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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?.
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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.
