pressure calibration
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TRIGON Management & Industrial Corp.
ContentsContents
Topics: Slide No:• Why measure pressure? 3• What is pressure? 4 - 5• Pressure terminology 6 - 11• Inferring non-pressure variables 12 - 29• Pressure measurement technology 30 - 44• Pressure calibrators 45• Exercises 46 - 48
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TRIGON Management & Industrial Corp.
Why measure pressure?Why measure pressure?4 Common Reasons4 Common Reasons
Safety• prevent pressurized pipes & vessels from burstingProcess Efficiency• variation of pressure below or above a set-point will result in
scrap rather than useable product in some manufacturing process
Cost Saving• preventing unnecessary expense of creating more pressure or
vacuum than is required saves moneyInferred Measurement of Other Variables• rate of flow through a pipe• level of fluid in a tank• density of fluid • how two or more liquids in a tank interface
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Pressure terminologyPressure terminologyEngineering UnitsEngineering Units
Pressure is defined as FORCE applied over a unit AREA.
P = F/AExamples of pressure units:Units of force per unit areaPascals Pa N / m2 (Newtons / square metre)psi lbs/in2 (Pounds / square inch)Bar Bar = 100,000 Pa
Units referenced to columns of liquidsins. water gauge in H2Omm water gauge mm H2O
ins. mercury in Hg mm mercury mm Hg
Atmosphere atm
Pressure applied by a 1 inch column of mercury with a density of 13.5951 g/cm³.
Pressure exerted by the earth’s atmosphere at sea level (approximately 14.6959psi)
Pressure applied by a 1 inch column of water at 20°C.
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What is pressure?What is pressure?The Same Weight, Different PressureThe Same Weight, Different Pressure
Weight = 100lb
100 sq ins1 sq ins
100 sq ins 1 sq ins
Pressure = 1lb/in² Pressure = 100 lb/in²
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What is pressure?What is pressure?Liquid & Gas PressuresLiquid & Gas Pressures
LIQUIDS The pressure exerted by a liquid is influenced by 3 main factors.
1. The height of the liquid.2. The density of the liquid.3. The pressure on the surface of the liquid.
GASESThe pressure exerted by a gas is influenced by 2 main factors.
1. Volume of the gas container.2. Temperature of the gas
Note. Gases are compressible whereas liquids are not
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Pressure terminologyPressure terminologyPressure Control LoopPressure Control Loop
I/P
PT
PIC • Pressure Loop Issues:– May be a Fast Process
» Liquid» Small Volume
– May Require Fast Equipment
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Pressure terminologyPressure terminologyReference PressureReference Pressure
Atmospheric PressureApprox. 14.7 psia
AbsoluteGage Compound
RangeBarometric
Range
Total Vacuum(Zero Absolute)
Pressure
Gage(psig) - Level of pressure relative to atmospheric– Positive or negative in magnitude
Absolute(psia) - based from zero absolute pressure - no massTypical atm reference: 14.73 psia
Compound Range (psig) - Gage reading vacuum as negative value
Differential(psid) - difference in pressure between two points
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Pressure terminologyPressure terminologyQuizQuiz
?Psia 19.7 5 psig
Atm. Pressure 14.7 psia5 psi vacuum
?Psia
?Psig -5 9.7
Absolute Zero
Total Vacuum
Assume: Patm = 14.7psia; 28 inches H2O per psi
1000 in H2O = ___________ psi35.71
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Pressure terminologyPressure terminologyMeasurable PressuresMeasurable Pressures
The four most common types of measurable pressures used in the process control industries are:
1. Head Pressure or Hydrostatic Pressure.Head Pressure or Hydrostatic Pressure.Pressure exerted by a column of liquid in a tank open to atmosphere, HEAD PRESSURE = HEIGHT x DENSITY
2. Static Pressure, Line Pressure, or Working pressureStatic Pressure, Line Pressure, or Working pressurePressure exerted in a closed system
3. Vapor PressureVapor PressureThe temperature at which a liquid boils, or turns into a vapor varies depending on the pressure. The higher the pressure, the higher the boiling point.
4. VacuumVacuumAbsolute pressure below atmospheric pressure ( a compound range gage transmitter will read a negative pressure)
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Pressure terminologyPressure terminologyMeasurable PressureMeasurable Pressure
Typical Vapor Pressure Curve
Pres
sure
(log) liquid
gasHigher Altitute
Lower Altitute(Sea Level)
T1 T2
Vapor pressure increases with temperature. • Liquid boils when its vapor pressure equals
atmospheric pressure.
