PRINCIPLES OF PRESSURE MEASUREMENTPrepared by : Sakinah AnsaryAssisted by : Ahmad NizarReviewed by : Ezatolah MardasiDate : 11th of March 2004

Instrument & Control Department

TPCP Descriptor

INSTALLATION CONSIDERATIONMAINTENANCE & CALIBRATIONDEVICE SELECTIONREFERENCESVariable Inductance Pressure SensorVariable Differential Transformer TransducerVariable Capacitance TransducerPotentiometric TransducerBourdon Tube

Bellows Sensor

Diaphragm Sensor

DATASHEETSStrain GaugeMECHANICAL PRESSURE ELEMENTELECTRICAL PRESSURE ELEMENTELECTRONIC PRESSURE ELEMENTBASIC PRINCIPLES

PRESSURE MEASUREMENT

Pressure TransmitterPressure GaugeResonant Wire Pressure TransducerOptical Pressure TransducerPiezoelectric

BASIC PRINCIPLES

What is PRESSURE??Pressure is Force exerted over a unit Area

P = F / A Where P = Pressure F = Force A = Area

UNITS of PressurePounds per square inch (PSI)Newton force per square meter / Pascal (Pa); where kPa is commonly usedKilogram per square centimeter (kg/cm2)Inch of water (inH2O)Millimeter of water (mmH2O)Inch of mercury (inHg)Millimeter of mercury / torr (mmHg)Atmosphere (atm)Bar

Conversion of common UNITS of Pressure

Absolute & Gauge PressureAbsolute Pressure; denoted as a: - Measured with reference to ZERO pressure, eg. psia

Gauge Pressure; denoted as g: - Measured with reference to ATMOSPHERIC pressure, eg. psig

Thus, Pressureabsolute = Pressuregauge + Pressureatm

PTPressure AppliedHLDiaphragmAtmosphericPlugged but not sealedReference pressureGauge PressurePTPressure AppliedHLDiaphragmVacuumedPlugged and sealedReference pressureAbsolute Pressure

Differential PressureDifference in pressure measurements taken at TWO related points; denoted as d, eg. psid P = Phigh Plow

Vacuum

Measured BELOW atmospheric pressure

Comparison of Pressure Modes

Pascalss LawStates that whenever an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of the that pressure

Application of Pascals Law: Hydraulic presses, jacks or breaks Instruments used for measurement and calibration

MECHANICAL PRESSURE ELEMENTS

Mechanical Pressure ElementsBase on pressure acting on a surface area inside the element to provide a force that causes a mechanical deflection

Common elements used are: - Bourdon tubes - Bellows elements - Diaphragms

Bourdon Tubes ElementsMost common type of pressure sensor

Increased pressure causes the flattened cross-section to straighten and move the closed end

Closed end is attached to mechanical linkage

Linkage is connected to a pointer or other output device

There are three types of forms: - C-Tube - Spiral - Helical

C-Tube (Low:015 psig & High:0-10000 psig)Most commonly used element

Simple and lease expensive to manufacture

Bulky and subject to damage from over pressuring

Two bourdon tubes are used to measure differential pressure; (P1 P2)

Spiral (Low:0-10 psig & High:0-10000 psig) Helical (Low:0200 psig & High:0-6000 psig)Much longer than C-Tube and is coiled into a spiral and helix respectively

More costly to manufacture

Occupies less space for given amount of movement

Multiple turns allow thicker material for a given sensitivity; less chance to damage

Bellows SensorsAxially flexible, cylindrical enclosure with folded sides [A]

Increased pressure causes the closed end to extend axially

Movement rotates a pointer or actuates a controller or transmitter by mechanical linkage [B]

Movement of the bellows is opposed by - Spring action of the bellows material - Pressure surrounding the bellows - External force by spring [C] or another bellow [D]

Accurately measures LOW pressure compared to Bourdon tubes as well as Vacuum and differential pressure [E] - Absolute ranges : 0 100 mmHg - Gauge ranges : 0 5 inH20

Bellows Sensors[A][D][C][B][E]

