Pressure Metrology and Calibration

Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface

Mathematically:p is the pressure F is the normal force A is the area.

Pressure is a scalar quantity

In SI units, the unit of Pressure is Pascal denoted by Pa

Other commonly used Units are kPa, mbar, bar, kg/cm2 , psi, mmWC, mmHg

2)(

1metreArea

NewtonForcePa

Definition

Expressed in three forms

Gauge pressureGauge pressure is zero referenced against ambient air pressure, it is equal to absolute pressure minus atmospheric pressure

Absolute pressureAbsolute pressure is zero referenced against a perfect vacuum, It is equal to gauge pressure plus atmospheric pressure

Differential pressureDifferential pressure is the

difference in pressure between two points

Terminology

• pressure gauge – usually refers to a self-contained indicator that converts the

detected process pressure into the mechanical motion of a pointer

• pressure transducer– combine the sensor element of a gauge with a mechanical-to-

electrical or mechanical-to-pneumatic converter and a power supply

• pressure transmitter – a standardized pressure measurement package consisting of

three basic components: a pressure transducer, its power supply, and a signal conditioner / re-transmitter that converts the

transducer signal into a standardized output

Terminology

• The following Basic Gauge Definitions are given in the IS : 3624 the Indian Standard “Specification for Pressure and Vacuum Gauges” :

• Pressure Gauge – A gauge to measure and indicate pressure greater than ambient using ambient pressure as the datum point. Ambient pressure is the pressure surrounding the measuring element.

• Vacuum Gauge – A gauge to measure and indicate pressure less than ambient using ambient pressure as the datum point.

• Compound Gauge – A gauge to measure and indicate pressure both greater than and less than ambient using ambient pressure as the datum point

• Differential Gauge – A gauge having two connections and a means to measure and indicate the difference between two pressures

Terminology

• Accuracy refers to the degree of conformity of the measured value to an accepted standard. It is usually expressed as a percentage of either the full scale or of the actual reading of the instrument. In case of percent-full-scale devices, error increases as the absolute value of the measurement drops.

• Repeatability refers to the closeness of agreement among a number of consecutive measurements of the same variable.

• Linearity is a measure of how well the transducer output increases linearly with increasing pressure.

• Hysterisis describes the phenomenon whereby the same process pressure results in different output signals depending upon whether the pressure is approached from a lower or higher pressure.

Terminology

Generally the following techniques are used to measure pressure

Barometer – For the measurement of atmospheric pressure

Manometer – using difference in the level of 2 liquid column

Dead weight testers

Bourdon tube

Pressure transducers - all electronic pressure instruments use mechanical sensing and subsequent conversion to electronic means by several methods

Measuring Instruments

Barometer

Aneroid Barometer

Manometers

Inclined Tube Manometer

Dead weight tester1 Hand pump, 2 - Testing Pump, 3 - Pressure Gauge to be calibrated, 4 – calibration, Weights, 5 - Weight Support, 6-Piston, 7-Cylinder, 8 - Filling Connection

Dead Weight Pressure Tester

A Bourdon tube is C-shaped with an oval cross-section with one end of the tube connected to the process pressureThe other end is sealed and connected to the pointer or transmitter mechanism Most commonly used instrumentTypical accuracy 0.2 % FSUsed for dynamic pressure alsoOil filled designsSmall between the source of pressure and bourdon tube

Bourdon Pressure Gauge

Mechanical sensing elements

• Some of the techniques employed in electronic type sensors are – Strain gauges,

– Capacitive sensors,

– Potentiometric (LVDT)

– Resonant wire sensors,

– Piezolectric sensors,

– Magnetic sensor

– Optical sensors

Transducers

• A strain gage is used to measure the displacement of an elastic diaphragm due to a difference in pressure across the diaphragm

• Strain gage-type pressure transducers are widely used

• Strain-gage transducers are used for narrow-span pressure and for differential pressure measurements

• Available for pressure ranges as low as 3 inches of water to as high as 200,000 psig (1400 MPa)

• Inaccuracy ranges from 0.1% of span to 0.25% of full scale.

Strain gauge Transducers

Optical Transducers

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

A light emitting diode (LED) is used as the light source, and a vane blocks some of the light as it is moved by the diaphragm.