Temperature
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
Flow Restriction in Line cause a differential Pressure
Line Pressure
Orifice Plate
QV= K DP
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
Theoritical equations come from 3 sources:
Continuity Equation• Flow into pipe equals flow out of pipe and is the same at all pipe
cross sections (Conservation of Mass)
Bernoulli’s Equation• (Conservation of Energy for fluid in a pipe)
Experimentally Determined Correction Factors• Discharge Coefficient• Gas Expansion Factor
Qm= K DP
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
Continuity EquationThe volume flowing into a pipe equals the volume flowing out of pipe, assuming constant density
FlowA1V1 Flow
A1v1 = A2v2A = area of pipe cross sectionv = velocity
A2V2
v1 = A2/A1 x v2 ⌫ πd2/4 x πD2/4 v1 = d2/D2 x v2 ⌫ d/D = βv1 = β2 x v2
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
Bernoulli’s EquationThe total energy before the restriction in the pipe
⌫ cancel - off for level pipe
Three energies:Kinetic (1/2ρv2)Potential (ρgh)Static Pressure (P)
v1 v2
P1
must equal the total energy after the restriction.P2
Flow D d
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
P 1 P 2..1
2ρ v 2
2 ..12
ρ v 12
P1 ..12ρv1
2 ..ρg h1 P2 ..12ρv2
2 ..ρg h2Before restriction After restriction
common
2 / ρ x dP = v22 - v1
2 V12 = (β2 x V2)2
2 / ρ x dP = v22 - β4 x v2
2
2 / ρ x dP = (1- β4)v22
common
dP = ½ ρ (v22 - v1
2)
subject
Re-arrangedv22 = (2 / ρ x dP) / (1- β4)
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Inferring nonInferring non--pressure variablespressure variablesFlowFlow
v2 = [(2 / ρ x dP) / (1- β4)] ½
v2 = (2)½ x (1/ρ)½ x 1/ (1- β4)½ x (dP)½
Qv2 = A2 x v2
Qv2 = (πd2/4) x (2)½ x (1/ρ)½ x 1/ (1- β4)½ x (dP)½
constant constant assumed constant
velocity of approachconstant - “E”
Volumetric Flow Qv2 = k (dP/ρ)½
Mass Flow k (dP/ρ)½ x ρ Qm2 = k (dP x ρ)½
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(i) What would be the differential at 10m³/s?
Quiz:If an orifice plate creates a differential of 50 kPa at 30m³/s
(ii) What would be the flow rate at 30kPa differential?
Qv = K √DP
Qv1 √DP1--- = ----Qv2 √DP2
30/10 = √50/ √DP2
Qv = K √DP
Qv1 √DP1--- = ----Qv2 √DP2
Inferring nonInferring non--pressure variablespressure variablesFlowFlow
30/Qv2 = √50/ √30
Qv2 = 23.26m³/sDP2 = 5.6kPa
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
H
P P P P
D
Liquid
Hydrostatic Pressure - The liquid will rise to the same level in each vessel regardless of its diameter & shape.
Unit Area (eg. per cm2)
Which shape gives higher pressure at the bottom of the vessel?
Similar height of column will have same mass acting on the same unit area
SAME PRESSURE
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
The hydrostatic pressure exerted by the column of liquid depends on the S.G. (or density) of the liquid and its vertical height.Density of liquid = DAverage cross-section area of vessel = AVertical height of liquid = HVolume of liquid, V =Total weight of liquid, M =
=Pressure at the bottom of liquid = weight of liquid
cross-section area==
H x AD x V
A x H
D x HS.G x H
D x
(D x A x H) / A
With reference to inches or mm WATER
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
P = r x g x height x area / area
mass x g
r x volume
P= force / area
g = gravitational acceleration
height x area
Phead = r x g x h Pascal
Density = mass/volume = r
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
XMTR
HL
Ullage or Vapor
S.G
Phead
0%
100%
Hei
ght
Cancelled off since both L & H sides of transmitter experience it.
DP Transmitter at the bottom of the tank measures HEAD.HEAD = pressure at the bottom of a column of liquid with known relative density (S.G)Phead = S.G x Height
Height = Phead / S.G
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
Quiz: Open TankWhat is the level if Pmax = 120 inH2O, s.g.= 1.2?
?Height = Phead / S.G
Height = 120 / 1.2XMTR
Height = 100 inchesL H
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Inferring nonInferring non--pressure variablespressure variablesLevelLevel
Quiz: Closed TankDry leg: no fluid in low side impulse piping, or legPh = 105 psiPl = 100 psiWhat is level if s.g. = 1.0?
