

he Gravimeter Electronics Package (GEP-3) includes electronics that sense and control

age

t houses the analog circuit cards, multiplexer, A/D converter,and microprocessor for digital I/O and data buffering:

re applied to the upper and lower capacitor platesurrounding the sphere. The resulting signal from the center capacitor plate indicates the

esign linearizes the response of the

transducer and minimizes effects of gain changes inside the feedback loop that may occur

due to temperature effects in the electronics or component aging.

A. GEP-3 - Gravimeter Electronics Package

T

the gravimeter, temperature, tilt, refrigeration & dewar subsystems. The pack

includes the following components:

1. Control chassis tha

2. Ultra Low noise Linear Power Supply housed in a dedicated chassis;3. Pre-amplifiers mount inside the head of the gravimeter4. All necessary cabling.

1. Gravimeter Control Electronics

The Gravity circuit card is mounted in the GEP-3 control chassis. This card generates

precisely matched drive signals that as

spheres displacement from its null position.

The signal from the center plate is connected to a preamplifier via a cryogenic triax cable.

This low-noise high input impedance preamplifier is mounted inside the head of the

gravimeter. By minimizing the distance the low level signal travels, noise pickup and

losses are also minimized. To further reduce the capacitance of this very low level signalto ground, the central shield on the cryogenic triax cable is driven at the signal potential.

The signal from the output of the preamplifier is carried to the gravity circuit cardthrough a coaxial cable. There, it is detected using a phase sensitive lock-in amplifier.

This signal is then applied to an integrator which generates a feedback current through

the feedback coil in the GSU. Using this technique, the position of the sphere can bedetected to within a few angstroms. The closed loop d


19/36


On the Gravity card, an eight pole Bezel filter with a corner frequency of 8 seconds isprovided as an anti-aliasing filter for digitizing the gravity signal. This filter, named the

GGP1 filter, is intended for sampling at 1 second intervals. It was designed to meet the

specifications of the Global Geodynamics project (GGP)1. An additional 2 pole Bezel

filter with a corner frequency of 5 seconds is provided for sampling the gravity signal at

faster rates.

The gravity card also includes circuitry so that the system can be modulated by a voltagesource. The Feedback Modulator circuit allows the frequency response and phase

characteristics of the instrument to be precisely measured at the site of operation. This is

an important parameter especially when comparing data sets from multiple SGs.

2. T

Tw d compensate for tilts seen

at t electronics operate on the same principle as the gravity

circuits also include circuitry for adjusting the ratiotes. This allows the user to adjust the null

compensate for tilts by changing the heights

Control electronicsilt

o separate tilt circuits (X an

he GSU. The tilt sensing

d Y axis) continuously sense an

sensing electronics. However, the tiltof the excitation signals applied to the side pla

position of the tilt signals to coincide with the optimum tilt positions defined by the

gravity sensor. Like the gravity controller, the tilt circuits operate in a closed feedback

loop where a current is generated that drives the tilt system to a null position. Unlike thegravity feedback current which drives a magnetic coil, the tilt feedback currents drive two

(X & Y) linear actuators which continuously

of the dewar support points. The tilt circuit cards also contain low-pass anti-aliasingfilters for monitoring the tilt balance signals.

3. Temperature control electronics

The temperature control circuit uses a germanium thermometer in a wheatstone bridge to

sense the temperature of the superconducting elements within the gravity sensing unit(GSU). The signal is detected using a phase sensitive lock-in amplifier, then fed back to

control the temperature of the GSU slightly above the ambient temperature of the liquid

helium bath. Using this technique, the core temperature of the sensor is controlled to

within a few micro Kelvin (OK).

Table 6: The characteristics of the GGP1 filter

Filter type : Bessel, 8 pole

ner frequency (f -3dB) : 61.5 mHz. (16.3 Second period).

Attenuation (ultimate) : -160dB / decadenuation at .5 Hz;

Topology : Sallen-Key unity gain

Intended sampling rate : 1 sample / second

Cor

Attenuation at fNyq : 100 dB atte

Phase Lag : Linear, 0.034 degrees / cpd

Time Delay : Constant in pass band, 8.204 sec.Flat to within 1% of unity gain (+/- .086dB) below 0.01 Hz (100 sec. period).

Flat to within 4% of unity gain (+/- .341dB) below 0.02 Hz (50 sec. period).


20/36


. Auxiliary electronics

serialS232 port is used for remotely controlling the GEP-3 and for uploading subsystem data.

This circuit card all GEP3 electronics

either n the local UIPC or rem

The r itialization. A

summ

r r otely controlling all GEP3 front panel functions.

ess e con oller.

iu leve ndica r.

priorenergizing the levitation magnets.

ply.

supply.

4

An auxiliary circuit card provides excitation and conditioning for several sensors

including various cryogenic temperature sensors. A step function generator is included to

measure the step response of the gravimeter or other sub-systems. This circuit accepts aTTL pulse (provided by the DAC-3) and outputs a step function with variable gain and

offset.

