Pressure Measurement Terms Glossary
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)
Setraceramâ"¢: 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)
