1 Pressure
Contents Topics: Slide No: Pressure measurement technology 03-17 Pressure calibrators 18 Exercises 19-20 2
Pressure Gauges Barometer Used to measure Barometric Pressure Reference is 0 psia, due to low vapor pressure of Hg. General operating principle: 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. P head = P atm P atm P head 3 Barometric Pressure = Atmospheric Pressure What is the barometric Pressure? 29.9 inhg
Pressure Gauges Manometers U-tube with one side reference, one side measured pressure H 4 How to check for dp? Reference side can be: Sealed (AP reference) Open to atmosphere(gp reference) dp = H (SG fill fluid - SG process fluid ) Connected to reference pressure(dp reference) Typically used for low pressures, non process control
Pressure 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). 5
Pneumatic Pressure Cells Pneumatic Controller Relay s modulated output is the controller output which is usually a pneumatic signal that Process Pressure adjusts the final control element (Control valve) Nozzle Flapper Bourdon Tube 6 Constant flowrate maintained (Compressed air) Output to Actuator (or Relay)
Pneumatic Pressure Cells Pressure Transmitter Produce a linear output proportional to input pressure Zero Scale: Full Scale: 3 psig 15 psig Disadvantages Reconfiguration costly Losses occur over long piping runs Performance levels are not comparable to electronic instrumentation 7
Electronic Pressure Transmitters Made up of 2 main elements: Transducer - Electronic sensor module that registers process variable and outputs a corresponding usable 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. electrical 8
Electronic Pressure Transmitters Sensing Diaphragm Process Variable (Standard signals) Signal To Controller Transmitter Signal from sensor module (Transducer) (Line / Static Pressure) Example of Application Transmitter configured to operate from: 0 to 50 psi Electronic Output: 4 to 20 ma This mean 0% reading (0 psi) represents 4 ma and 100% reading (50 psi) represents 20 ma. 9 What will be the output current at 25 psi reading? 4 + (25/50)*16 = 12 ma
Electronic 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 10
Electronic 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. 11
Electronic Pressure Sensor Modules Variable Resistance / Piezo-Resistive 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. Thin Film Strain Gauge 12 Diffused Strain Gauge
Electronic Pressure Sensor Modules Piezoelectric Process Pressure 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 Crystal Control Signal Diaphragm Amplifier & electronics 13
14 Pressure measurement technology Electronic Pressure Sensor Modules Variable Inductance 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.
Electronic 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 15
16 Pressure measurement technology Electronic 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
Electronic Pressure Sensor Modules Output Electronics Output Electronics 17 Sensor Module Diaphragm Seal Sensor Module 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.
Pressure calibrators ISO 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? 18 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/cm 2 )?
Exercise 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 [ AP 94.7psi 80.psi GP Transmitter Transmitter Line pressure = 80 psig Liquid flow 2. If a customer wants to measure vacuum, what type of transmitter should be used? (A) AP (B) DP (C) GP [ ] 19
Exercise c a b 80 psig 50 psig 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): [ ] 20
Exercise S.G of Process Fluid @ Temp + Pressure = 1.0 200mm P1 P2 S.G. = 13.6 (Note 1 mm H 2 O = 9.8 Pa) 6. What is the differential pressure (P1 - P2) in kpa being applied to the manometer in the the above application? 21
Reference A Smith-Corripio [1997]. Principles and Practice of Automatic Process Control 2 nd Edition, John Willey & Sons Ogata [2010]. Modern Control Engineering 5 th Edition, Prentice Hall Rosemont [2002]. Fundamental Control Training http://www.isa.org 22