Title: Principles of Pressure Measurement
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3Units of Measure
4How 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.
5How 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
6Mechanical Methods of pressure measurement
- Manometry
- Bourdon
- Bellows
- Aneroid
- Diaphragm
- Diaphragm Capsule
- Strain gauge
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8Manometry
9Bourdon
10Bourdon Tube Types
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15Aneroid
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17Diaphragm Types
18Diaphragm Types (cont.)
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21From 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.
22From 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
24Strain 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.
25Capacitance-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.
26Capacitance-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
27Potentiometer-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
28Resonant Wire Pressure Transducer
Introduced in the late 1970s.
Based on resonant frequency
29Resonant 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
30Piezoelectric 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).
31Optical 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.
32Magnetic Pressure Transducer
33Electronic Pressure Sensors Range
34Practical 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.