Absolute Pressure Formula

Absolute pressure is a type of pressure measurement which is always referred to as a perfect vacuum. Absolute pressure ranges are often having a label with an ‘abs’. When we are walking outside on a windy day and the wind is strong enough that we can feel its force. This feeling is caused by the air molecules which are bouncing off of you. We may call this as the collective force of the air molecules per unit area the atmospheric pressure. Often, this pressure is measured in pounds per square inch (psi). In this topic, we will discuss the Absolute Pressure Formula with examples. Let us learn it!

                                                                                                                                         Source: en.wikipedia.org

Absolute Pressure Formula

What is Absolute Pressure?

As we know that there is no pressure in the space. Why? This is because there is no atmosphere in the space. There are no air molecules in space, so we will not feel any pressure at all. In other words, there is absolutely zero pressure in space i.e. in a vacuum.

We have two ways of measuring pressures, depending on which we use as a reference. These are:

• Absolute pressure
• Gauge pressure

One way to measure the pressure is to use absolute zero pressure such as in space. This will be like the base value against which other pressures are measured.

Therefore, the pressure measured relative to this absolute zero pressure in a vacuum is called absolute pressure. A pressure measurement below this atmospheric pressure is called negative pressure, or also as vacuum pressure.

So we might be surprised that how do we make measurements based on the vacuum on our earth?” We use an absolute pressure sensor. Scientists have found such a vacuum through behind a diaphragm in the sensor. They do this by removing the air and then sealing it to prevent it from direct contact with the outside.

So, absolute pressure is also known as sealed-pressure. It’s a unit of psia i.e. pounds per square inch absolute. This is very helpful because even if the outside ambient pressure changes, we may have an accurate method to measure the pressure.

The Formula for Absolute Pressure:

When any pressure is detected above this absolute zero of pressure, it is referred to as absolute pressure. We may measure it by using a barometer, and it is equal to the sum total of measuring pressure and the atmospheric pressure.

Thus,

Absolute pressure formula \( (P_as) \) is :

\(P_abs=P_atm+P_gauge\)

P_gauge	Gauge pressure
P_atm	Atmospheric pressure
P_abs	Absolute Pressure

 Vacuum Pressure = Atmospheric Pressure – Absolute Pressure

At the sea level it is approx. 14.7 pounds per square inch.

Solved Examples

Q.1:  A pressure gauge measures the reading as 29 psi. If the atmospheric pressure is 13 psi. Compute the absolute pressure that corresponds to this gauge pressure reading.

Solution:

Given parameters are:

Atmospheric pressure, \(P_atm = 31 psi\)

Gauge pressure, \(P_gauge = 14.2 psi\)

Now applying the formula,

Absolute pressure, \(P_abs=P_atm+ P_gauge\)

\(P_abs = 13 psi + 29 psi\)

\(P_abs = 42 psi\)

Thus Absolute pressure will be 42 psi.

Q.2: The psia pressure instrument gives the reading as 36.8 psi. If the atmospheric pressure is 16 psi, then calculate the corresponding gauge pressure.

Solution:

Given values are:

Atmospheric pressure \(P_atm  = 16 psi\)

Absolute pressure  \(P_abs  = 36.8 psi\)

The Gauge pressure is

\(P_gauge = 36.8 psi – 16 psi\)

= 20.8 psi.

Thus Gauge pressure will be 20.8 psi.