As the process pressure moves the vane between the source diode and the measuring diode, the amount of infrared light received changes

available with ranges from 5 psig to 60,000 psig (35 kPa to 413 MPa)

0.1% full scale accuracy

• Deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit

• Diaphragm is usually metal or metal-coated quartz and is exposed to the process pressure on one side and to the reference pressure on the other

• Either balanced or unbalanced mode• wide range ability • Can detect pressures between 5 and

10,000 psig (35 KPa to 70 MPa). • Their accuracy is between 0.1% and

1% of full scale

Capacitance Transducers

• a change in pressure produces a movement, which in turn changes the inductance or reluctance of an electric circuit.

• The LVDT operates on the inductance ratio principle

• ranges from 0-30 psig (0-210 kPa) to 0-10,000 psig (0-70 MPa)

• LVDT-type pressure transducers are available with 0.5% full scale accuracy

• They can detect absolute, gauge, or differential pressures

• Limitations are susceptibility to mechanical wear and sensitivity to vibration and magnetic interference

Reluctance Transducers

• a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element

• movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit

• Potentiometric transducers can detect pressures between 5 and 10,000 psig

• accuracy is between 0.5% and 1% of full scale

Potentiometric Transducers

• a wire is gripped by a static member at one end, and by the sensing diaphragm at the other

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

• 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• detect absolute pressures from 10 mm

Hg, differential pressures up to 750 in. water, and gauge pressures up to 6,000 psig

• Typical accuracy is 0.1% of calibrated span

Resonant Sensor Transducers

Piezo electric Transducers

• Pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal and is proportional to the force applied

• Electric signal generated by the crystal decays rapidly

• Unsuitable for the measurement of static forces or pressures but useful for dynamic measurements

• Crystal sensor can be electrostatic, piezoresistive, or resonant depending on characteristics.

• Piezoresistive pressure sensors operate based on the resistivity dependence of silicon under stress

• These sensors also provide high speed responses (30 kHz with peaks to 100 kHz)

• detect pressures between 0.1 and 10,000 psig (0.7 KPa to 70 MPa)

• Typical accuracy is 1% full scale

Low pressure measurement

Figure 4-7: Hot-Cathode Vacuum Gauge

Low pressure measurement

Ionisation sensors

Vacuum measurement

Calibration of Pressure Measuring instruments

Hierarchy of units, standards and measuring equipment

SI

Units

Primary

Standards

Secondary

Standards

Reference Standards

Working Standards

Testing and Measuring Equipment

Legal units

NationalMetrologyLaboratory

Accredited calibrationLaboratories, stateLegal metrology laboratories

End user in trade, industry or testing laboratory

• The accuracy of pressure measurement and its reliability are of prime importance to all major industries

• Therefore the selection of proper measuring instrument and their periodic calibration are very important.

Calibration of Pressure Measuring Instruments

Traceability, Precision and Accuracy• In pressure terms, traceability is defined as the ability to trace

the calibration of a given measurement either directly or indirectly to national Standards of mass and length, such as the NPL, New Delhi in India and, National Institute of Standards and Technology (NIST) in the United States.

• Precision the limit of error or agreement within which the instrument will reproduce measurements when the same input (pressure in this instance) is repeatedly applied to it under the same environmental conditions.

• Accuracy is the degree of conformity to some standard and combines traceability and instrument precision. Accuracy is the difference between the true value and the measured value

• The accuracy specification is an aid in the initial selection of the instrument or transducer, but it tells little of the actual performance in a particular situation

High Accuracy Pressure Calibration • High accuracy pressure calibration is required to

achieve a total uncertainty of less than 0.05 % of the full scale pressure.

• Many calibration methods and pressure standards equipment are available.

• Method used normally depends on the equipment available to perform the test.

• The constraints usually relate to the budget for equipment and the volume of testing to be performed.

Calibration of Pressure Measuring Instruments

• The first decision to make is to determine what type of standard is appropriate for the application.

• The choice is between primary and secondary standards.

• A primary pressure standard is a pressure measuring or generating instrument, which can reduce pressure measurements into measurements of mass, length and temperature and gravity. – Examples are dead weight testers and mercury

manometers.

