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Pressure

Contents

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Topics: Slide No:• Pressure measurement technology 03 - 17• Pressure calibrators 18• Exercises 19 - 20

Pressure measurement technologyPressure Gauges

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Barometer Used 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 Gauges

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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

dP = H (SGfill fluid - SGprocess fluid)

ManometersU-tube with one side reference, one side measured pressure

H

How to check for dP ?

Pressure measurement technologyPressure Gauges

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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 technologyPneumatic Pressure Cells

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Pneumatic Controller Relay’s modulated output is the controller

output which is usually a pneumatic signal that adjusts the final control element (Control valve)

Output to Actuator (or Relay)Constant flowrate maintained

(Compressed air)

NozzleFlapper

Bourdon Tube

Process Pressure

Pressure measurement technologyPneumatic Pressure Cells

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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 technologyElectronic Pressure Transmitters

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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.

Pressure measurement technologyElectronic Pressure Transmitters

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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 technologyElectronic Pressure Sensor Modules

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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 technologyElectronic Pressure Sensor Modules

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• 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.

Variable Capacitance

Pressure measurement technologyElectronic Pressure Sensor Modules

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• 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.

Variable Resistance / Piezo-Resistive

Thin Film Strain Gauge

Diffused Strain Gauge

Pressure measurement technologyElectronic Pressure Sensor Modules

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• 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 technologyElectronic Pressure Sensor Modules

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• 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 technologyElectronic Pressure Sensor Modules

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• 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 technologyElectronic Pressure Sensor Modules

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• 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 technologyElectronic Pressure Sensor Modules

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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 calibratorsISO Requirement

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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)?

Exercise

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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

Exercise

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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

Exercise

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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)

Reference

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A Smith-Corripio [1997]. Principles and Practice of Automatic Process Control 2nd Edition, John Willey & Sons

Ogata [2010]. Modern Control Engineering 5th

Edition, Prentice Hall Rosemont [2002]. Fundamental Control Training http://www.isa.org