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Principles of Pressure Measurement

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Title: Principles of Pressure Measurement


1
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  • ???? ?????

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Units of Measure
4
How Much is a Pascal (Pa)
  • A Newton is the force necessary to accelerate a
    mass of 1 kg at a rate of 1 meter per second per
    second.
  • The acceleration of gravity is 9.8 m/sec2
  • The force due to gravity on a 1 kg mass is 9.8 N
    is 1 kg weight.
  • 1 Newton is 0.102 kg weight.

5
How Much is a Pascal (Pa)
  • 1 n/m2 is a very small pressure
  • Therefore kilopascal (kPa)
  • 1 atmosphere (14.7 psi, 750mmHg) is approximately
    100 kPa 1 bar
  • 1 kPa is about 7 mmHg

6
Mechanical Methods of pressure measurement
  • Manometry
  • Bourdon
  • Bellows
  • Aneroid
  • Diaphragm
  • Diaphragm Capsule
  • Strain gauge

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Manometry
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Bourdon
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Bourdon Tube Types
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Aneroid
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Diaphragm Types
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Diaphragm Types (cont.)
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From Mechanical to Electronic
  • After the 1920s, automatic control
  • By the 1950s pressure transmitters and
    centralized control rooms.
  • Bourdon tube, bellows, and diaphragm no longer
    had to be connected to a local pointer, but
    served to convert a process pressure into a
    transmitted (electrical or pneumatic) signal.

22
From Mechanical to Electronic
  • Mechanical linkage, pneumatic pressure
    transmitter, 3-15 psig output signal, for
    transmission over distances of several hundred
    feet.
  • Later, as solid state electronics matured and
    transmission distances increased, pressure
    transmitters became electronic.
  • The early designs generated dc voltage outputs
    (10-50 mA 1-5 V 0-100 mV), but later were
    standardized as 4-20 mA dc current output signals.

23
  • 1930s, the first unbonded-wire strain gages.
  • bonded thin-film strain gages.
  • diffused semiconductor strain gages.
  • Semiconductor pressure sensors are sensitive,
    inexpensive, accurate and repeatable.
  • Capacitance

24
Strain Gage Based Pressure Transducer
  • Narrow-span pressure
  • Gauge Pressure
  • Differential pressure
  • Absolute Pressure
  • Full Scale
  • 3 inches of water to 200,000 psig(1400 MPa).
  • Accuracy 0.1 to 0.25 of full scale.

25
Capacitance-Based Pressure Transducer
  • Capacitance change results from the movement of a
    diaphragm element.
  • Stainless steel is the most common diaphragm
    material used.
  • Balanced or unbalanced mode
  • Two Capacitors
  • Single Capacitor with moving plate on a diaphragm.

26
Capacitance-Based Pressure Transducer
  • Widespread in part because of their wide
    rangeability.
  • Full Scale from high vacuums in the micron range
    to 10,000 psig (70 MPa).
  • Differential pressures as low as 0.01 inches of
    water
  • In compared with strain gage transducers, they do
    not drift much.
  • Accuracy within 0.1 of reading or 0.01 of full
    scale.
  • A typical temperature effect is 0.25 of full
    scale per 1000 F.
  • Applications low-differential and low-absolute
    pressure

27
Potentiometer-Based Pressure Transducer
  • Electrical output from mechanical pressure gauge.
  • Unavoidable errors
  • Temperature effects cause additional errors
  • Strong output
  • Low power applications
  • Full scale 5- 10,000 psig
  • Accuracy 0.5-1

28
Resonant Wire Pressure Transducer
Introduced in the late 1970s.
Based on resonant frequency
29
Resonant Wire Pressure Transducer
  • Advantage Inherently digital output
  • Limitations
  • sensitivity to temperature variation
  • nonlinear output signal
  • sensitivity to shock and vibration
  • Absolute pressures from 10 mm Hg
  • Differential pressures up to 750 in. water
  • Gauge pressures up to 6,000 psig (42 MPa).
  • Accuracy 0.1

30
Piezoelectric Based Pressure Transducer
  • Dynamic Pressure
  • NO Static Pressure
  • FS 0.1 and 10,000 psig
  • Accuracy 1 FS
  • Temp. Compensated
  • FS 3 psi to 14,000 psi
  • (21 KPa to 100 MPa).

31
Optical Pressure Transducer
  • Infrared light
  • Compensation for the LED light source by means of
    a reference diode.
  • Immune to temperature effect
  • Movement is very small (under 0.5 mm)
  • hysteresis and repeatability errors are nearly
    zero.
  • No much maintenance
  • Excellent stability
  • Long-duration measurements
  • FS 5 psig to 60,000 psig (35 kPa to 413 MPa)
  • Accuracy 0.1 fullscale.

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Magnetic Pressure Transducer
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Electronic Pressure Sensors Range
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Practical Considerations
  • In industrial applications, good repeatability
    often is more important then absolute accuracy.
  • For a wide range, transducers with good linearity
    and low hysteresis are the preferred choice.
  • Ambient and process temperature variations also
    cause errors in pressure measurements,
    particularly in detecting low pressures and small
    differential pressures. In such applications,
    temperature compensators must be used
  • Power supply variations reduce the performance of
    pressure transducers.
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