• A secondary standard is an instrument which must be calibrated to relate the output to pressure.

• Both standards have advantages and disadvantages.

Calibration of Pressure Measuring Instruments

Advantages• Pressure measurements are traceable to measurements of

mass and length and therefore more directly to national standards.

• They have good long term stability- Each measurement must be corrected for temperature,

local gravity and in some instances air buoyancy. These corrections make this type of standard slower to use and the probability for errors increases as the corrections are applied

• Dead weight testers generate incremental pressures. There is a combination of weights for each pressure to be generated. This type of standard cannot be used to measure unknown pressures except within the increments of the weights.

Calibration of Pressure Measuring Instruments Primary Pressure Standards

• Dead weight testers are difficult to use to generate absolute pressure (that is below ambient pressure)

• the pressures must be corrected for the loss of air buoyancy on the weights.

• The piston and weights are in a bell jar which must be evacuated each time the weights are changed to generate a new pressure.

• pressure in the bell jar is the reference pressure and added to the generated pressure to obtain the absolute pressure.

• Measuring the vacuum introduces another measurement uncertainty.

• Mercury manometers, since they contain mercury are a health and environmental hazard.

• Over pressuring a mercury manometer can result in the mercury being blown out of the tube, and should the mercury contaminate the device being tested it would probably be ruined due to mercury's tendencies to amalgamate with other metals.

Calibration of Pressure Measuring Instruments Primary Pressure Standards

Disadvantages

• Today pressure transducers are manufactured in large quantities and with high accuracy.

• Automated pressure standards are required during the manufacturing process and for final calibration.

• Because of corrections, manipulating the weights and controlling the environment, primary pressure standards are difficult and expensive to adapt to production testing where automatic calibration is necessary.

Calibration of Pressure Measuring Instruments Secondary Pressure Standards

Advantages• Faster and easier to use. • Usually no measurement corrections. • Non-incremental measurements. Easier to

adapt to automatic operation. • Generally less expensive.

Disadvantages• Must be periodically recalibrated by a standard

traceable to national standards. • Pressure measurements cannot be reduced to

measurements of mass, length or temperature

Calibration of Pressure Measuring Instruments Secondary Pressure Standards

.

Methods of Pressure Calibration

• Pressure calibration always involves applying a known pressure to the device under test and recording the following– the known pressure – the output signal or reading of the device under

test.– Additionally, the temperature of the device under

test, – time of reading – whether the pressure is ascending or descending.

• There are many variations of calibration --from the most simple manual methods, to completely automated calibration systems. However, the heart of each system is a pressure measuring instrument.

Dead Weight Pressure tester

• Also called Dead Weight piston gauge or Pressure balance.• One of the fundamental method - force per unit area of the

piston. • consists of an accurately machined piston of known weight

which is inserted into a closed fitting cylinder (clearance between piston and cylinder will be an order of few microns), both of known cross-sectional area.

• Weights of known mass loaded on one end of the piston and fluid pressure applied to the other end of the piston until enough force is developed to lift the piston-weight combination

• When the piston is floating freely within the cylinder (between limit stops), the piston is in equilibrium with the unknown system pressure. Applied pressure = the ratio of force due to the weights-piston and the area of cross section of the piston-cylinder.

Calibration of Pressure Measuring Instruments

Hydraulic Dead Weight Tester

04/11/23

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Absolute Dead Weight Tester

• (1)     Measuring Range: The maximum pressure to be measured by dead weight tester to be selected from the following two series– 1x10n, 1.6x10n , 2.5x10n , 4x10n , 6x10n (MPa)– 1x10n, 2x10n , 5x10n(MPa)

• (2)     Accuracy classes: Dead Weight Testers are classified into six accuracy classes as 0.005, 0.01, 0.02, 0.05 , 0.1 and 0.2 and the class is determined by calibration.

• (3)     Free Rotation Time and Fall rate of the piston: It should not be less than the value mentioned in the following tables.

Metrological and technical requirements of Dead Weight Tester as per OIML R110 standard.

