PRESSURE MEASUREMENTS

Chapter 6Experimental Methods for Engineers, 6th edition, J.P. Holman, 1994

OVERVIEWIntroduction

What is Pressure?Why measure Pressure?How to measure Pressure?

Measuring DevicesHigh Pressure SensorsCalibratingElastic Deformation SensorsLow Pressure SensorsUltra Low Pressure Sensors

WHAT IS PRESSURE?Pressure is a normal force exerted by a fluid over a surface areaAbsolute, Gage, Vacuum PressureStatic & Dynamic PressurePa, Bar, atm, Psi

Patm

Pabs = 0

Pvac

Pabs

Pabs

PGage

Absolute, gage, vacuum pressures

WHY MEASURE PRESSURE?Pressure negates the properties of a fluid:State, flow, forcesQuality and Safety of Operation:Tire, compressors, etcPressure measurements is used in various general, industry and research applications

INDUSTRY APPLICATIONDrilling Technology utilise pressure sensors for real time downhole data transferWeather forecastingMedicineAviationPressure VesselsMany other application

you are guarantee to seeworking as an engineer

Pressure Chart

Sphygmomanometer

Fluid Manometer

HOW TO MEASURE PRESSURE?

P&ID Pressure Sensor Symbol

atmabs PghP += ρ

2

21 vq ρ=

RTP ρ=

εσ

=E

PDI: Differential Pressure IndicatorPI: Pressure Indicator

Hydrostatic Level TypeSymbol

General Instrument Symbol

HIGH PRESSURE SENSORS & CALIBRATIONRoger Marley

INTRODUCTIONTypes of medium pressure sensors

Manometer & BarometerMicrophone

Types of high pressure sensorsBourdon-tube GaugeSchrader Gauge

Calibration of pressure sensors

MANOMETERSThe manometer consists of a tube filled with liquid of known densityA pressure difference across the tube causes the liquid to shift positionThe change in position can be measured to give the pressureBest suited to static pressure measurementDifficult to use for small pressure changes, unsuitable for very large pressures Very simple manometer

PA = ρgh1

PB = ρgh2

MANOMETER TOPOLOGIES

PA = ρmangh2

PA – PB = ρmangh P1 – P2 = ρgz2 (If D >> d)

P1 – P2 = ρgx sinθ

BAROMETEROne common application of the manometer is the barometerThe barometer measures atmospheric pressureThis barometer uses a reference gas separated from the atmosphere by a liquidIf the atmospheric pressure changes, the reference gas expands/contractsStatic pressure gauge Dodgy barometer

How it works

MICROPHONEThe most common pressure sensor in daily useDesigned for use at around 1 atm. pressureOnly measures fluctuations – measures dynamic not static pressureHas several different constructions of varying complexity Condenser Microphone construction

BOURDON-TUBE GAUGEInvented by Eugene Bourdon in 1849Can be used to measure pressures up to 100,000 psiUses an elastic tube as its primary elementThe tube straightens out with increasing pressure, moving the pointer via mechanical linksMeasures static pressure

Diagram

Commercial bourdon-tube gauge

SCHRADER GAUGEUses a piston connected to a springSimple & sturdy constructionNot particularly accurateCommon use is in simple tyre pressure gaugesPerforms better than bourdon-tube under dynamic loads

Tyre pressure gauge

Pressurised gauge

Construction

F = PA

k x = F

x = (A/k)P

CALIBRATIONThe most common way to calibrate pressure sensors is with a dead-weight testerHas accuracy in the 0.005% to 0.1% rangeAllows pressure tests up to 10kBar (~145,000 psi)

1 - Hand pump2 - Testing Pump3 - Pressure Gauge to be calibrated4 - Calibration Weight5 - Weight Support6 - Piston7 - Cylinder8 - Filling Connection

ELASTIC DEFORMATION SENSORSHaidyn McLeod

BELLOWS GAUGEUses the elastic deformation of a convoluted unit which expands and contracts with changes in pressure.Either electrical or mechanical output.Doesn't work well with dynamic pressures due to mass and large displacements.

