01 pressure basic1
TRANSCRIPT
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Level 1 - Pressure 1RMT Training - 05 /98
Fundamental TrainingFundamental TrainingLevel 1
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Level 1 - Pressure 1RMT Training - 05 /98
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
ContentsContents
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Level 1 - Pressure 1RMT Training - 05 /98
Why measure pressure?Why measure pressure?4 Common Reasons4 Common Reasons
Safety• prevent pressurized pipes & vessels from bursting
Process 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 money
Inferred 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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Level 1 - Pressure 1RMT Training - 05 /98
What is pressure?What is pressure?The Same Weight, Different PressureThe Same Weight, Different Pressure
1 sq ins 100 sq ins
1 sq ins100 sq ins
Weight = 100lb
Pressure = Pressure =1lb/in² 100 lb/in²
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Level 1 - Pressure 1RMT Training - 05 /98
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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Level 1 - Pressure 1RMT Training - 05 /98
I/P
PT
PIC • Pressure Loop Issues:– May be a Fast Process
» Liquid» Small Volume
– May Require Fast Equipment
Pressure terminologyPressure terminologyPressure Control LoopPressure Control Loop
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Level 1 - Pressure 1RMT Training - 05 /98
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 H2O mm 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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Level 1 - Pressure 1RMT Training - 05 /98
Gage(psig) - Level of pressure relative to atmospheric– Positive or negative in magnitude
Atmospheric PressureApprox. 14.7 psia
Absolute
Gage CompoundRange
Barometric Range
PressureTotal Vacuum(Zero Absolute)
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
Pressure terminologyPressure terminologyReference PressureReference Pressure
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Level 1 - Pressure 1RMT Training - 05 /98
Absolute Zero
Total Vacuum
Atm. Pressure 14.7 psia
5 psig ?Psia 19.7
5 psi vacuum
?Psia
?Psig -5 9.7
Assume: Patm = 14.7psia; 28 inches H2O per psi
1000 in H2O = ___________ psi35.71
Pressure terminologyPressure terminologyQuizQuiz
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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 pressure
Pressure exerted in a closed system
3. Vapor PressureVapor Pressure
The 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. Vacuum Vacuum
Absolute pressure below atmospheric pressure ( a compound range gage transmitter will read a negative pressure)
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Level 1 - Pressure 1RMT Training - 05 /98
Pressure terminologyPressure terminologyMeasurable PressureMeasurable Pressure
Typical Vapor Pressure Curve
Pre
ssur
e(lo
g)
Temperature
liquid
gasHigher Altitute
Lower Altitute (Sea Level)
T1 T2
Vapor pressure increases with temperature.
• Liquid boils when its vapor pressure equals atmospheric pressure.
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Level 1 - Pressure 1RMT Training - 05 /98
Flow Restriction in Line cause a differential Pressure
Line Pressure
QV= K DP
Orifice Plate
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
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
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
The volume flowing into a pipe equals the volume flowing out of pipe, assuming constant density
A1V1 A2V2Flow Flow
v1 = A2/A1 x v2
v1 = d2/D2 x v2
� πd2/4 x πD2/4
Continuity Equation
A1v1 = A2v2
A = area of pipe cross sectionv = velocity
� d/D = β v1 = β2
x v2
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
Bernoulli’s Equation
� cancel - off for level pipe
v1 v2
P1 P2
D d
Three energies:Kinetic (1/2ρv2)Potential (ρgh)Static Pressure (P)
Flow
The total energy before the restriction in the pipe must equal the total energy after the restriction.