5. GEP-3 Remote Control Card, A/D converter and Setup circuitry

The GEP electronics is provided with a Remote Control Card which includes a

microprocessor, and a 16 bit A/D converter with a 40 channel multiplexer. AR

ows the operator to view a virtual front panel of the

otely via a VNC connection.o

emote Control Card also contains circuits used during the SG in

Con ol Car below.ary of the GEP3 Remote tr d functions is shown

16 Bit A/D converter and 40 channel for monitoring system health signals.

Control Logic fo em

dewar pr ur tr

Liquid hel m l i to

Heater pulses used for trapping super currents in the levitation magnets.

Demagnetization circuit used for demagnetizing the Mu Metal shield

Getter Heater power sup

Body Heater power

Refrigeration system control relays.

Helium gas pressure monitoring circuits.

Helium liquefaction control relays. Electronics PCB and chassis and temperature monitoring circuits.

Room temperature monitoring, for 3 external temperature sensors.

Two spare single ended 16 bit A/D inputs for soil moisture monitoring or otheruser defined inputs.

Table 7: GEP-3 Gravimeter Electronics Package Specifications:

Power requirements: 140 W (with automatic leveling system installed), 120/240 VAC, 50/60 Hz

erature limits 0-30 OC

10 kg.

Operating conditions: Ambient temp

Dimensions:

1) Control Chassis: 483 mm wide x 133 mm high x 305 mm deep, Weight: 11 kg.

2) GEP-3 Power Supply Chassis: 483 mm wide x 133 mm high x 245 mm deep, Weight:

(1& 2 each utilize 3U of height for 6 U total in a standard 19" rack)


21/36

B


emote monitoring and control of the SG and its subsystems. The DDAS-3 data

t multiplexer that

s include a GPS

to UTC through aPS c The

syste i feredand it can beautom tically uploaded via FTP to an external data archiving site.

The t . These

ports m1. U e and gateway to the internet)2 GEP3 (SG monitoring and control)

3 DVM 1 Agilent 34420A NanoVolt Meter (High precision Gravity signal)

4 DVM 2 Agilent 34420A NanoVolt Meter (Dual Digitizer option)

5 T (uses two serial ports)

6 P ter, temperature, humidity)7 L o GEP-3)

8 T9 U

he DA essor board has been upgraded in 2007 to support bothompact Flash (CF) and Ethernet. The addition of the CF card increases the data

T is buffering cap ,

histor

T rd simplifying the firmwareupgrading procedure. Firmware upgrades can now be installed remotely with only ashort (approxim

TA gravity channel. A

ultiplexer and 16 bit A/D converter located in the GEP-3 is used for all other status

hannels. The controller also records the output from a digital barometer.

ing is provided through the generation of hardware trigger pulses

. DDAS-3 Data Acquisition System

The DDAS-3 data acquisition system was designed specifically for controlling the GEP-3

and logging SG and related meteorological data. This turn-key system allows operatorsto record the highest quality data from the SG, provides automated archiving of data and

llows ra

acquisition system is controlled by the DAC-3, an intelligent serial por

oordinates all time critical tasks and buffers data. Other componentcreceiver, a precision volt meter, a Paroscientific Met-3 metrological station and a

personal computer running custom software. Sample timing accuracy is maintainedwithin a few milliseconds, more than 10 times better than the specification recommended

by the GGP.

. Data Acquisition Controller (DAC)1The primary function of the DAC-3 is to record uninterrupted data from the gravity

sensor and barometric pressure sensor. Additional signals are logged to verify system

ealth and for maintenance purposes. Data sampling time is referencedhG re eiver which communicates via a precision time pulse and a serial interface.

m s designed to comply with all GGP specifications. Data is internally bufcontinuously exported to a PC where it is written to disk. From the PC

a

Da a Acquisition Controller contains a processor and ten (10) serial I/O ports

co municate with:IPC (Host PC for hard disk storag

.

.

.

. rimble GPS receiver precision time base

. aroscientific Met-3 Station (barome. iquid helium level monitor (2006 onward LHe monitor is integral t

. ransportable Voltage standard

. PS (power monitoring).

C controller u-procTC

buffering capacity of the DAC from approximately two hours to approximately 90 days.

acity virtually eliminates the dependence on the PCs hard diskh

ically the weak link in data acquisition systems.

he DAC controller firmware now also resides on the CF ca

ately one minute) data gap

wo independent digitizers are used on systems with single sphere gravity sensors. Angilent 34420A 7 digit digital voltmeter (DVM) is dedicated to the

m

c

Precise sample tim


22/36


resented directly into the DVM external trigger input. During normal operation a

is used to initiate a controller routine that

are combination allows the operator full control over virtually all of the

monitoring for maintenance and data logging

ent setup. For

mai d before accessto p tricting

acce everal

exam

p

precision timing pulse from the GPS receiversults in independent hardware triggers asserted for each DVM. During the unlikely lossre

of GPS synch, triggers are generated by the controllers internal clock.