Diaphragm SensorsUsed to detect slight changes in pressure

Axially flexible, thin, flat and corrugated disk held in place

Increase pressure on one side causes the other side to deflect

Force opposing the pressure is the sum of: - Spring constant of the diaphragm - Pressure on the opposite side of the diaphragm - Spring constant of opposing spring

Axial movement can measure liquid level, rotate a pointer or activate a controller by attaching the free end to linkage [A]

There two types of elements: - Elastic element - Limp element

Elastic ElementUsually metallic - Single; either flat or concentric corrugations [B] - Capsular; consists of two diaphragms welded together at their perimeter [C]

Limp Element

Metallic or non-metallic - Does not give an opposing force to an applied pressure - Single diaphragm form - Used to contain pressure and exert a force on an opposing spring [D]

Able to measure VERY LOW pressure

Common to measure absolute/gauge pressure down to 0 0.2 inH2O

Does not provide much movement which limits their use in gauges and controllers

Diaphragm Sensors[A][B][D][C][E]

ELECTRICAL PRESSURE ELEMENTS

Electrical Pressure ElementsElectro-mechanical pressure transducers convert the motion produced by mechanical sensing elements into changes in electrical signals for monitoring and controlling process pressure

Generally, the electrical output is directly proportional to the applied pressure

Types of electrical pressure elements are: - Potentiometric transducer - Variable Capacitance Transducer - Variable Differential Transformer Transducer - Variable Inductance Pressure Sensor

Potentiometric TransducerUtilizes a Wheatstone bridge circuit in which one of the bridge resistors is replaced by a potentiometer

Potentiometer is a wire-wound resistor with a movable slide on it

When the slide moves, the resistance value of the potentiometer changes which changes the resistance in the bridge circuit

But, this transducer has a limited response to pressure changes and can easily wear and damage the windings

Potentiometric Transducer

Variable Capacitance TransducerConsists of two metal plates; one of which is linked to a movable mechanical pressure element ie. diaphragm and the other is a static plate

Both plates are separated from each other by dielectric material

When distance between plates changes, capacitance changes resulting in changes in opposition to current flow in the AC circuit

Thus, by measuring the current flow, changes in pressure can be detected

Ideal for measuring low input levels

Advantages:- Low hysteresis- Good linearity, stability and repeatability- Fast response Disadvantages:- High impedance output- Complex electronics

Linear Variable Differential Transformer (LVDT) TransducerLVDT operates on the inductance ratio principleThree coils are wired onto an insulating tube containing an iron core, which is positioned within the tube by the pressure sensorAlternating current is applied to the primary coil in the center, and if the core also is centered, equal voltages will be induced in the secondary coilsBecause the coils are wired in series, this condition will result in a zero outputAs the process pressure changes and the core moves, the differential in the voltages induced in the secondary coils is proportional to the pressure causing the movement

LVDT-type pressure are available with ranges 0 - 30 psig to 0 - 10000 psigIt can detect absolute, gauge or differential pressures

Advantages:- Rugged; will not be easily damaged- Do not need to compensate for friction; movable core not in touch

Disadvantage:- Susceptible to mechanical wear and sensitivity to vibration and magnetic

Variable Inductance Pressure SensorTwo coils are wired in opposition to form two legs of an AC circuit

A diaphragm made of a magnetic material is placed between the two coils

Pressure form the measured process is applied to one side of diaphragm while the other side is exposed to a reference pressure ie. atmospheric pressure

Changes in process pressure will cause the diaphragm to flex and move towards one of the coils and away from the other

As the diaphragm moves, the relative inductance of the coils changes which change the circuit output, thus can be measured as pressure

Advantages:- Rugged and stable in performance- High output and dependable overload protection

Disadvantages:- Frequency response is limited- No linear output at times

ELECTRONICS PRESSURE ELEMENTS

Strain GaugeThere two types of strain gauges: - Bonded Strain Gauges - Unbonded Strain Gauges

Bonded Strain Gauges

Discrete metal/silicon foil bonded or glued to a piece of insulating material (usually some movable part of a mechanical pressure sensing element eg. diaphragm) that will bend or flex with pressure changes