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
P 1 P 2..1
2ρ v 2
2 ..1
2ρ v 1
2 common
P 1..1
2ρ v 1
2 ..ρ g h 1 P 2 ..1
2ρ v 2
2 ..ρ g h 2Before restriction After restriction
� �
dP = ½ ρ (v22 - v1
2)
2 / ρ x dP = v22 - v1
2 V12 = (β2
x V2)2
2 / ρ x dP = v22 - β4
x v22
2 / ρ x dP = (1- β4) v22
commonsubject
v22 = (2 / ρ x dP) / (1- β4) Re-arranged
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
v2 = [(2 / ρ x dP) / (1- β4)]
½
v2 = (2)½ x (1/ρ)½ x 1/ (1- β4)½ x (dP)½
Qv2 = (πd2/4) x (2)½ x (1/ρ)½ x 1/ (1- β4)½ x (dP)½
Qv2 = A2 x v2
constant constant assumed constant
velocity of approachconstant - “E”
Qv2 = k (dP/ρ)½Volumetric Flow
Qm2 = k (dP x ρ)½Mass Flow � k (dP/ρ)½ x ρ �
�
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
(i) What would be the differential at 10m³/s?
Quiz:If an orifice plate creates a differential of 50 kPa at 30m³/s
DP2 = 5.6kPa
(ii) What would be the flow rate at 30kPa differential?
30/Qv2 = √50/ √30
Qv = K √DP
Qv1 √DP1--- = ----Qv2 √DP2
30/10 = √50/ √DP2
Qv2 = 23.26m³/s
Qv = K √DP
Qv1 √DP1--- = ----Qv2 √DP2
Inferring non-pressure variablesInferring non-pressure variablesFlowFlow
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Level 1 - Pressure 1RMT Training - 05 /98
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.
Which shape gives higher pressure at the bottom of the vessel?
Unit Area (eg. per cm2)
Similar height of column will have same mass acting on the same unit area
SAME PRESSURE
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
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Level 1 - Pressure 1RMT Training - 05 /98
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 HWith reference to inches or mm WATER S.G x H
D x
(D x A x H) / A
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
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Level 1 - Pressure 1RMT Training - 05 /98
P = r x g x height x area / area
mass x g
r x volume Density = mass/volume = r
P= force / area
g = gravitational acceleration
height x area
Phead = r x g x h Pascal
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
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Level 1 - Pressure 1RMT Training - 05 /98
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
XMTR
HL
Ullage or Vapor
S.G
Phead
Phead = S.G x Height 0%
100%
Hei
gh
t
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)
Height = Phead / S.G
Cancelled off since both L & H sides of transmitter experience it.
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Level 1 - Pressure 1RMT Training - 05 /98
Quiz: Open Tank
What is the level if Pmax = 120 inH2O, s.g.= 1.2?
XMTR
HL
?Height = Phead / S.G
Height = 120 / 1.2
Height = 100 inches
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
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Level 1 - Pressure 1RMT Training - 05 /98
Quiz: Closed Tank
Dry leg: no fluid in low side impulse piping, or leg
Ph = 105 psi
Pl = 100 psi
What is level if s.g. = 1.0?
Ptop= Ullage
XMTR
HL
dP = 5 psi = 5 x 28 inH2OHeight = 140 / 1.0
Height = 140 inches
Phead
Inferring non-pressure variablesInferring non-pressure variablesLevelLevel
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Level 1 - Pressure 1RMT Training - 05 /98
Pbottom =
Ptop =
Pbottom - Ptop =
Hence,
S.G =
Ptop
Phead(top)
Pbottom
Ptop
Phead(bottom)
h1
h2
Liquid level must be above the Top transmitter tap.
H
H
S.G X h2
S.G X h1
S.G (h2 - h1)
diff. Pressure / dist. betw. taps
Inferring non-pressure variablesInferring non-pressure variablesDensityDensity
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Level 1 - Pressure 1RMT Training - 05 /98
Ullage
Pbottom
Ptop
50”
H
H
Quiz:
Determined the S.G of the process fluid if
Ptop = 20 psi
Pbottom = 22 psi
Distance between taps = 50 inches
Assuming 1 psi = 28”H2O
S.Gprocess = DP / dist. betw. Taps= 56 / 50= 1.12
DP = (22 -20) = 2 psi = 56”H2O
Inferring non-pressure variablesInferring non-pressure variablesDensityDensity
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Level 1 - Pressure 1RMT Training - 05 /98
At 0% Liquid Interface (4mA)
DP = Hside - Lside
= (SG1*h1) - [(SGf*(h1-h2)) + (SG1*h2)]
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
Inferring non-pressure variablesInferring non-pressure variablesInterfaceInterface
h1
h2
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Level 1 - Pressure 1RMT Training - 05 /98
Total Liquid level must always be above the Top transmitter tap.