Fall Rate of the Piston as per Accuracy Class and Measuring range Pressure Upper limit of Maximum Piston Fall rate (mm/minutes) medium the measuring for Accuracy class range (MPa) 0.005 0.01 0.02 0.05 0.1 0.2 gas 0.1 to 1 1 1 1 2 2 - gas more than 1 2 2 2 3 3 - liquid 0.6 to 6 0.4 0.4 0.4 1 2 3 liquid 6 to 500 1.5 1.5 1.5 1.5 3 3

Free Rotation Time of the Piston as per Accuracy Class and measuring range Upper limit of the Free Rotation Time ( minutes) for Accuracy Calss Measuring range (MPa) 0.005 0.01 0.02 0.05 0.1 0.2 0.1 to 6 4 4 3 2 2 2 6 to 500 6 6 5 3 3 3

Metrological and technical requirements of Dead Weight Tester as per OIML R110 standard.

Calibration of Dead Weight pressure Tester

• Dead weight pressure testers are normally calibrated against reference standard dead weight pressure tester by cross float method.

• Both the reference and test testers are connected through a differential pressure cell ( null indicator)

• For high accurate work, the capacitance sensor can be used for monitoring the vertical movement and position of the floating piston.

• The dead weight pressure testers are normally calibrated either in terms of pressure value itself or in terms of effective area.

• The uncertainty of the reference standard is known and is calibrated with traceability to National/International Standard.

Calibration of Dead Weight pressure Tester

(i) Corrections on Piston-Cylinder Area (a) Effect of Temperature:

The Temperature corrected area = Ao [ 1+ (c +p)(t-tr)](b) Pressure distortion Effect:

b= [2 + 1/(k2-1)]/E Aeff. = Ao [ 1+ bp+ (c +p)(t-tr)]

(ii) Corrections on Applied Force(a) Gravity correction:

Gravity corrected force = (Mo + m) gl , (b) Air buoyancy correction: Buoyancy corrected = (Mo + m) (1- a/m) gl

(c) Surface tension correction: Surface tension force = .c

Feff.= (Mo + m) (1- a/m) gl+.c (iii) Friction between piston and cylinder

Corrected Pressure,

Calibration of Dead Weight pressure Tester

Absolute Pressure Standard

• Absolute pressure standards have a permanent vacuum in the reference chamber.

• evacuating the pressure chamber to a pressure less than the resolution of the instrument

• measuring the residual pressure with a vacuum gauge and setting that pressure by adjusting the zero.

• Absolute pressure standards are sometimes "zeroed"at higher pressures by applying a pressure from another standard or by measuring atmospheric pressure with both instruments and setting the reading with the zero adjustment of the instrument being set up for test.

Gauge Pressure Standards

• Gauge pressure standards use atmospheric pressure as the reference pressure.

• These instruments are easy to zero. • Atmospheric pressure is applied to

the pressure chamber and the instrument is adjusted to zero output.

Typical Uncertainties in pressure calibration

• Type A – Repeatability

• Type B –- Due to Accuracy of the Reference Standard- Due to the Calibration uncertainty of the Reference Standard- Due to the Resolution of the Ref. / Test gauge- Due the acceleration due to gravity- Due to zero setting- Due to Hysterisis

1 Pneumatic Dead Weight tester 30mbar to 20bar ± 0.02 % rdg

2 Vacuum Dead Weight tester100 to 1000 mbar ± 0.02 % rdg

3 Oil Dead Weight Tester 1 to 1200 bar ± 0.025 % rdg

4 Oil Dead Weight Tester 1 to 600 bar ± 0.04 to 0.06 % rdg

5 Multifunction pressure indicator 0 to 20 bar (abs) ± 0.025 % rdg

6 Portable Pneumatic Calibrator -3.6 to 30.6 psi ± 0.02 psi

7 Portable Hydraulic Calibrator 0 to 400bar ± 0.04%

8 Absolute Pressure gauge 0 to 200 psi ± 0.08 psi

9 Absolute Pressure gauge 0 to 25 psi ± 0.025 psi

10 Water Column Manometer 0 to 2200 mmWc ± 0.5 mmWc

11 Mercury Manometer 0 to 1600 mmHg ± 0.6 mmHg

FCRI pressure Standards

References