As an active sensing element strains or bends, the bonded foil will be strained

The tension of the foil will change its resistance

Ideal for taking dynamic measurements

Advantages:- Fast response- Low source impedance- Minimum mechanical motion, size and weight- Works well with AC and DC power

Disadvantages: Loss of accuracy due to hysteresis Costly output measurement devices

Unbonded Strain GaugesConsists of insulated posts that support stretched small wires that will respond to changes in pressure These posts are linked to a mechanical pressure element eg. diaphragm which deforms under pressure

The resulting changes in tension in the wire will change the resistance indicating changes in process pressure

Advantages:- High sensitivity- Moderate accuracy- May be used in high temperature; no adhesive required

Disadvantages:Frequent recalibration due to hysteresisZero tends to shift; long term changes in wire resistivity and stress relief

Resonant Wire Pressure TransducerIn the design, a wire is gripped by a static member at one end, and by the sensing diaphragm at the other

An oscillator circuit causes the wire to oscillate at its resonant frequency

A change in process pressure, changes the wire tension, which in turn changes the resonant frequency of the wire

A digital counter circuit detects the shift

Because this change in the frequency can be detected quite precisely, this type of transducer can be used for low differential pressure applications as well as to detect absolute and gauge pressures

Advantage:- It generates inherently digital signal; can be sent to a stable crystal clock in microprocessor

Disadvantages:- Sensitive to temperature, shock and vibration variation- Nonlinear output signal

Optical Pressure TransducerAn LED is used as light source and a vane which blocks some of the light as it is moved by a diaphragm

As process pressure changes the vane between light source and measuring diode, amount of IR received changes as compared to reference diode which is never blocked by the vane

Detects the effects of minute motions due to changes in process pressure and generate a corresponding electronic output signal

Advantages:- Immune to temperature effects because all diodes are effected equally by changes in temperature- Hysteresis and repeatability errors are nearly zero due to small movements (< 0.5 mm)- Low maintenance- Stable and designed for long duration measurements

PiezoelectricWhen pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied.The fundamental difference between these crystal sensors and static-force devices such as strain gages is that the electric signal generated by the crystal decays rapidly.This characteristic makes these sensors unsuitable for the measurement of static force or pressures but useful for dynamic measurements.Pieoelectric device can further be classified according to whether the crystal's electrostatic charge, its resistivity, or its resonant frequency electrostatic charge is measured.Depending which phenomenon is used, the crystal sensor can be called electrostatic, piezoresistive or resonant.The desirable features of peizoelectric sensor include their rugged construction, small size, high speed, and self-generated signal.Piezoelectrics are sensitive to temperature variations and require special cabling and amplification.

Piezoelectric (Working Principle)When pressure is applied to a crystal, it is elastically deformed.This deformation results in a flow of electric charge (which lasts for a period of a few seconds).The resulting electric signal can be measured as an indication of the pressure which was applied to the crystal.This sensor cannot detect static pressures, but are used to measure rapidly changing pressures resulting from blasts, explosions, pressure pulsations or other sources of shock or vibration.

INSTALLATION CONSIDERATIONS

Installation ConsiderationsManner in which a pressure measuring instrument is installed is an important factor in achieving accurate and reliable operation

Measuring device is normally placed in direct contact with the process and should be installed in visible and readily accessible locations

In some applications, it is desirable to prevent the process fluid from coming in contact with the sensing element

The process fluid may be noxious, poisonous, corrosive, abrasive, freeze or decompose at ambient temperature or hotter/colder than the sensor can tolerate

Other reasons are to filter out potentially plugging solids or to remove potentially damaging pressure spikes or vibrations Special installation precautions must be taken to protect the measuring instrument and reading accuracy

Some of the protection devices used include: - Seal pots - Mechanical pressure seals - Chemical seals - Pulsation dampeners - Snubber

Seal PotsUsed when measuring steam pressure or in processes in which a vapor will condense into liquid at ambient temperature

Acts as a condensate chamber and provides a large area of liquid contact between process and measuring instrument

Lead lines to the measuring instrument should be full of liquid to protect the instrument from high temperatures