2. User Interface PC, and UIPC softwareA high quality PC running the windows O.S. is provided fully configured. A typical

configuration includes a 1U high rack mounted Dell server with RAID mounted in the

electronics rack along with a rack mounted keyboard and LCD monitor. The PC isequipped with A Gigabit Ethernet port and 4-port RS232 card for communicating with

various sub systems.

The User Interface PC runs a custom software program called UIPC. This

ardware/softwh

SG functions including setup procedures,and archiving. The UIPC software includes numeric and graphical displays of all data

channels. A Users log allows the operator to enter notes when visiting the system. A

comprehensive alarm system can be easily configured to trigger when any channels

exceed normal limits. Alarm triggers can be configured to notify the operator in anumber of ways including automatic email notification. An automated FTP routine can

be configured to automatically upload data to up to five archival sites on a daily basis.

Configurations are easily accomplished through an intuitive graphical user interfaceGUI).(

Several Virtual Instrument screens are provided to allow remote control of the GEP-3Levi itat on Current Supplies, and other power supply used during instrum

users to login with a passworntaining secure operation UIPC requiresrogram features is granted. Users are assigned one of 5 permission levels res

tures. Sss to anywhere from demo capabilities to full access to the all fea

ples of the UIPC screens are shown in the figures below.


23/36

Figure 11: Screen shot of the UIPC Main Page, showing Gravity and Barometer

Data in real time, Digitally filtered data as it is calculated, GPS status, LHe status,

DAC status and Alarm Status.

Figure 12: Screen shot of the UIPC Residuals F60 Page, showing Observed Gravity,

Modeled Gravity and the Residual in real time (less filter delay).

Figure 13: Screen shot of the UIPC GEP-3 Page, showing the GEP3 virtual front

panel.

The PC is connected to the DAC through either a serial RS422 port or Ethernet



24/36


ed

tom a remote location. All user interface and remote

functions can be accomplished without degrading the timing accuracy or interrupting the

Data Acquisition Controller.

The UIPC can be viewed via a TCP/IP connection using a virtual network connection

(VNC). A broadband connection is required for accessing the system with the Graphical

User Interface (GUI) and is highly recommended for properly maintaining the system.

3. High Resolution 7 Digit DVM (Agilent 34420A Nano Volt Meter)A high precision voltmeter manufactured by Agilent Technologies is provided for precise

sampling of the analog Gravity signal. Specifications for the Agilent 34420A Nano VoltMeter can be found at the manufacturers web site. This device is equipped with a two-

channel multiplexer. The primary channel is used for sampling the gravity control card

GGP filter output. The second input can be used with the optional transportable voltage

standard for calibration of the voltmeter without disconnecting the gravity signal from theinput connector.

. Trimble GPS Receiver

ime synchronization

fter power up. In order to maintain precise time synchronization, the 1 PPS signal fromthe GPS is used to generate a hardware interrupt in the DAC -processor. This allows

timing accuracy to be maintained to within a few milliseconds of UTC. Accurate timing

facilitates comparison of gravity data from gravimeters located at distant locations and isessential when stacking data records.

5. Optical isolators and lightening arrestors for digital dataDigital lines from the GPS receiver and the host PC RS422 port are optically isolated. In

addition, lightening arrestors are provided for the GPS receiver to minimize thepossibility of damage to equipment during electrical storms.

6. Interconnect cablingAll necessary interface cables are provided to insure rapid installation. Users should

specify the length of cabling required from the DAC3 to the GPS antenna and Met-3Station at the time of ordering.

. DDIG-3 - OSG Dual Digitizer Package

be

d for redundant logging of the gravity signal. This provides backup protection inase the primary DVM fails or needs service or calibration. For Dual sphere sensors, this

connection. The UIPC is used for data storage as well as a user interface. Data is logg

to a hard disk at 1 minute intervals. The UIPC is equipped with a Gigabit Etherneinterface allowing easy access fr

4

Timing accuracy is maintained by synchronization with a Trimble GPS receiver. Thisreceiver is capable of tracking eight satellite vehicles enabling rapid t

a

C

To increase reliability, a second high resolution Agilent 34420A 7 DVM can

providec

option is required for digitizing the signal from the second gravity sensor.


25/36


n Agilent E3648A current supply is used to energize the levitation coils in the GSU.

he current supply is controlled through an RS232 interface connected to the userly to simplify

e initialization process.

n order to verify and measure the stability of the DVMs, a high precision transportablel ovides a stable voltage that can be sampled periodically.

d is better than 20 ppm/year and temperature coefficient isO

losure

ted by a factor of more thangaskets provides protection against contaminates that may be

ent. Additional EMI gaskets provide an effective faraday

it is

fects whenever possible. Although care is taken in theronics, at some level, changes in

al. The TREE is designed to operate

hroughout the

D. DPS-4 - Current Supply for initializing Levitation Magnets

A

This device is designed with very quiet and stable linear power supplies. Two outputs

with a maximum output of 5 amps at 8 volts provide ample power for initializing the SG

levitation magnets.

