Gauge pressure
The difference between absolute or real pressure and atmospheric pressure is called gauge pressure or relative pressure. It is applied only in those cases in which the pressure is higher than atmospheric pressure; when this quantity is negative it is called vacuum pressure {Cengel, Y. (2006). Chapter 3, Fluid Mechanics}.
Many of the devices used to measure pressure use atmospheric pressure as a reference level and measure the difference between real or absolute pressure and atmospheric pressure, This value is called gauge pressure.
The devices used to measure manometric pressure are called manometers and work according to the same principles as mercury barometers and aneroids. Gauge pressure is expressed either above or below atmospheric pressure. Manometers used to measure pressures below atmospheric are called vacuum manometers or vacuometers.
Explanations
When pressure is measured relative to a perfect vacuum, it is called absolute pressure; when measured relative to atmospheric pressure, it is called gauge pressure.
When the pressure measured by the manometer is equal to that of the atmosphere, the manometric pressure is equal to zero, so there is no difference in pressure between the analyzed system and the atmospheric environment. When a manometer is connected to the system or enclosure whose pressure is to be measured, they measure the excess pressure with respect to atmospheric pressure. If the pressure in said enclosure is less than or equal to atmospheric, it indicates zero.
A perfect vacuum would correspond to zero absolute pressure. All absolute pressure values are positive, because a negative value would indicate tensile stress, a phenomenon that is considered impossible in any fluid.
Pressures below atmospheric are called vacuum pressures and are measured with vacuum gauges (or vacuometers) that indicate the difference between atmospheric pressure and absolute pressure. The absolute, gauge and vacuum pressures are positive quantities and are related to each other by:
- pman=pabs− − patm{displaystyle p_{text{man}}= p_{text{abs}}}- p_{text{atm}}{,} (for pressures above patm)
- pvac=patm− − pabs{displaystyle p_{text{vac}}= p_{text{atm}}- p_{text{abs}}{,} (for pressures below patm)
where
- pman{displaystyle p_{text{man},} = Manometric pressure
- pvac{displaystyle p_{text{vac}},} = Vacuum pressure
- pabs{displaystyle p_{text{abs}},} = Absolute pressure
- patm{displaystyle p_{text{atm}},} = Atmospheric pressure