SPRING LOADED BELLOWS GAUGEA spring opposes the bellows.Limits the expansion of the unit and prolongs the bellows life.Resulting deflection is theforce acting on the bellowsand the opposing spring force.

DIAPHRAGM GAUGES

Uses the elastic deformation of a flexible membrane that separates two different pressures.The deformation of the diaphragm is dependent on the difference in pressure between the two faces.

DIAPHRAGM GAUGESCan be used to measure gauge, differential, vacuum or absolute.Can be measured using mechanical, electrical, piezoresistive and capacitive means.Follows a linear variation with Δp when the deflection is less than 1/3 the diaphragm thickness.Good Dynamic sensor.

DIAPHRAGM GAUGESThe natural frequency of a circular diaphragm is

ELECTRICAL PRESSURE SENSORConsists of foil strain gauges.Typically contains 4 gauges in a Wheatstone bridge formation.Accuracies of ± 0.5%

PIEZORESISTIVE PRESSURE SENSOR

Piezoresistive consist of a silicon diaphragm with a semiconductor strain gauge bonded to the diaphragm.Pressure sensitivity: S = ∆R/ (∆p*R) (mV/V-bar)Advantages:- High sensitivity- Good linearity at constant temperature.

MEMS DIAPHRAGM SENSORCurrent MEMS pressure sensors can contain onchip compensation and amplification.

Can perform signal conditioning and compensate for temperature.

CAPACITIVE PRESSURE SENSORS The variable gap created by a moving diaphragm can be used as a capacitance sensor.The capacitance of the sensor is related to pressure.

CAPACITIVE PRESSURE SENSORS Basic concept: C = ɛ A/xSensitivity: ΔC/Δx = -ɛ A/xAdvantages

more sensitive than piezoresistiveless temperature dependantgreat dynamic pressure sensor

Disadvantagesrequires special electronicsdiaphragm mechanical properties

CORRUGATED DIAPHRAGM SENSORCreates a larger linear response over a larger range of deflections.Is better suited for mechanical sensing devices as amplification may not be required.

LVDT-DIAPHRAGMThe motion of a diaphragm sensed by a Linear Variable Differential Transformer or (LVDT).http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm

BRIDGMAN GAUGE

BRIDGMAN GAUGEThe wire is typically Manganin (84% Cu, 12%Mn, 4%Ni).Where b = 2.5 x 10¯¹¹ Pa¯¹Resistance is less affected by temperature change.Resistance can respond to variations in pressure in the megahertz range.The total resistance of the wire is about 100Ωand is usually employed in a Wheatstone bridge.

LOW PRESSURE SENSORSAdam Millen

Absolute Pressure below 133 Pa

Textbook 6.8-6.10

LOW PRESSURE MEASUREMENT

THE MCLEOD GAUGE

Lower the reservoir to take a sample of the vacuum.

Vc = ay

pc = p (VB/Vc)

pc – p = y

p = ay2/ (VB – ay)

p = yVc / (VB – ay)

p = ay2 / VB

Sensitive to condensed vapors

Measures pressures for 0.0013 to 13.3 Pa

Electronic vacuum gauges are now more common, but the McLeod Gauge is often used to calibrate them.

PIRANI GAUGEMeasures thermal conductivity of a gas

0.1 to 100 Pa

ULTRA LOW SENSORSJames McBride

ULTRA LOW PRESSURE (ULP)Also known as Ultra High Vacuum (UHV)Pressures below 100 nPa (10~9 torr)Extreme conditions so require extensive measures to ensure accuracy. Issues include:

High Speed Pumps. No one single pump is capable of operating from standard pressure to UHV so need several.Seals – Need special metal seals to prevent trace leakage.Extremely Clean.Minimal Surface areaOutgassing. Construction materials absorb other chemicals.