Tinterface PC. The UIPC program provides a virtual front panel for the supp

th

After injecting currents into the SG magnet circuit, heater pulses are applied topersistent switches causing the current to be trapped inside the superconducting

magnet. The heater pulses are generated on the GEP-3 Remote Control Card.

E. VOTS-3 - Voltage Transfer Standard Package

IVo tage Transfer Standard pr

The accuracy of the standar

better than 2 ppm/ C. The standard can be disconnected and sent out for calibration at a

NIST qualified calibration site as part of regular system maintenance. Removal of thestandard does not affect logging of all other channels. This important feature allows the

user to periodically calibrate the DVMs with minimal interruption of the data record.

This need for long term periodic calibration is often overlooked in other data acquisition

systems. The voltage standard allows one to correct data for changes in DVM calibrationfrom aging, or for the exchange of DVMs in the event of a failure.

F. TREE-3 - Temperature Regulated Electronics EncThe Temperature Regulated Electronics Enclosure (TREE) provides a stable environment

for the GEP-3 Control chassis, GEP-3 Power supply, analog to digital converters, DAC

and PC. It consists of a sealed rack with a thermostatically controlled heat exchanger.

hile operating, external temperature fluctuations are attenuaW10 inside the case. Rubber

resent in a harsh environmp

shield providing additional immunity to electrical interference.

Changes in temperature affect all electronic components to some degree. Therefore,

mportant to minimize these efidesign and manufacture of the gravimeter elect

temperature and humidity will affect the gravity sign

slightly above the ambient temperature. This prevents the possibility of condensation

appearing on the electronics which can cause changes in their characteristics. Theenclosure is cooled by two isolated air circulation systems. The external system sucks in

air and then passes it over a finned/heat-pipe heat exchanger. The internal system

circulates the internal air over the opposite side of the heat exchanger and t


26/36


nclosure. In this manner dust and other contaminants are not allowed to enter the

nclosure. To regulate the internal temperature, a thermostat is used to vary the speed ofe external circulating system depending on the internal temperature.

e

eth

Figure 14: Temperature Regulated Electronics Enclosure (TREE)

GWR provides the

igiquartz MET3 Measurement System manufactured by Paroscientific. This systemrovides the ultimate in precision meteorological measurements. Barometric pressure

. Temperatureesolution is 0.01C with total accuracy of 0.5C and relative humidity performance is

midity sensor are fully

mperature compensated. The MET3 system utilizes a Gill Barometric Pressure Port to

G. PRE-5 - Paroscientific Met-3 Meteorological Measurement System

Because the SG sensor is housed inside a vacuum can and operated at approximately 4.2Kelvin, it is virtually free from errors induced by changes in atmospheric pressure.However the SG easily measures the real Newtonian effect of the atmospheric mass

above the SG sensor. For this reason accurately measuring the barometric pressure is an

essential component of obtaining a meaningful gravity time series. Refer to Figure 3.

For recording barometric pressure, temperature and humidity

Dp

resolution is better than 1 microbar with total accuracy of 0.08 hPar

better than 2%. The instrument is enclosed in a durable weatherproof package suitable for

mounting outdoors on a pole or rooftop. Mounting hardware and interface cabling are

included for easy installation.

Data is provided by a Digiquartz

barometric pressure transducer, an external precisionthin film platinum 1000 ohm RTD temperature sensor, and a monolithic IC capacitance

humidity sensor. The barometric pressure sensor and hu

tereduce dynamic pressure errors caused by wind. The Multi-Plate Radiation Shield

protects the temperature / humidity sensor from error-producing solar radiation and

precipitation.

A microprocessor-based electronics provides fully compensated and linearized outputs

via a two-way RS-232 interface. The MET3 interfaces directly with the GWR DAC3

where data is logged with a precision time stamp and displayed along with other critical

SG data.


27/36


III. Un-interruptible Power Supply (UPS) and Power

Conditioning.

GWR offers a variety of equipment for dealing with failures, voltage instabilities,

spikes and surges on the AC power mains. The type of equipment required at any given

site is determined by the operating conditions, and by infrastructure already in place atthe installation site.

Because the SG is used for studying very long period signals (approaching DC) it isimperativ ipment.

ailure to do so can degrade the quality of the data set to a point where it has littlen-

lso,

. At some locations power mains fluctuations can greatly

m lightning strike. In this approach different zones are defined which

the mains to the equipment, more sensitive components can be used wherenecessary without the risk of damage every time a strike occurs.

e that high quality un-interrupted power be maintained to the equ

Fscientific value. For this reason every precaution should be taken to maintain uinterrupted power, especially to the control electronics and data recording system.