In some cases, the seal pot, line and instrument are filled with a sealing fluid to prevent freezing

When a sealing fluid is used, the measuring instrument should be zeroed with the lines full of the seal fluid; wet leg installation

Seal Pots

Mechanical Pressure SealsInstalled between the measuring instrument and the process

Isolates the instrument from corrosive, toxic or plugging effects of the process

Operation is based on Pascals Law; force applied to a liquid in a confined area is transmitted undiminished throughout the liquid

Designed so that process pressure exerts a force on the diaphragm, which is transferred via the fill liquid to the measuring instrument

Mechanical Pressure SealsType of fill liquid used will depend on the particular application

In all cases, the liquid should have a: - Low freezing point- High boiling point- Low viscosity- Low coefficient of thermal expansion

SiphonSiphon or pig tail is a type of mechanical seal commonly used

Used to protect pressure gases from thermal shock in high process temperatures

Steam and other vapors condense and form a liquid trap in the lower portion of the siphon

Usually mounted vertically to prevent thermal shock

When mounted horizontally, siphon will absorb some mechanical shock and vibration

Chemical SealsChemical seals is also known as a diaphragm protector which uses diaphragm seals

Designed for use where the process fluid being measured would normally clog the pressure system or might freeze due to changes in ambient temperature and to withstand corrosive effects of certain process fluids

The diaphragm seals the pressure system from process fluid

Any movement of the diaphragm will change the process pressure accordingly and indicated by the gauge pointer

Pulsation DampenersUsed in applications where there is rapid fluctuation of process pressure

Sudden changes in pressure make it difficult to read the actual value of the pressure measurement and will cause unnecessary damage on the measuring instrument

Fluctuations can be minimized by placing a restriction in the impulse line to reduce the response rate of the instrument

Pulsation DampenersAvailable in a variety designs:- Contains a porous filter disc at the inlet to the restriction; rate of response is delayed by a fixed time of about 10 seconds [A]- Contains a piston that absorbs shock and surge by rising and falling with pressure impulses [B]- Design provide a means for adjusting the amount of restriction to obtain the best damped response [C&D]

SnubberPressure snubber has the following functions:

- Eliminates pressure instrument failure due to hydraulic or pneumatic shock- Smoothes out pressure impulses and fluctuations- Removes harmful solids from actuating fluid- Assures steady average pressure readings

Throttling ScrewSimplest means of providing a restriction in the socket

This device is threaded into a tapped hole

Its size is selected base on the viscosity of the pressure fluid, rapidity of pressure fluctuations and the amount of dampening effect desired

Pressure Tap for GasFor non-condensing gasses, tapping point should be made to the side or upper quadrant of the process line and fitted with a suitable process isolation valve

The pressure transducer (transmitter) should be positioned above the tapping point with its process connection downwards to allow any condensate that may form to drain back into the process without blocking the impulse line

To ease service and commissioning the transducer should be fitted with an isolation valve and vent (or vent valve

Pressure Tap for Vapor/SteamFor vapor or steam service, the tapping point should be made to the side or upper quadrant of the process line and fitted with a suitable process isolation valve

The pressure transducer (transmitter) should be positioned below the tapping point so that the impulse line will stay filled with condensate in service

The distance below the impulse line should be chosen to ensure that adequate cooling occurs to prevent thermal damage to the transducer

For steam service the impulse line must be filled with water prior to startup to prevent possible thermal damage to the transducer by live steam

To ease service and commissioning the transducer should be fitted with an isolation valve and vent (or vent valve)

Pressure Tap for LiquidFor liquids, the tapping point should be made to the side or lower quadrant of the process line and fitted with a suitable process isolation valve. If sediments may be present, do not fit to the bottom of the process lineThe pressure transducer (transmitter) should be positioned below the tapping point with its process connection upwards (if possible) to allow any gasses that may form to vent back into the processTo ease service and commissioning the transducer should be fitted with an isolation valve and vent (or vent valve)

MAINTENANCE& CALIBRATION

MaintenancePressure sensors require scheduled, periodic maintenance and/or recalibrationIt is necessary to periodically remove the transducer from the process and to make sure that this procedure does not require shutting down the process and does not cause injury or damageBecause the process fluid may be toxic, corrosive, or otherwise noxious to personnel or the environment, it is necessary to protect against the release of such fluids during maintenanceA three-way manifold can provide such protection. In the illustration,

- Valve P is used to isolate the process - Valve D serves to discharge the trapped process fluid from the instrument into some safe containment- Valve T is to allow the application of a known calibration or test pressure to the instrument

CalibrationThe dead weight tester is generally regarded as the most accurate method in calibration of pressure instruments

It is used for calibration of pressure gauges, transmitters, transducers, etc.

