Glossary

Absolute position measurement: Position measured from a fixed point.
Absolute pressure: Pressure measured with reference to a perfect vacuum.
Accelerometer: A sensor for measuring acceleration or the rate of change of velocity.
Accuracy: A measure of the difference between the indicated value and the true value.
Actuator: A device that performs an action on one of the input variables of a process according to a signal received from the controller.
Analog to digital converter: A device that converts an analog voltage or current into a digital signal.
Aneroid barometer: A barometer that uses an evacuated capsule as a sensing element.
Anticipatory action: See also Derivative action.

Bellows: A pressure sensor that converts pressure into linear displacement.
Bernoulli equation: A flow equation based on the conservation of energy, which includes velocity, pressure, and elevation terms.
Beta ratio: The ratio of the diameter of a restriction to the diameter of the pipe containing the restriction.
Bimetallic: A thermometer with a sensing element made of two dissimilar metals with different thermal coefficients of expansion.
Bode plot: A plot of a transfer function (gain in decibels and phase in degrees) versus frequency (log scale).
Bourdon tube: A pressure sensor that converts pressure to movement. The device is a coiled metallic tube that straightens when pressure is applied.
British Thermal Unit: A measure of heat energy, which is defined as the amount of heat required to raise 1 lb of water 1 F at 68 F and 1 atm.
Buffer amplifier: A circuit for...

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Introduction to Instrumentation, Sensors, and Process Control