Example Vacuum Chamber

USES FOR ULTRA LOW PRESSUREUses for UHV generally revolve around research:

X-ray photoelectron spectroscopy (XPS)Analyse the composition, chemical and electrical state within a material.

Auger electron spectroscopy (AES)Used to study surfaces for material sciences.

Secondary ion mass spectrometry (SIMS)Measure the composition of thin films and solids.

Thermal desorption spectroscopy (TPD)Measure adsorption binding energy.

Angle resolved photoemission spectroscopy (ARPES)Analyse the density and distribution of electrons.

Particle acceleratorsAtomic Physics Experiments involving ‘cold atoms’

UHV is necessary for these applications to reduce surface contamination, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (10−6 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.

ULP SENSORSTypes of ULP Sensors

Ionisation GageKnudsen GageAlphatronOthers

ULP Requires specialised sensors:High Precision & accuracy.Indirect Pressure Measurement – measure some property of the vacuum rather than the vacuum itself.Mustn’t contaminate environmentOnly concerned with gases.

IONISATION GAGEThree Types – Hot Cathode, Cold Cathode and Spinning RotorGeneral Range of 0.13 Pa – 1.3x10-6Pa. Can measure as low as .13 nPa.Detect Ionisation of gas.

IONISATION GAGE

g

p

ii

Sp 1=

p – Pressureig – Grid Currentip – Plate CurrentS – ‘Sensitivity’ (Chemical Dependant)

Works by emitting electrons from the cathode. These collide with gas atoms and ionise them. The electrons and negative ions are then attracted to the positively charged grid to produce the grid current. The positive ions are attracted to the plate and produce the plate current. Hot cathode generates electrons by heat, cold cathode generates electrons by a large potential difference

IONISATION GAGE CALIBRATIONMost ultra-low pressure gages measure pressure indirectly, hence are non-absolute.

Eg. Ionisation gage measurements are dependant on the chemical properties of the gas in the vacuum.

Non-Absolute pressure measurement requires calibration.

Use either McLeod, Knudsen or deadweight.

Must also be used in conjunction with a mass spectrometer to calculate chemical composition.

KNUDSEN GAGEWorks by heating panels slightly so that Tg –T is small. Gas atoms striking the panels are energised and leave at a greater velocity than they arrived.This difference in velocity results in a net momentum imparted to the vanes.The force can calculated by measuring the angular displacement of the mirror. Force is related to pressure by:

g

g

TTT

Fp−

= 4

Consists of two vanes with heated panels that rotate freely in a pressure chamber, suspended by a filament from a fixed point with a mirror attached.

KNUDSEN GAGERange between 1Pa and 10-6Pa

Key Advantages –Can be configured to be absolute. i.e. Doesn’t require calibrationUnlike McLeod Gauge doesn’t introduce foreign contaminants such as Mercury and oil and is therefore better for high precision environment.Doesn’t require expensive auxiliary sensors like the ionisation gage.Filaments can’t burn out and suspension is not delicate – so more versatile.

Disadvantages –Cannot measure pressure as accurately as Ionisation.

ALPHATRON & OTHERSAlphatron (Shown Below) is a special radioactive Ionisation Gage.Emits radiation and determines the resulting ionisation of gas.No filament, so can measure pressures up to 1 atm as well.

Other Type:Langmuir Gage – measures pressure in terms of damping on high frequency vibration of quartz fibres

SUMMARYWhat is Pressure and Why Measure it?How to Measure Pressure?Measuring Devices

High Pressure SensorsCalibratingLow Pressure SensorsElastic Deformation SensorsUltra Low Pressure Sensors

SUMMARYWhat is Pressure and Why Measure it?How to Measure Pressure?Measuring Devices

High Pressure SensorsCalibratingLow Pressure SensorsElastic Deformation SensorsUltra Low Pressure Sensors

QUESTIONS?