When fluid pressure generated by a screw pump acts on the bottom of a vertically free floating piston, the force produced pushes the loaded free piston vertically upwards

The piston floats freely in its cylinder and the pressure in the circuit will be determined by the weights loaded on the piston divided by the effective area of the piston with corrections for value of acceleration due to gravity, air buoyancy and surface tension

Dead-Weight Tester

DEVICE SELECTION

Electrical/Electronics Pressure Element SelectionFigure below shows the selection of various electrical/electronic pressure elements at a certain pressure

Pressure Transmitter SelectionPressure transmitter is used where indication, monitoring and/or controlling of pressure is required at a location not adjacent to the primary elements eg. panel or control room

The pressure range selection is made base on:- Process data given by process engineers - Referring to Line List- Obtained from Heat and Material Balance

There are two types of transmitters: - Pneumatic: 3-15 psig - Electronic: 4-20 mA

Pneumatic TransmitterThe differential pressure to be measured is applied across a pair of metal diaphragms welded to opposite sides of a capsule

Space between the diaphragms and core member is filled with liquid

The force developed on the diaphragm by differential pressure is brought out of the transmitters by a rigid rod passing through a metal seal diaphragm

This force is opposed by a balancing force developed by pneumatic bellows

Imbalance between capsule force and pneumatic bellows force is sensed by a pneumatic nozzle-baffle; which in turn a servomechanism responsive to nozzle pressure re-establishes the balance

As a result, pneumatic pressure is maintained exactly proportional to differential pressure and is used as output signal

Pneumatic TransmitterElectronic Transmitter(Rosemount)

Electronic TransmitterThis particular type utilizes a 2-wire capacitance technique

Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm in the center of the cell

The sensing diaphragm is a stretched spring element that deflects in response to differential pressure across it The displacement of the sensing diaphragm is proportional to the differential pressure

The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm

The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 420 mA

The operating range, maximum and minimum pressure should be considered

Gauge pressure transmitters should have an overpressure rating of at least 150% of the maximum rating operating pressure

Parameters used in transmitters:- Span: actual pressure range to be measured after the transmitter adjustment- Range: pressure range within which the span can be adjusted

Most transmitters have two adjustments- Zero: output minimum is adjusted as 4 mA, 3 psig or 20 kPa at zero pressure- Span: output maximum is adjusted as 20 mA, 15 psig or 100 kPa when pressure as at top of the span

(Rosemount)

(Rosemount)(Foxboro)Functional Overview Block Diagram for Pressure Transmitter

Pressure Transmitter Sensor Units(Rosemount)The sensor is isolated mechanically, electronically and thermally from the process medium and the external environmentMechanical and thermal isolation is achieved by moving the sensor cell away from the process flange to a position in the neck of the electronics housingThis design relieves mechanical tress on the cell, thereby improving static pressure performance and removes the sensor from direct process heatGlass-sealed pressure transport tubes and insulated cell mountings provide electrical isolation, and thus improve electronic circuitry's flexibility, performance and transient protection

Capacitance Principle (1967)The pressure applied to the transmitter high and low pressure sides is transmitted by a sealed fill-fluid to both sides of a sensing diaphragmAs the applied differential pressure causes the sensing diaphragm to move, the capacitor of the cell changes as the pressure changesThe amplifier unit converts the change in capacitance to a 4 to 20 mA signalThe sensing diaphragm is a stretched spring element that deflects in response to differential pressure across itThe maximum sensing diaphragm movement is 0.1mmA 1m distortion of a sensing diaphragm is equivalent to a 1% error

