Principles of Pressure Measurement

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Units of Measure
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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.

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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

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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.

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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.

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• 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

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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.

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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.

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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

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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

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Resonant Wire Pressure Transducer
Introduced in the late 1970s.
Based on resonant frequency
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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

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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).

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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.