At most installation sites, power failures are the primary concern. When power fails to

the SG electronics, the best-case scenario results only in loss of data during the outage. Aworse scenario occurs when power failure results in an offset in the data record. This

is a random event and is normal when the SG looses power and goes out of temperature

regulation. A worse scenario occurs when the outage is of sufficient duration to acause a substantial loss of liquid helium. If the power failure is of sufficient duration or if

the failures are frequent enough then the SG may run out of liquid helium, allowing the

instrument to warm above the critical superconducting temperature. If this occurs, thesensor must be re-initialized, a procedure which requires considerable effort and

expertise.

In some areas low voltage or high voltage conditions can be common. Most equipment israted at the nominal voltage, -5% / +10%. For 230 volt mains this corresponds to voltage

limits of 219VAC to 253VAC

exceed these limits. If these voltage extremes reach the SG electronics or refrigerationsystem the systems may turn off or in extreme cases damage may result.

In addition to power failures, equipment must be protected from spikes and surges.

These problems typically result from lightning storm activity when lightning strikes anoverhead utility line or when lightning strikes the building or Earth nearby the SGinstallation. Typically a zoned approach is the most reliable method of handling spikes

and surges fro

contain protection devices to limit the spike or surge passed to the next zone. Thisapproach is needed because it is impossible to handle the large power dissipation

requirements of the protection device and limit the surge voltage to an acceptable level,

with a single device. By incrementally limiting the peak voltage as the power makes its

way from

Table 8: PRE-5 Specifications

Pressure Accuracy : +/- 0.08 hPaTemperature Accuracy : 0.5 OC

Relative Humidity Accuracy : 2%


28/36


GWR recommends th transformer forprotecting the SG elec

e use of a high quality UPS with isolationtronics at all installa n In addition, s

may be s

tio sites. park gaps, MOVs and

filtering should be use ener een on the power

mains. If the mains A nominal then an additionalvoltage controller is reco duration power outages are

expected, extended battery banks, refrigeration system UPS and backup generator should

e considered. A summary of recommended equipment is given below. More detailed

escriptions of each item listed is provided in the following sections.

ower conditioning equipment and requirements

d at locations where high gy spikes

ofC voltage level may exceed +/-5%mmended. In cases where long

b

d

Table 9: Summary of p

Power Conditions LPZ-1 LPZ-2 LPZ-3 GensetUPS UPS Batter

Refrigeration Electronics Elect. UPS

ies

Advanced Country

Rare lightning activity

Rare Power FailuresShort Duration Power Failures

MIN

Developed Country

Rare lightning activity

Frequent Power Failures

Short Duration Power Failures

MIN

Developed CountryRare lightning activity

Frequent Power FailuresLong Duration Power Failures

MAX

Developing Country

MINSevere lightning activity

Occasional Power FailuresShort Duration Power Failures

Developing Country

Severe lightning activity

Frequent Power Failures

Long Duration Power Failures

MAX

A worst case power backup, power conditioning, and lightning protection zone schemes

is described in Figure 15 below.

n this diagI

th

ram power arrives from the utility via the distribution transformer shown at

s routed to an

e left. The output of the transformer provides a nominal voltage of 230 VAC to the linerouted to the SG hut. At the power meter the utility has installed a 6KV spark gap to

shunt high voltages to Earth in case the power line is struck by lightening.

The utility power normally enters the hut at the service entry power panel. In this design,because frequent power failures are anticipated, the utility power i

automatic power transfer switch. A diesel generator with electronic governor is installed

in parallel with the power mains providing an alternate power source to the automatic

transfer switch. When a power failure condition is sensed on the main, the automatictransfer switch automatically starts the generator. After a brief (programmable)

stabilization period the transfer switch re-checks the condition of the mains and switchesthe input to the generator if the mains are still out. In this manner power is interrupted

for only a short period, during which the UPS must handle load requirements.


29/36

Surge ArrestorOn Utility Line(SPARK GAP)

COMPREHENSIVE POWER LINE CONDITIONING AND SURGE SUPRESSION FOR SUPERCONDUCITNG GRAVIMAT LOCATIONS WITH UNSTABLE POWER MAINS AND/OR SEVERE ELECTRICAL STORM EXPO

HAll metal enclosure with w

provides m ximum shield

enclosure may be sufficlocations. Consult lo

S

a

IELDED H

SerPo

vice Entrywer Panel

LiSu

gp

L1

L2

POWERSPAR

(6

MK GKV)

ETERAP

Diesel FueledPower Genera

(6KVA)tor

AutoTraSw

maticnsferitch

Batery BankFor

6kVA UPS(100AH x 216VDC max)

Buck/BoostTransformer

UtilityPowerLine

UtilityDistributionTransformer

Power MeterEarth Bond

May not bepresent or may

great distancefrom service ent

panel

Utility

owereter

PM

be

rycuitkers

CirBrea

Lightning conductorsto have sweeping

bends

LPZ-1

4 KV Spark Gap atice EnServ trance

d y

r

LPZ-2

ILC-5500

Provide

GENSETFacility Wiring to be

provided by End Use

(NOT BY GWR)

By Power Utilit

Ser try Panelroundancem maxeferred

vice EnEarth/GResist


30/36


The transfer switch is followed by the power entry panel to the SG hut. Inside the panelbox a 4KV spark gap is installed limiting spikes to 4000 volts. The ground bus in this

panel must be adequately earthed so it can handle shunting the spark over energy to

Earth. The Earth line installed should have less than 1 ohm resistance to the Earthingpoint. The adequately Earthed spark gap constitutes lighting protection zone 1 (LPZ-1).

