# What is gauge pressure

## What is gauge pressure - definition

### Gauge pressure

The one discussed aboveprint is calledcalled absolute pressure . Often it is important betweenAbsolute pressure andGauge pressure to distinguish. In this article, the term pressure refers to absolute pressure unless specifically stated otherwise. In engineering, however, we are often concerned with pressures generated by some devicesmeasured become. Although absolute pressures must be used in thermodynamic relationships, showPressure gauges often thedifference between the absolute pressure in a system and the absolute pressure of the atmosphere outside the measuring device. You measure theGauge pressure .

• Absolute pressure.When pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia). Pounds per square inch absolute (psia) is used to indicate that pressure is relative to a vacuum rather than ambient atmospheric pressure. Since the atmospheric pressure at sea level is about 101.3 kPa (14.7 psi), this is added to any pressure reading in air at sea level.
• Gauge pressure.When pressure is measured relative to atmospheric pressure (14.7 psi) it is called gauge pressure (psig). The term gauge pressure is used when the pressure in the system is greater than the local atmospheric pressure patm is . The latter pressure scale was developed because almost all pressure gauges register zero when open. Gauge pressures are positive when above atmospheric pressure and negative when below atmospheric pressure.

ptrack = pabsolutely - pabsolutely; ATM

• Air pressure.Atmospheric pressure is the pressure in the ambient air at or near the surface of the earth. Atmospheric pressure varies with temperature and altitude above sea level. TheCorresponds to standard atmospheric pressure approximately the average pressure at sea level at 45 ° N. TheStandard atmospheric pressure will be at sea level273 O K (0O C) defined and amounts to:
• 101325 Pa
• 1.01325 bar
• 14.696 psi
• 760 mmHg
• 760 torr
• Negative pressure - vacuum pressure.If the local atmospheric pressure is greater than the pressure in the system, the term becomesVacuum pressure used. A perfect vacuum would correspond to an absolute zero pressure. It is certainly possible to have a negative overpressure, but not to have a negative absolute pressure. For example, an absolute pressure of 80 kPa can be described as a gauge pressure of -21 kPa (ie 21 kPa below an atmospheric pressure of 101 kPa).

pvacuum = pabsolutely; atm - pabsolutely

For example, a car tire that is pumped up to 2.5 atm (36.75 psig) above local atmospheric pressure (say 1 atm or 14.7 psia local) has an absolute pressure of 2.5 + 1 = 3, 5 atm (36.75 + 14.7 = 51.45 psia or 36.75 psig).

On the other hand, condensing steam turbines (in nuclear power plants) vent steam at a pressure well below atmosphere (e.g. at 0.08 bar or 8 kPa or 1.16 psia) and in a partially condensed state. In relative units it is a negative overpressure of about -0.92 bar, -92 kPa or -13.54 psig.