(Yokogawa)

Piezoresisitive Principle : (1982) The solid state sensor consists of a Wheatstone Bridge circuit which has resistors diffused into a silicon chip, thereby becoming a part of the automatic structure of the silicon

As pressure is applied to the IC chip diaphragm, strain is created in the bridge resistors

Piezoresistive effects created by this strain, change the resistance in the arms of the bridge, producing a voltage proportional to pressure

Output from the bridge is typically in the range of 75 to 150mV at full scale pressure for a bridge excitation 1.0 mA

(Yokogawa)

Single Crystal Silicon Resonant Principle : (1992)In the single resonant sensor principle, the sensor has two H-shape resonators on a diaphragm chip, one in the center of the chip and the other is located slightly off to the sideThe resonating element consists of a silicon beam, which is deflected by the application of pressureAs pressure is applied, the center resonator goes into compression, and the outer resonator goes into tension altering its natural frequencyThe output of the compression resonator increases from 90 to 110 kHz and the output of the tension resonator decreases from 90 to 70 kHzThis produces a high differential output directly proportional to the pressure being appliedThe digital frequency is easily managed by the CPU as a time based function (Yokogawa)

SENSOR COMPARISION

Transmitter SelectionConsiderations that should be viewed when selecting a pressure transmitter:

i. Functional Specification- Temperature; process, range- Pressure; operating and min/max- Environment; humidity- Hazardous Location; Zone, Group, Ex-Proof, IS- Damping; time constant and corner frequency- Output; range and no of wire- Zero Elevation or Suppression- Power Supply and Load Limits

ii. Performance Specification- Accuracy- Linearity- Hysteresis- Repeatability- Temperature Effect- Overpressure Effect- Static Pressure Effect- Vibration Effect- Power Supply Effect- Load Effect

iii. Material

iv. Feature

Pressure Gauge Selection(Ashcroft)

Pressure Gauge SelectionType of Pressure Element Bourdon, Bellows or DiaphragmType of Gauge Gauge Pressure, Absolute Pressure, Differential Pressure, Compound Pressure Pressure Range and Unit operating pressure bet. 50-75% of scaleDial Size - 2 1/2 up to 12; 4 (commonly used)Mounting - Bottom, Lower/Center Back or FlushPressure Element Material Stainless Steel, MonelCase Material Phenolic, Stainless SteelLiquid Filled eg. siliconAccessory - Pulsation Dampener, Snubber, Throttle Screw, Siphon or Diaphragm SealBlow Out Vent A feature which relieves pressure in the gauge caseConnection Size 1/2 or 1/4 NPT

Types of Mounting(Swagelok & Ashcroft)

DATASHEET SAMPLES

ADDITIONALINFORMATIONTEMPERATURE COMPENSATION ON PRESSURE TRANSMITTER

TEMPERATURE COMPENSATION ON PRESSURE TRANSMITTERThe sensor of the smart transmitter is uniquely temperature compensated to ensure high performanceAfter the basic transmitter has been assembled, it is tested throughout its input pressure range at various ambient temperatureData from these tests are translated into compensation coefficients and entered into a memory chip (EEPROM) in the sensor assembly to provide precise temperature error compensation during transmitter operationThe coefficients make the transmitter accurate with changes in pressure and changes in ambient temperatureThis compensation is proposed because temperature changes will cause the pressure element to expend or contractAs a result instability of output will occur

REFERENCESBasic Instrumentation 3rd Edition Chapter 5: Pressure Measurement and ControlSKG 14: Pressure, Flow, Level and Temperature Training ModuleYokogawa BrainWave NewsletterRosemount Measurement Model 3051 Smart Pressure Transmitter Family BrochureFoxboro Intelligent Pressure Transmitter Product SpecificationAshcroft Pressure Gauges Product CataloguesSwagelok Pressure Gauge Technical BrochureEndress and Hauser Industrial Automation 2003 Product Catalogue www.sciencemadesimple.net

Thank You

Hope that this presentation will help you take the PRESSURE off yourself and put it on the right SENSOR for your application