### Scales

October 17, 2012

#### by SetraSystems

Often we hear terms used to describe pressure transducers or terms commonly used in the HVAC/R and industrial industries, but there may be some confusion as to their meaning. Here are a few commonly used terms and their definitions.

Absolute Pressure Transducers

Absolute Pressure:
Pressure measured relative to high vacuum. Referred to as pounds per square inch absolute or (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 the 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.

Ba:
Unit of pressure (or stress). 1 bar= 750.07 mm of mercury at 0°C, lat. 45°

Barometric Pressure Transducers

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 physical damage to 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 with changes in applied pressure.

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

Compound Pressure Transducers

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 or (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 measure and 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/ 0 to 5 VDC, FS is 5 VDC, 800-1100 MB, FS is 300 MB.

Gauge Pressure Transducers

Gauge Pressure:
Pressure measured relative to ambient atmospheric pressure. Referred to as pounds per square inch (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. (Setra uses the best straight line method).

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 1 m/sec2 to a mass of 1 kg.

P/I:
Term common to process industries meaning pressure-in/current-out. (3-15 PSIG Input to 4-20 mADC Output).

Pascal (Pa):
The standard unit of pressure (or stress) in the SI system: equal to 1 newton per square meter (1 N/m2).

Pressure Transducer:
An electromechanical device thatprovides a linear D.C. voltage output proportional to applied pressure

Pressure Transmitter:
An electromechanical device for translating pressure values into currents (generally 4-20 mA) into a low-impedance load output proportional to applied pressure

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

PSIA:
Pounds per square inch absolute

PSIV:
Pounds per square inch vacuum

Range:
The spread between the maximum and minimum pressures 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.

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; i.e. 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

Vacuum Pressure Transducers

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 in. Hg Vacuum. Referred to as pounds per square inch vacuum (psiv)