chapter 4(equipment and accessories)
TRANSCRIPT
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Equipment andAccessories
(M. Anees Akhtar)
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Vacuum Boxes Vacuum Boxes are used for Bubble Leak testing in the
field
Vacuum boxes are available for rounded surfaces,
corner seams and vertical seams. The box should be able to withstand an external pressure
of 100 kPa.
Flexible gaskets are provided to seal the enclosure to thetest surface when pressure is applied to the vacuum box.
The vacuum box should be of convenient size such as 15cm by 75 cm (6 inches wide by 30 inches long).
Each box contains a window in its top opposite the openbottom.
Suitable connections, valves, lighting and gauges arethe accessories of the vacuum boxes.
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Vacuum Boxes
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Vacuum Boxes
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Desirable features of Vacuum Boxes Desirable features for a vacuum box are. Ability to readily admit natural or artificial light Close proximity of viewing window to the surface of the
weldment being inspected. Capability for easy initial seating when starting evacuation,
and good sealing properties to hold the vacuum.
Equipped with a highly portable, readily availableevacuation device with sufficiently high flow to be able toovercome a certain amount of seal leakage and rapidlyevacuate the box to the required pressure differential.
Easily read dial gauge Quick acting valve for either shutting off the evacuation
device or controlling the level of pressure differential. A vacuum box for bubble leak testing should be
checked for workability before performing leak testing todetermine that the condition of the box components issuch that it is capable of attaining a higher pressuredifferential than is required for the test.
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Application of Foaming Solutions following are the methods listed in order of
decreasing preference
Spray application using a pump-type garden spray
can with a fine orifice.
Spray application using a plastic squeeze bottle or anoil squirt can
Brush application using a short handle painters brush,
1 to 3 cm in width.
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Gaskets The gasket system used with a vacuum box is
critical to the ease with which the box can behandled and sealed to the test surface so as to be
able to hold a vacuum. The gasket must be shaped so that the initial area
of the gasket contact with the test surface is smallto make seating of the vacuum box easier.
Neoprene gaskets of 20 to 40 durometer providegood flexibility.
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Dial Gauges for Vacuum BoxA pressure or vacuum dial gauge must be readily
visible to the operator controlling the pressure withinthe vacuum box during leak testing.
Pressure indicating gauges used in testing shouldpreferably have dial graduations covering a rangeof about double the intended maximum pressure.
In no case should the range be less than one and
one half not more than four times that pressure.All gauges used shall be calibrated against a
standard dead weight tester a calibrated mastergauge, or a mercury column, and recalibrated atleast once each 12 months.
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Pirani Guages
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Pirani Guages Pirani gauges employ a wheatstone bridge circuit,which serve to heat a filament and to balance itsresistance against a standard resistor sealed off in highvacuum.
A change of pressure causes a change of filamenttemperature and, consequently, of the filamentresistance, thus unbalancing the bridge.
The pressure can then be measured in terms of theunbalanced voltage. Alternatively, the power required
to maintain the filament temperature at a constant levelis a measure of pressure. The temperature in this case iskept constant by means of a feedback circuit.
produce fairly accurate absolute pressure readings fromatmosphere to 0.1 or 0.01 Pa (1.0 or 0.1 mtorr).
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Hot Cathode Ionization Guage The high filament temperature (1500 C to 2200 C)causes electrons to escape and become free electrons.
The free electrons create gas ions as they acceleratetowards positively charged grid.
They are absorbed by the grid; forming electron currentcalled emission current.
Gas ions are attracted to negatively charged ioncollector where they pick up electrons and becomeneutral molecule. This neutralizing electron current iscalled ion current.
The magnitude of the ion current is an indicationof pressure.
A conventional hot cathode ionization gauge measurespressures in the range of 10-3to 10-10mbar.
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Hot Cathode Ionization
Gauge(BayardAlpert Ionization
gauge)
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Cold Cathode Ionization Gauge
(Penning Gauge) Measuring Range
10-210-7mbar The cold cathode ionization gauge is basically a direct current (dc) glow
discharge tube . The tube has two metallic electrodes(an anode and a cathode), one at
each end of the tube. A dc voltage is applied in series with a large current limiting resistor. The
resistor ensures discharge stability and prevents excessive current flow. To strike a discharge, an initial supply of electrons is needed from the
cathode. Field emission occurs when the cathode is held at a high voltage and
electrons are freed. Once electrons are released from the cathode, they accelerate towards
the anode creating positive gas ions by impact ionization. The positive
ions accelerate towards the cathode. The energy from positive ionsimpacting the cathode releases an additional supply of secondaryelectrons sufficient in quantity to sustain the discharge process. Thequantity of electrons released by ion impacts on the cathode dependson the number of ions produced in the discharge.
The number of ions produced depends on the gas pressure in the tube. Measuring the ion current at the cathode is an indication of pressure.
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cold Cathode Ionization Gauge
(Penning Gauge)
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Radioactive Ionization Gauge
The ions are produced by radiations (usually alphaparticles) emitted from a radioactive source.
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Evacuation system for Vacuum Box
Leak TestingA small box mounted air ejector connected to a
compressed air supply.
A small portable vacuum pump.
The vacuum box, when placed over theexamination area, should be evacuated at least35 kPa pressure differential with respect toatmospheric pressure.
The 35 kPa pressure differential can be verified bythe dial gauge.
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Lighting Good lighting is necessary for the safety and reliability of
the test results.
Two general sources of lighting:
Artificial Lighting
For continuous shift operation, artificial light is essential.
For other types of operation, it must be relied upon from 20to 50% of the total working hours, excluding overtime work ornight work.
