Pressure Measurement

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?