Glossary of Pressure Terms

Absolute Pressure: pressure measured relative to full vacuum

  • Referred to as pounds per square inch absolute (PSIA)

Accuracy: combined error of linearity, hysteresis, and repeatability

  • Setra uses the root sum of the squares (RSS) method

Ambient Conditions: the conditions (pressure, temperature, etc.) of the medium surrounding the case of the transducer

Atmospheric Pressure: pressure of the atmosphere at Earth's surface

  • At sea level this pressure is not constant (it varies with the weather), but averages around 100 kilopascals (kP)
  • Ambient pressure may, in other circumstances, be measured in pounds per square inch (psi) or in atmospheres (atm)
    • One atmosphere is also approximately the ambient pressure at sea level and is equal to 14.7 psi

BAR: Unit of pressure (or stress)

  • 1 bar = 750.07 mm of mercury at 0°C, lat. 45°

Barometric Pressure: atmospheric pressure, often measured in millibars, in Hg (inches of mercury), or hectopascals

Bourdon Tube: an early, mechanical pressure gauge consisting of a flattened tube that tends to straighten under internal pressure

  • Today usually used with a potentiometer to produce an electrical output

Burst Pressure: the maximum pressure that may be applied to the positive pressure port without rupturing the sensing element

Capacitive Sensing: detection and measurement of pressure through the change in voltage across a capacitor, one plate of which is a diaphragm which deflects slightly which changes in applied pressure

Compound Pressure: pressure measured from full vacuum (-14.7 PSIV) to gauge pressure, referencing atmosphere

Differential Pressure: pressure measured relative to a reference pressure

  • Referred to as pounds per square inch differential (PSID)

Excitation: the external electrical voltage and/or current applied to a transducer for its proper operation

FS (Full Span or Full Scale): the range of measured values over which a transducer is intended to measure, specified by the upper and lower limits

  • Ex: 0 to 100 PSIG
    • FS is 100 PSIG to 5 VDC
    • FS is 5 VDC
    • 800-100 MB FS is 300 MB

Gage Pressure: pressure measured relative to ambient atmospheric pressure

  • Referred to as pounds per square inch gauge (PSIG)

Hysteresis: the maximum difference in output, at any measured value within the specified range, when the value is approached first when increasing and then decreasing pressure

Linearity: the maximum deviation of any calibration point on a specified straight line, during any one calibration cycle

  • Best Fit Straight Line (BFSL) Method: relationship of a calibration curve to a specified straight line

  • End Point Method: relationship of a calibration curve to a specified straight line through its end points

  • Terminal Method: relationship fo a calibration curve to a specified straight line with end points at zero and full scale

Long Term Stability: the ability of a transducer to reproduce output readings obtained during its original calibration at room conditions for a specified period of time, usually one year

Manometer: an early instrument for measuring pressure

  • Originally a U-shaped tube containing liquid (water, oil, or mercury), one limb opening to the gas volume to be measured, the other closed or connected to a registering or recording instrument
  • Modern versions utilize diaphragms, bellows, or other devices for sensing relative pressures

Millibar (mbar): unit of pressure generally used in barometric measurements

  • 1 mbar = 100 N/m2, or 1~ dyn/cm2

Newton (N): the unit of force in the International System of Units (SI)

  • The force required to impart an acceleration of 1m/sec2 to a mass of 1 kg

Pascal (Pa): the standard unit of pressure (or stress) in the SI system

  • Equal to 1 newton per square meter (1 N/m2)

P/I: term common to process industries meaning pressure-in/current-out

  • 3-15 PSIG Input to 4 to 20 mA DC Output

Position Effect: gravity effect on the output of a transducer when mounted with the diaphragm in different orientation than when it was calibrated

  • Effect is usually minimal and can be zeroed out with the zero potentiometer (on units with a zero pot) with no effect on accuracy

Pressure Transducer: an electromechanical device for translating fluid pressure valves into voltages across a high-impedance (5k ohms or greater) load

Pressure Transmitter: an electromechanical device for translating fluid pressure values into currents (generally 4 to 20 mA) in a low-impedance load

Proof Pressure: the maximum pressure that may be applied without changing performance beyond specifications (typically 0.5% FS zero shift)

  • If the proof pressure is constantly applied over time, there may be an offset in output attributed to it

PSIA: pounds per square inch absolute

PSIV: pounds per square inch vacuum

Range: the spread between the maximum and minimum pressure between which the transducer has been designed to operate

Repeatability: the ability to reproduce output readings when the same pressure value is applied consecutively, under the same conditions, and in the same direction

Response Time: the length of time required for the output to rise to a specified percentage of its final value as a result of a step change in pressure

RSS Accuracy: the root sum of the squares

  • Combined error of linearity, hysteresis, and repeatability (at a constant temperature)

Setraceram™: a patented ceramic/glass sensing element used by Setra in barometric transducers and instruments requiring the highest degree of measurement precision and stability

Span: the algebraic difference between the limits of the range

  • Ex: 0.1 to 5.1 Volts DC
    • Span is 5 VDC
  • Sometimes used to designate full scale output (ex: 5 VDC)

Thermal Error: the maximum change in output, at any pressure value within the specified range, when the temperature is changed from room temperature to specified temperature extremes

Thermal Zero Shift: the zero shift due to changes of the ambient temperature from room temperature to the specified limits of the operating temperature range

Torr: a unit of low pressure equal to a head of 1 mm of mercury or 133.3 N/m2

Vacuum: generally refers to pressures between 0 and atmospheric

  • Often measured in 0-30 Hg Vacuum
  • Referred to as pounds per square inch vacuum (PSIV)
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