Phead
Ptop= Ullage
dP = 5 psi = 5 x 28 inH2O XMTRHeight = 140 / 1.0L H
Height = 140 inches
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Inferring nonInferring non--pressure variablespressure variablesDensityDensity
Phead(top)
Pbottom
Phead(bottom)
h1
h2
H
2 - h1)
diff. Pressure / dist. betw. taps
PtopPbottom =
Ptop =
Pbottom - Ptop =
Hence,
S.G =
S.G (h
S.G X h2
PtopS.G X h1
H
Liquid level must be above the Top transmitter tap.
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TRIGON Management & Industrial Corp.
Inferring nonInferring non--pressure variablespressure variablesDensityDensity
Pbottom
Ptop
50”
H
H
Quiz:Determined the S.G of the process fluid if Ptop = 20 psiPbottom = 22 psiDistance between taps = 50 inchesAssuming 1 psi = 28”H2O
S.Gprocess = DP / dist. betw. Taps= 56 / 50= 1.12
DP = (22 -20) = 2 psi = 56”H2O
Ullage
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Inferring nonInferring non--pressure variablespressure variablesInterfaceInterface
Indirectly measures liquid Interface
Pbottom
Ptop
L H
Remote Seal
Vapor
0%
100%
SG1
SG2
Dist. Betw. Taps
(h1 - h2)
Total Liquid level must always be above the Top transmitter tap.
SGf
h1
h2
At 0% Liquid Interface (4mA)
DP = Hside - Lside
= (SG1*h1) - [(SGf*(h1-h2)) + (SG1*h2)]
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TRIGON Management & Industrial Corp.
Inferring nonInferring non--pressure variablespressure variablesInterfaceInterface
Indirectly measures liquid Interface
Total Liquid level must always be above the Top transmitter tap.
Pbottom
Ptop
L H
Vapor
0%
100%
h1
h2
Remote Seal SG1
SGf
Dist. Betw. Taps
(h1 - h2)
SG2
At 100% Liquid Interface (20mA)
DP = Hside - Lside
= [SG2*(h1-h2) + SG1*h2)] - [(SGf*(h1-h2)) + (SG1*h2)]
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Inferring nonInferring non--pressure variablespressure variablesInterfaceInterface
Application Example:
• Transmitter calibrated from 120”H2Oto 132”H2O
• Determine % of interface of Liquid A with respect to Liquid B
Vapor
0%
100%
SG1= 1.0
SG2= 1.1
Pbottom
Ptop
L H
Remote Seal
10 ft
Liquid A
Liquid BIf transmitter reads 123 inH2O
% interface = (3/12) * 100%= 25%
123 inH2O
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Pressure measurement technologyPressure measurement technologyPressure IndicatorsPressure Indicators
BarometerUsed to measure Barometric Pressure
Reference is 0 psia, due to low vapor pressure of Hg.General operating principle:
PheadPatm
Barometric Pressure = Atmospheric Pressure
29.9 inHgWhat is the barometric Pressure?
• Tube completely filled with mercury & Invert into the container filled with mercury.
• The mercury level in the tube will drop until it reaches an equilibrium.
• This equilibrium height is a measure of atmospheric pressure. Phead = Patm
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Pressure measurement technologyPressure measurement technologyPressure IndicatorsPressure Indicators
ManometersU-tube with one side reference, one side measured pressure
H
dP = H (SGfill fluid - SGprocess fluid)How to check for dP ?– Reference side can be:
• Sealed (AP reference)• Open to atmosphere(GP reference)• Connected to reference pressure(DP reference)
– Typically used for low pressures, non process control
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Pressure measurement technologyPressure measurement technologyPressure GaugesPressure Gauges
Mechanical
The mechanical element techniques convert applied pressure into displacement.
The displacement may be converted into electrical signal with help of Linear Variable Displacement Transformer (LVDT).
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Pressure measurement technologyPressure measurement technologyPneumatic Pressure CellsPneumatic Pressure Cells
Output to Actuator (or Relay)Constant flowrate maintained
(Compressed air)
NozzleFlapper
Bourdon Tube
Process Pressure
Pneumatic ControllerRelay’s modulated output is the controller output which is usually a pneumatic signal that adjusts the final control element (Control valve)
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Pressure measurement technologyPressure measurement technologyPneumatic Pressure CellsPneumatic Pressure Cells
Pressure TransmitterProduce a linear output proportional to input pressure
Zero Scale: Full Scale:
3 psig15 psig
Disadvantages– Reconfiguration costly– Losses occur over long
piping runs– Performance levels are not
comparable to electronic instrumentation
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Pressure measurement technologyPressure measurement technologyElectronic Pressure TransmittersElectronic Pressure Transmitters
– Made up of 2 main elements:• Transducer - Electronic sensor module
that registers process variable and outputs a corresponding usable electrical signaleg. resistance, millivolts, capacitance, etc.
• Electronics - Convert transducer output to a standard output signaleg. 4 - 20 mA, 1 - 5 V dc, digital signal, etc.