LPZ-1 is followed by an isolation transformer with L/C and MOV filtering. This LPZ-2

device limits output spikes to 10V/s with an input spike of 6KV/s. In addition it

provides a Neutral to Ground Bond eliminating common mode voltage spikes.

Because voltage variations exceeding +/-5% are expected, a precision voltage regulator is

installed in LPZ-3. This device also includes additional filtering.

The voltage regulator output is fed to two separate UPS systems, one for the SG

electronics and another for the Refrigeration system. Separating the UPS into twocomponents allows the UPS systems to be optimized for the different load requirem

The UPS loads are summarized in the table below.

Table 10:

ents.

Summary of UPS Load Requirements for SG electronics and Refrigeration System

Equipment and Condition Load (VA)

TREE enclosure including all standard SG electronics: Maximum for normal operation 565

TREE enclosure including all standard SG electronics: During Instrument Setup 888

Refrigeration System: Maximum for normal operation 1440

Refrigeration System: STARTING 5760

Because of the variability of environmental and soil conditions, GWR recommends thatall users call upon a local expert when installing Earthing systems at the SG installation

site. Generally a SINGLE low impedance (less than 1 ohm) earth line should be installed

at the power service entrance to enclosure housing the SG. All grounds including the SGhut lightning protection system (lightning rod) should be tied to this Earth point.

A.Lightning Current Arrestor (LPZ-1)Lightning Protection Zone 1 (LPZ1) is the first line of defense between the utilities power

ins and the SG equipment. The purpose of LPZ1 is to dissipate the bulk of the energytransmitted along the power mains when a lightning strike occurs. The voltage spike is

limited to a threshold where LPZ2 devices will not be damaged. For this purpose 4KV

spark gaps are supplied for installation at the service entrance to the building whichhouses the SG. If the current return line is NOT bonded to Earth at this point, devices are

installed from: Line-1 to Line-2; Line-1 to Earth; Line-2 to Earth. If the current return

line IS bonded to Earth at this point, devices are installed from L to Earth only.

ma


31/36


itioner (LPZ-2)ightning Protection Zone 2 (LPZ2) is a three-stage surge protection system that consists

lation

nd Earth. Even in cases where Neutral is bonded to Earth at the service

ics is desirable. Th eriesn with a capacitor and MOV limit

10V/us with an input test waveform of 6K .

rotection Zone 3 (LPZ3) is a Precision Automatic Voltage regulator withadditional surge protector and noise filtering. This device uses pulse width modulated

(PWM) switching of a Buck Boost transformer to regulate the output voltage. Stability iswithin +/-3% with the input voltage between 184-264VAC and load of 0-100%. To

maintain the -5% to +10% specification required by the Refrigeration compressor the

input limits are 175 to 300 VAC. (waiting for final spec from TSI).

B. Isolation Transformer and Power Cond

Table 11: LPZ1 Component Specification

Manufacturer : Phoenix Contact

Model : FLASHTRAB FLT-PLUS

Arrestor Rated Voltage : 440 VAC (50/60 Hz)Nominal DC Spark-over Voltage : 2.9KV (+25% - 45%)

Nominal Discharge Surge Current : 50 KA

Lightning Test Cur. Peak : 50 KA

charge : 25 Asspec. energy: 625 KJ / ohm

Protection Level :


32/36


eatching with the availability of a precision electronic governor. When choosing a

genset, it is important

an inductive load such as the refrigeration com

insu o large,we ey

are operated for prolonged periods with in

air/ ome

D. Generator Set (Genset)In order to maintain continuous power during prolonged power outages a backupgenerator should be used. For this purpose GWR offers a carefully matched high quality

diesel fueled genset manufactured by Lister Peter. This manufacturer was chosen

because of its worldwide service capabilities, reputation for high quality and proper siz

Table 13: LPZ3 Component Specification

Manufactu

Model

Capacity : 5500 WNominal V

Operating 6 +/-3% reg. (223-237 VAC)

Operating 1 5 30 +/-5% reg. (218241 VAC)

Operating +5% (208241VAC)Power Efficiency

Automatic : Automa on system failureDimensio : 2 0mmW

Weight : 22.7 KG

Ambient Temperature : 0 to 40 deg. C

rer : TSI Power Corporation

: VRp-5500

oltage & Frequency : 230 V, 47-63 Hz

Voltage (0-100% load) : 184 2 4 VAC @

Voltage : 7 0 VAC @

Voltage : 165 300 VAC @ -10%: 96% (typical)

Bypass tic bypassns 7 x 159mm H x 368mm D

Relative Humidity : 90% max., non-condensing

m

to pay close attention to proper sizing, especially when operating

pressor. If the capacity of the generator is

fficient the compressor will not start reliably. However if the generator is tol engines when tht-stacking will occur. Wet-stacking is a phenomenon of diese

sufficient load, a condition which causes the

fuel mixture to bec to rich.