Pbottom
Ptop
L H
Remote Seal
Vapor
0%
100%
SG1
SG2
Dist. Betw. Taps
(h1 - h2)
At 100% Liquid Interface (20mA)
DP = Hside - Lside
= [SG2*(h1-h2) + SG1*h2)] - [(SGf*(h1-h2)) + (SG1*h2)]
Indirectly measures liquid Interface
Inferring non-pressure variablesInferring non-pressure variablesInterfaceInterface
h1
h2SGf
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Level 1 - Pressure 1RMT Training - 05 /98
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 B
123 inH2O
If transmitter reads 123 inH2O
% interface = (3/12) * 100%= 25%
Inferring non-pressure variablesInferring non-pressure variablesInterfaceInterface
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Level 1 - Pressure 1RMT Training - 05 /98
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
Pressure measurement technologyPressure measurement technologyPressure GaugesPressure Gauges
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Level 1 - Pressure 1RMT Training - 05 /98
dP = H (SGfill fluid - SGprocess fluid)– 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
ManometersU-tube with one side reference, one side measured pressure
H
How to check for dP ?
Pressure measurement technologyPressure measurement technologyPressure GaugesPressure Gauges
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Level 1 - Pressure 1RMT Training - 05 /98
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).
Pressure measurement technologyPressure measurement technologyPressure GaugesPressure Gauges
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Level 1 - Pressure 1RMT Training - 05 /98
Output to Actuator (or Relay)
Constant flowrate maintained(Compressed air)
Nozzle
FlapperBourdon Tube
Process Pressure
Pressure measurement technologyPressure measurement technologyPneumatic Pressure CellsPneumatic Pressure Cells
Pneumatic Controller
Relay’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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Level 1 - Pressure 1RMT Training - 05 /98
Disadvantages– Reconfiguration costly– Losses occur over long
piping runs– Performance levels are not
comparable to electronic instrumentation
Pressure TransmitterProduce a linear output proportional to input pressure
Zero Scale: Full Scale:
3 psig15 psig
Pressure measurement technologyPressure measurement technologyPneumatic Pressure CellsPneumatic Pressure Cells
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Level 1 - Pressure 1RMT Training - 05 /98
– Made up of 2 main elements:• Transducer - Electronic sensor module
that registers process variable and outputs a corresponding usable electrical signal
eg. resistance, millivolts, capacitance, etc.
• Electronics - Convert transducer output to a standard output signal
eg. 4 - 20 mA, 1 - 5 V dc, digital signal, etc.
Pressure measurement technologyPressure measurement technologyElectronic Pressure TransmittersElectronic Pressure Transmitters
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Level 1 - Pressure 1RMT Training - 05 /98
Transmitter
Signal fromsensor module(Transducer)
Signal To Controller
Process Variable
(Standard signals)
Sensing Diaphragm
(Line / Static Pressure)
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.
What will be the output current at 25 psi reading?
4 + (25/50)*16 = 12 mA
Pressure measurement technologyPressure measurement technologyElectronic Pressure TransmittersElectronic Pressure Transmitters
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Level 1 - Pressure 1RMT Training - 05 /98
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
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
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.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
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.
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
• 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
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
• 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
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
• 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
Variable Reluctance
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
• 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
Vibrating Wire
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
– 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
Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules
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Level 1 - Pressure 1RMT Training - 05 /98
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)?
Pressure calibratorsPressure calibratorsISO RequirementISO Requirement
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Level 1 - Pressure 1RMT Training - 05 /98
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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Level 1 - Pressure 1RMT Training - 05 /98
ExerciseExercise
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): [ ]
50 psig80 psig
c a b
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Level 1 - Pressure 1RMT Training - 05 /98
ExerciseExercise
6. What is the differential pressure (P1 - P2) in kPa being applied to the manometer in the the above application ?
S.G of Process Fluid @ Temp + Pressure = 1.0
P2P1
S.G. = 13.6200mm
(Note 1 mm H2O = 9.8 Pa)