Pressure Measurement
• Why is it important?
Pressure Measurement
• Asses the situation
– What is the range of pressures to be
measured?
– Is pressure dynamic or static?
– What is more important: absolute accuracy or
good repeatability?
– How much overpressure protection is
required?
Types of Measurement
• Mechanical
– U-tube manometer, Bourdon tube, Diaphragm
and Bellows
• Electrical
– Strain Gauge, Capacitive sensor,
Potentiometric, Resonant Wire, Piezoelectric,
Magnetic, Optical
Mechanical
• Mechanical pressure measurement devices are
large and cumbersome.
• Not suited for automated control loops typical in
industry.
• Mechanical devices:
– U-tube Manometer
– Bourdon tube
– Diaphragm and Bellows element
U-tube Manometer
• Measures difference in
pressure between two points in
a pipe.
• Typical in laboratories.
Bourdon Type
• Flexible element used as sensor.
• Pressure changes cause change in element
position.
• Element connected to pointer to reference
pressure.
Diaphragm and Bellows Element
• Similar concept to Bourdon type.
• Widely used because they require less space
and can be made from materials that resist
corrosion.
Electrical
• Have become more common with increased
reliance on computerized control systems.
Strain Gauge
• Measures deflection of elastic diaphragm due to
pressure difference across diaphragm.
• Widely used in industry.
• Used for small pressure ranges.
• Measurements tend to drift.
Capacitive Sensor
• Measures changes in capacitance of electrically
charged electrodes from movement of metal
diaphragm due to pressure difference across
diaphragm.
Capacitive Sensor, cont.
• Can be operated in balanced or
unbalanced mode.
– Balanced always has capacitance of zero.
Measures pressure indirectly by measuring
drift in capacitor arms.
– Unbalanced measures ratio between output
voltage and excitation voltage.
• Widely used in industry.
• Large rangeability.
Resonant Wire
• Wire is oscillated at resonant frequency
by oscillator circuit.
• Pressure changes cause change in wire
tension which changes oscillatory
frequency.
• Generates digital signal.
• Very precise, used for low differential
pressure measurements.
• Sensitive to temperature variation and
has non-linear output
Piezoelectric
• Measures the charge developed
across quartz crystal due to change
in pressure.
• Charge decays rapidly making
unsuitable for static pressure
measurements.
• Sensors are very rugged. Pressure
can be applied longitudinally or
transversally.
• Used to measure dynamic pressure
changes associated with explosions
and pulsations .
Magnetic
• Measures induced current caused by movement of
magnetic components from pressure changes.
• Used in applications where high resolution in small range is
desired due to very high output signals.
• Sensitive to stray magnetic fields and temperature
changes.
Optical
• Detects effects of minute motions due to process pressure changes
through partial blocking of an LED.
• Immune to temperature effects.
• Excellent stability and long-duration capability.
Various types of pressure gauges
Questions?