Table 14: Genset Specification

M etterM

C

C :E :

Operating Voltage : 230 VAC

Voltage regulation (0% - 100% load step) : +/- 3% (within 250 ms)

/- 2%

te item

Dimensions : 72cmW x 104cm H x 198cm L

Weight : 331 KG

anufacturer : Lister Podel : GS8D

apacity (prime) : 6.0 KW

apacity (standby) 5.8 KWngine Governor Electronic

Voltage regulation (steady state, all loads) : +Operating Frequency : 50 Hz

Frequency Regulation (0% - 100% load) : 0.5%

Frequency Regulation (steady state) : 0.25%Cooling : Liquid Cooled (pressurized)

Rotor/Stator Insulation : Class H / Class H

Enclosure : Sound Attenuated EnclosureAutomatic Transfer switch : Included as a separa


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E. Compressor UPS

In order to main in op ation f the S erconducting state it is imperative that

th r this purpose, a liquid heliumd p n

s ea enter g the

fThe refrigeration system is m de up commonly

r th cold ead. he C of 14.5A @

1 avy Industries (SHI) is suppliedb on at voltages ranging from 100

240 VAC. See Table 3 for details and

power to the refrigeration system fails, the liquid helium ballast begins to vaporize

until the cryogen is exhausted. The 35 liters of liquid

ecause of the large power consumption of the refrigeration system it is not practical tobackup the system for l ries. For power failures with

durations greater than 2 hours a backup generator is recommended. However forbr time for starting or

m refrigeration system.

A so consu es approxim during normal operation, the

starting load is considerably higher. The starting current is specified at 57A @ 100 VAC

fo p er li cycl . Fopr

G . The exact unit is decided uponduring consultation with the end user. The best choice depends upon the specific site

co

ta er o G in its sup

e sensor be maintained at 4.2 Kelvin (K) indefinitely. Foewar with a capacity of 35 liters is rovided. During normal operation the refrigeratio

ystem removes all h t in dewar. This eliminates the loss of liquid helium

rom the system thereby allowing the system to operate in a steady state.

a of a compressor and expander unit,

eferred to as e h T ompressor and coldhead draw a total

00VAC. The equipment manufactured by Sumitomo Hey SHI with a built in step down transformer for operati

specifications on this system.

fI

maintaining the contents at 4.2 K

ballast provides sufficient thermal capacity for maintaining the system at 4.2 K for more

than twenty days without power. However during transition to or from refrigerated to un-refrigerated operation, a disturbance on the gravity can be observed for several 10s of

minutes. For this reason it is desirable to minimize the number of power losses to the

refrigeration system even if the down period is short.

When power returns to the refrigeration system up to 24 hours of settling time is required

before the systems cools to a point where it returns to steady state operation. If powerfailures are common, a situation can arise where the liquid helium is gradually lost due to

insufficient refrigeration on-time. If this situation might occur, it is critical the power to

the refrigeration be protected by a UPS and backup generator.

B ong periods with a UPS and batte

idging power failures of less than 2 hours and to provide adequate

aintaining a backup generator, a UPS is recommended for the

lthough the compres r m ately 1.5KW

r approximately 20 ow ne es r this reason a 6KVA UPS is required toovide fail safe starting of the compressor.

WR offers several UPS configurations for this purpose

nditions.


34/36


uisition system are backed up by a

edicated UPS. There are several important factors that were considered when choosing

the load is between 30%

and 90% of rated capacity. With loads of less than 30% the efficiency drops

Even if the compressor load is turned off (a condition referred to as load

The liquid helium acts as a ballast to maintain the SG at 4.2K during power

F. Electronics UPS

The SG electronics including the GEP3, and data acq

Table 15: Compressor UPS Specification

Purpose : Backup short duration power outages

: Final voltage stabilizer and conditioner

Operating Voltage : 230 VAC

Standard Battery configuration : TBD

Manufacturer : Toshiba or FalconModel : TBD

Capacity : 6 KVA at startup

Operating Principal : Double Conversion

Nominal Voltage & Frequency : 230 VAC

Automatic Bypass : YES on UPS failureDimensions : TBD

Weight : TBD

Ambient Temperature : TBD

Relative Humidity : TBD

Extended Battery configuration : TBD

dan architecture which utilizes separate UPS systems for the electronics and refrigeration

system as apposed to one large UPS. These considerations are outlined below.

The SG electronics consumes less than 600 watts of power compared to nearly6KW required by the compressor at startup.

