Mechanical and Electrical Pressure Instruments

Vocabulary flashcards covering the definitions, operating principles, pressure ranges, and installation considerations for mechanical and electrical pressure instruments.

Pressure Instrument

The complete assembly that includes the primary sensing element plus the mechanism or electronics to convert deformation into a readable or transmittable signal (e.g., pressure gauge, pressure transmitter, pressure switch).

Pressure Element (Primary Element)

The part of an instrument that is directly in contact with the fluid and deforms or changes in response to pressure (e.g., Bourdon tube, diaphragm, bellows, capsule, strain gauge).

Gauge Pressure

Pressure measured relative to atmospheric pressure.

Absolute Pressure

Pressure measured relative to a perfect vacuum.

Differential Pressure

The difference between two distinct pressures, referenced to one of the two pressures.

Vacuum Pressure

Pressure that is below atmospheric pressure, referenced to atmospheric pressure.

Bourdon Tube

A mechanical sensing element consisting of an elliptical tube with one open end and one closed end; internal fluid pressure causes elastic deformation and displacement of the closed tip. Operates at up to $29\,\text{bar}$ and is not suitable for pressures below $0.3\,\text{bar}$.

Bellows

A convoluted metal can with a fixed open side that expands or contracts along its axis when internal pressure is applied; operates at up to $7\,\text{bar}$ and provides good sensitivity for low-pressure applications.

Diaphragm Element

A flexible disk that deflects when pressure is applied to one side; suitable for low-pressure ranges and corrosive fluids.

Capsule

A mechanical pressure element made of two diaphragms welded together, providing a higher operating range of up to $300\,\text{bar}$.

Capacitance Pressure Sensor

An electrical pressure sensor in which applied pressure alters the distance between capacitor plates in a capsule, producing a capacitance change that is converted into a $4\text{--}20\,\text{mA}$ output.

Piezoelectric Sensor

An electrical sensor that generates an electrical charge or voltage when a quartz or crystal material is mechanically compressed; features a high frequency response but cannot measure static pressure.

Strain Gauge Pressure Sensor

An electrical pressure sensor featuring strain gauges bonded to a diaphragm; diaphragm deflection alters electrical resistance, which is measured across a Wheatstone bridge with an output in $\text{mV/V}$.

Differential Pressure (DP) Transmitter

An electrical instrument that senses the differential pressure between high and low pressure ports—such as across an orifice plate—utilizing differential capacitance or strain to produce a $4\text{--}20\,\text{mA}$ output signal.

Pigtail Seal (Siphon)

A looped or siphon pipe installed on instrument input lines to trap condensate and prevent direct exposure to live steam.

Pulsation Damper (Snubber)

A protective device installed on instrument input lines to attenuate and dampen high process pressure fluctuations.

Overpressure Stop / Blow Out (Rupture) Disc

Safety components designed to prevent instrument damage or hazardous conditions for personnel in the event of excessive process pressures.