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TRIGON Management & Industrial Corp.
Pressure measurement technologyPressure measurement technologyElectronic Pressure TransmittersElectronic Pressure Transmitters
Example of Application
Transmitter configured to operate from:
0 to 50 psiElectronic Output:
4 to 20 mAThis mean 0% reading (0 psi) represents 4 mA and 100% reading (50 psi) represents 20 mA.
Transmitter
Signal To Controller(Standard signals)
Sensing Diaphragm
Signal fromsensor module(Transducer)
Process Variable (Line / Static Pressure)
What will be the output current at 25 psi reading?4 + (25/50)*16 = 12 mA
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Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
Characterized by the type of sensing element:
– Variable capacitance– Variable Resistance (Wheatstone bridge)
• Strain gauge» Thin -film strain gauge» Diffused, strain gauge
– Variable inductance– Variable reluctance– Vibrating wire– Piezoelectric
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TRIGON Management & Industrial Corp.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
Variable Capacitance
• Process pressure transmitted thru isolating diaphragm
• Distortion of sensing diaphragm proportional to the differential pressure
• Position of sensing diaphragm detected by capacitor plates
• Differential capacitance translated to 4-20mA or 10-50mA output dc signal.
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TRIGON Management & Industrial Corp.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
Variable Resistance / Piezo-Resistive
Thin Film Strain Gauge
Diffused Strain Gauge
• Process pressure transmitted thru isolating diaphragm• Very small distortion in sensing diaphragm• Applies strain to a wheatstone bridge circuit• Change in resistance translated to 4-20mA or 1-5V dc signal• GP XMTRs - ref. side of sensor exposed to atm. Pressure• AP XMTRs - sealed vacuum reference.
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Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
• Piezoelectric crystal is a natural or a synthetic crystal that produces a voltage when pressure is applied to it.
• Voltage produce by crystal increases with increases in pressure and vice-versa.
• The produced small voltage is then amplified to a standard control signal.
Piezoelectric
Amplifier & electronics
Control Signal
Piezoelectric Crystal
Diaphragm
Process Pressure
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Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
• Inductance is the opposition to a change in current flow
• Alternating current pass through the coil
• Elastic element connected to core
• Applied pressure deflects elastic element
• Position of core changes relative to coil resulting in change in inductance
• Resistor connected in series with inductor to measure change in voltage.
Variable Inductance
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Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
Variable Reluctance• Reluctance is a property of
magnetic circuit• A moving magnetic element
located between two coils• Coil turn electromagnet when
excited by AC source• Position of element with respect to
the coils determines differential magnetic reluctance
• Thus differential inductance within the coils
• A bridge is used to measure changes in a circuit
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TRIGON Management & Industrial Corp.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
Vibrating Wire• Wire located in magnetic field
vibrate when current pass through it
• Wire movement within field induces current into it
• Induced voltage amplified as output signal
• Vibration frequency depends on wire tension
• Elastic element connected to wire.
• Frequency of wire vibration become a function of measured pressure
• Direct digital output signal
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TRIGON Management & Industrial Corp.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
– Sensor (transducer) module is part of the transmitter.– Sensor will become active only when the transmitter is
powered. (Attenuation)– Output Electronics in the transmitter translates the
userable electrical signal from the sensor into a standard output signal.
Output Electronics
Sensor Module
Output Electronics
Sensor Module
Diaphragm Seal
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Pressure calibratorsPressure calibratorsISO RequirementISO Requirement
ISO Require calibration device to be 4 times more accurate than the accuracy of the instrument being calibrated.If the reference accuracy of a 3051C transmitter is 0.075% of span,
– What should the accuracy of the C/V pressure source be?
– the equipment for calibrating the pressure source?
If the diameter of the ball on a dead weight tester is 0.75 inches. The weight of a plate is 723g.
– What is the pressure required to freely float that plate on the dead weight tester (g/cm2)?
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ExerciseExercise
1. If the atmospheric pressure drop by 0.1 % and the line pressure remains unchanged, what changes will occur in the
readings?(A) AP reading will change.(B) GP reading will change.(C) Both reading will change.(D) Both reading will not change.
[ ]
2. If a customer wants to measure vacuum, what type of transmitter should be used?(A) AP(B) DP(C) GP [ ]
Liquid flow
Line pressure = 80 psig
94.7psi 80.psi GP Transmitter
AP Transmitter
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ExerciseExercise
50 psig80 psig
c a b
Write down the readings in (psi) that are recorded by the transmitters in the above application (Atmosphere = 14.7 psi).
3. Differential Pressure Transmitter (a): [ ]
4. Gage Pressure Transmitter (b): [ ]
5. Absolute Pressure Transmitter (c): [ ]
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