In general the efficiency of a UPS is maximized when

rapidly. If the SG electronics were to be run from the same UPS as the

compressor, the load would be 10% of the UPS capacity. Considering the in-

efficiency of the UPS when operating at this load, adequate run times can not beachieved with a practical sized battery bank using a 6KVA UPS for the SG

electronics.

shedding), this does not the address the problem of the inefficiency whenoperating a UPS at 10% of load capacity.

Continuous operation of the electronics is critical to maintain the DC gravitybaseline.

failures. Therefore, maintaining uninterrupted power to the refrigeration systemis not as critical as it is for the SG electronics.


35/36


Table 16: UPS for SG Electronics, Specifications

Mfg. / Model TSI Power, UPS-1800E

Electrical

Capacity in VA (Watts) 1800 VA (1300W)

Transformer type output isolation transformerFerroresonant constant voltage

Isol secondary transformer windingsation 100% galvanic isolation through separate primary and

Input

Nom lt c single phaseinal voltage 230 vo s a

Ope t usingrating voltage 185 275 Volts withou batteriesNom % (outpu are linear with respect to frequency)inal frequency 50 Hz sinusoidal +/-5 t voltage variations

Ac Winput cord Hard ired

Output

Nominal voltage 230 volts ac +/-3%

Voltage regulation +/-5% upo s loadn change in line or

Crest Factor 3:1

Output waveform Pure sine wave, less than 5% THD with full linear load

Power efficiency 90%, under full load conditions

Pow itcher on/off Switch On/off rocker power sw

Transfer time Zero (continuous no-break)

B(option-

atteries & Backup Time Maintenance free sealed lead-acid batteries

Two EXT-800 battery pack configuration allows hot swapping of battery packs

60 minutes under typical SG electronic load of 565 VA

EXt-8000s battery recharge time is 8 hours to 90% capacity

1)

Batteries & Backup Time

(option-2)

Valve Regulated Lead Acid external battery banks

48VDC (4x12VDC) 134 Ah @ 48VDC Maximum of four (4) parallel banks @ 4-5 hours per bank Maximum backup time of 20 hours under typical SG electronics load 48 Kg/battery * 4 parallel batteries x 4 banks = 720 Kg max Customer to supply ventilated housing at the site of installation

Noise filtering Low-pass filter reduces noise over a wide frequency range.

Surg te pro ection A three-stage surge protection system consisting of:

Ferroresonant isolation transformer;

Capacitor

M.O.V.

Surge test conditions ANSI/IEEE C62.41-1991 test pulse. Category B3,

Combination Wave, 6000Volts, 3000Amps.

Surge let-through voltages Test pulse injection: Line-Neutral, Line-Ground & Neutral-Ground.

Combination Wave: L-N: 35V, L-G: 35V, N-G: 0.5V

Indicators ED bar-graph indicators:10 L

Output voltage;

battery voltage;

Output load 0 to 100% of rated power.

4 status LEDs indicating:

AC power present; inverter, status; Output mode;

AC load, and battery status.

Physical

Dimensions 431.8 x 533.4 x 165mm (NOT including battery packs or chargers)

(17"W x 21"D x 6.5"H)

Weight 59 kg (130 lbs) (NOT including battery packs or chargers)

Rack mounting options Rack mount available upon request

FET inverter circuit FET inverter circuit ensures efficiency and reliability.

Environmental

Ambient temperature 0to +40C, 90% relative humidity 30-90%, (non-condensing @ 25C).

Cooling method Forced air (fan cooled)


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V. GWR-OG-4KCS aintenance or Failure Backup 4 Kelvin

oler Sy

d tall a backup refrigeration system when a new SG is

nce the em it is more prone to failure

e other sys on systemdes coldhead, mpressor, hoses, and SG sealing flanges. It is installed and tested in

ng SG Once in place, this allows the coldhead and entire

to be ained personnel. The option includes a

ent ervices required at the time of

tion

es ar G is installed at a new location. A list of these

es and se w.

nc

Stable Support P r, Electrical power u

te ooling and filling of liquid heliumdewar. Consult W y depending on the

installation cond io

1 -2 cylinders U a least one cylinderemain at

rom ing if required

I M

Cryoco stem

In all cases it is esirable to ins

installed. Si refrigeration system is a mechanical syst

than th tems required for SG operation. The backup refrigeratiinclu co

parallel duri commissioning.

refrigeration exchange by minimally trcomplete redundant refrigeration system as described in Table 3.

V. Equipm , expendables & s

installa

Various suppli e required when an S

expendabl rvices is shown belo

Suitable E losure or hut of sufficient size to house the SG and its components.

ie ideally connected to basement rock.

35 - 100 li

so rce.

rs o iqf l uid helium used for initial cG R for exact amount as requirements var

it ns.

ltr High Purity (99.999% pure) helium gas atto r the operating site permanently.

Services f a a irqu lified electrician for site w

Important Note

st instrumentation

for their customers. So frequent improvements are made on the

ology and onents of equal or greater value or

serviceability may be substituted without notice

GWR Instruments Inc., is committed to provide the be

techn alternate comp

gwr observatory superconducting gravimeter_2007!10!02

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