This article establishes units for Pressure Measurement in process plants. Main keywords for this article are Definitions Pressure Measurement Units. Pressure Units. Range of Pressure. Differential Pressure. Atmospheric Pressure. Pressure Elements. pressure unit conversion.

**Definitions Pressure Measurement Units**

**Absolute Pressure.**

The pressure measure relative to zero pressure (vacuum).

**Ambient Pressure.**

The pressure of the medium surrounding a device.

**Atmospheric Pressure.**

The barometric reading of pressure exerted by the atmosphere. At sea level it is 14.7 lb/in².

**Differential Pressure.**

The difference in pressure between two points of measurement.

**Elevation, Elevated Zero.**

A range in which the zero value of the measured variable of measure signal is greater than the lower range value.

**Gauge Pressure.**

Pressure measured relative to ambient pressure.

**Pressure Elements.**

The portions of a pressure-measuring gauge that move or are temporarily deformed by the system pressure, the amount of movement or deformation being proportional to the pressure.

**Range.**

The region between the limits within which a quantity is measured received, or transmitted, and which is expressed by stating the lower and upper range values.

**Suppression, Suppression Zero.**

A range in which the zero value of the measured variable is below the lower range value.

**Vacuum.**

A low-pressure gaseous environment having pressure lower than ambient atmospheric pressure.

**Pressure Measurement Units**

Because of the relationship that pressure has to a system as fundamental parameter, its measurement is of crucial importance and wide ranges of this measurement with variable degrees of accuracy are

required.

** Pressure Units**

- In the MKS system of measurement, pressure is expressed as kilogram per square meter kg/m.
- Standard unit of Pressure in English (FPS) system is pound per square inch or ‘psi’
- In Standard International (SI) units, pressure is measured as Newton per square meter or Pascal (Pa).
- the larger unit kPa shall be used. Other pressure units preferred are mmH2O or mmHg.
- bar (Barometric Pressure) = one atmosphere.

**Range of Pressure**

Refer to Graph 1 for the range of different types of pressure.

* Based on standard atmospheric pressure = 14.696 psia or 29.92 inches Hg abs

(A) Absolute pressure measured above a perfect vacuum.

(B) Atmospheric pressure measured exerted by the earth’s atmosphere (= 14.7 psi).

(C) Gauge pressure measured above atmosphere.

(D) Differential pressure is the difference between two pressures.

(E) Vacuum is a pressure less than atmosphere.

## Pressure Unit Conversion

Refer to Table I for Pressure Unit Conversion.