Absolute pressure is the pressure measured from zero (0) value (absolute zero pressure).
ABSOLUTE PRESSURE EXPLAINED
let the manometer reads x atm, or x psi, x kgf/cm² pressure at a particular place.
We noted that the pressure is exist in two type, one is original local atmospheric pressure and other is external pressure (which may vacuum or some value
so the absolute pressure is the total pressure measured including local atmospheric pressure and extra pressure.
This extra pressure is known by gauge pressure. sometime it is vacuum when it goes in negative value from atmospheric pressure.
Absolute Pressure Diagram
Image Credit: Akhil ML
The above diagram of absolute pressure is clearly indicates that the absolute pressure is measured from zero as a datum to its peak point of A.
It also shows that the all other pressure is calculated within this range means the absolute pressure is total of all the pressure present.
In case of B point it is not showing that the absolute is total measured but it is total. As we discuss earlier the vacuum pressure is negative gauge pressure. lets clear with formula
Absolute Pressure Formula (P abs) is given by
P abs = P atm + P gauge
P atm = atmospheric pressure
P gauge = gauge pressure.
Absolute pressure = Atmospheric pressure + Gauge pressure
So in case of B point, The total absolute pressure is equal to the atmospheric pressure plus (-ve) vacuum.
P (abs) = Atmospheric pressure + (-ve Vacuum pressure)
i.e. P (abs) = Atmospheric pressure – Vacuum pressure
So what is the absolute pressure’s value ?
Before that we have to understand the gauge pressure and vacuum pressure
So what is Gauge pressure ?
Lets clear it
Gauge pressure is defined as the pressure measured by pressure reading instruments likely known as gauge.
Actually gauge measures the difference between the pressure they are measuring and atmospheric pressure surrounding the instrument.
That means the gauge cannot measure the atmospheric pressure or the atmospheric pressure is not included in the pressure measured by Gauge Instrunment.
So if we notice the reading of gauge as 0 psig (pounds per square inch gauage) in a vessel or pipe, then this is only gauge pressure.
If we want to know the absolute pressure in the vessel/pipe, then we have to add 0 psig + atmospheric pressure (in psi).