General Lighting
General lighting is the base or minimum amount of light
required. The uniform distribution of light to produce equivalent seeing
conditions throughout an interior.
Localized general lighting sources are arranged 3 meter ormore above the work to provide sufficient light for generalsafety and for ordinary visual needs.
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Absolute Pressure Dial Guages Aneroid Capsules Upto 350 kPa
Bourdon Tube PressureGauge Above 350 kPa
The ordinary dialpressure gauge hastypical accuracies inthe range of 0.25 to
0.33% of full scaleindication whenrecently calibrated.
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Barometers
Can be used to measureabsolute pressure.
The height of mercurycolumn indicatesatmospheric pressure.
These pressuremeasurements are absolute,because the pressure insidethe sealed end of the tubeis always zero.
There is only vacuum abovethe mercury column, sothere is no contribution dueto air pressure.
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Instruments for Pressure
Measurement Types of pressure gauges that provide visible
indications during leak testing include;
Absolute pressure dial gauges.
Aneroid barometers.
Ordinary dial gaugesindicating pressure relative to
ambient atmospheric pressure as well as water.
Manometer , UTube mercury manometers and
mercury column barometer.
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Instruments for Temperature
MeasurementDew point sensor
The dew pointis the temperature below whichthe water vapor in a volume of humid air at a
constant barometric pressure will condense into liquidwater.
Vapor pressure due to moisture adds to the totalpressure measured by the pressure measurementinstruments.
It works on the principle of balancing the moisturecontent in the element with the moisture content ofthe surrounding atmosphere.
Used to test volume subject to pressure changeleakage rate measurement tests.
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Vacuum Pumps
Vacuum pumps are classified into two
groups Gas transfer pumps
Capture pumps
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Low and Medium Vacuum
Pumps Rotary Vane Pump
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High and Ultra High Vacuum
PumpsDiffusion Pump
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High and Ultra High Vacuum
Pumps Turbomolecular Pump
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Vacuum ValvesValves are required to isolate either part or all
of the pumping group from the system which isbeing evacuated or are necessary to control
the pumping sequence.Vacuum valves must;
be free from leakage.
Offer minimum flow resistance and
Contain materials that do not outgas.
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Vacuum Valves
Diaphragm Valves Gate Valve
Closed
Open
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Vacuum Connector O Rings
The O ring are made of an elastomer materialwhich is trapped and compressed between two
surfaces to form the seal. Demountable joints for vacuum in the range
from atmospheric pressure down to 10-7mbarnormally utilize O ring for the sealing method.
Grooved flanges are used to issue the leak
tight system and less compression suchgrooves should meet the followingrequirements.
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Vacuum Connector O Rings
The O ring are made of an elastomer material
which is trapped and compressed between two
surfaces to form the seal. Demountable joints for vacuum in the range
from atmospheric pressure down to 10-7mbar
normally utilize O ring for the sealing method.
Grooved flanges are used to issue the leaktight system and less compression.
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Vacuum Connector Groovs in the flanges should meet the following
requirements. The cross-section of the groove should be of the order of 10
percent greater than the cross section of the O ring, toallow for a metal to metal flange contact withoutovercompressing the O ring.
To provide positive retention of the gasket, when the seal isnot made.
To provide the minimum volume of trapped gas.
To allow reuse of the O ring after the seal is broken. In thisrespect, overcompression or grooves with sharp corner oracute angles are of bad design as this leads to permanent
deformation or cutting of the O ring. To permit easy flange machining end to wide machine
tolerance.
Orings generally provide better sealing than flat gaskets.
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Vacuum Connector Hinged Clamps
These clamps are
used for quickrelease
applications
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Mass Spectrometer Common functions of commercially available
mass spectrometer leak detector instruments
are: Pumping tracer gas samples from leaks in test
objects into the vacuum of the instrument.
The ionization of gas sample molecules by
electron impact.
The sorting and identification of positive ions
according to their mass-to-charge ratios.
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Mass Spectrometer Tube
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Sniffer It is a simple tracer gas
collection device
It is connected to leak
detector with a pumping
device. The pump provides the
suction necessary to
draw tracer gas samples
to the sniffer inlet through
the connecting hose andinto the sensing element
of the leak detector.
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Compound mastics for sealing to vacuum
Apiezon Type
(hydrocarbon)Silicone type Fomblin
Good lubricant Yes No Yes
Vapour pressure (mbar) Low Approx. 10-6 Low
Temperature suitability 125C max. -40 to 200C -02 to 200C
Cost Moderate Low High
Chemical stability Moderate Good Very good
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Tuning and calibration Tuninga helium leak detector is a procedure
of adjusting the electronic controls of theunit to achieve maximum signal strength,highest reading on the leak rate meter whileadmitting a known flow of helium to the leak
detector.Calibrationof the instrument merely
correlates the output signal with a knownflow of helium standard calibration leak.
Calibration is usually accomplished byadjusting the gain, or amplifier feed back,until the leak rate meter reading agrees withthe value stamped on the standard leak.
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Calibrated Leak Standards
Reservoir Calibrated Leaks
Reservoir leaks contain their own tracer gas
supply.
NonReservoir calibrated Leaks
Non-reservoir leaks to which tracer gas is
added during testing.
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Classification of Calibrated
Leaks
LEAKS
Reservoir
Permeation Capillary
Fixed ValueVariable
Value
Porous Plug
Non -Reservoir
Capillary
Fixed ValueVariable
Value
Porous Plug
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Properties of calibrated LeaksAn ideal calibrated leak should have the
following properties:
The leakage rate should be constant, and should
remain unaffected by ambient conditions. The calibration should be accurate.
The physical size should be convenient.
The calibrated leak should not be too delicate or
fragile. The calibrated leak should have its own gas
supply.