Absolute pressure and gauge pressure.

What is Absolute pressure and gauge pressure.???

They are  very important terms in pressure measurement applications .Before understanding the concepts of absolute pressure and gauge pressure it is essential to know about  some terms .

Atmospheric pressure

Atmospheric Pressure.

What is atmospheric pressure?.

Earth’s atmosphere is mainly enclosed of gases like nitrogen and oxygen.Also  small quantity of  other gases . The weight of these gases above us is the cause of atmospheric pressure . We know pressure is the force acting on a unit area .The weight of the gases above us is acting on the every point on earth .And we measure these forces per unit area as pressure .This is called atmospheric pressure .As the altitude increases the amount of gas above  reduces .which causes the atmospheric pressure to reduce . Generally atmospheric pressure  at sea level is used as a reference .

 

Vacuum and perfect vacuum

 

Vacuum 

Any pressure below atmospheric pressure is vacuum or negative pressure .  we can achieve vacuum in a volume by reducing the matter or amount of air  in it. For example a vaccum pump connected to a chamber or vessel reduces the air in it by removing the air inside the chamber by suction.

Perfect  vaccum  or Absolute zero pressure

  Suppose you have a set up to create a vacuum in a region.for example a vacuum pump connected to a closed  cylinder.  you started the vacuum pump to create vacuum .The pressure in the cylinder will decrease (or vacuum increase).The vacuum builds up in the cylinder .This means the matter or air in the cylinder reduces .

What if  the complete matter in the cylinder or an enclosed volume removes ????.This state is called absolute vacuum or perfect vacuum  .There will be no matter inside and this point of state is called the perfect vacuum or absolute zero pressure.

 And one more thing. We can’t create it in lab or in any other set up.Because it is hard to find or create a space without any particle in it.It is a theoretical concept.We use this state as a reference point for scientific calculations . Although small spaces whose dimensions are in atomic range with no matter inside it can be assumed as a perfect vacuum state but it is not possible to isolate such a space .  interstellar space is only a close approximation of perfect vacuuum . Yet it is not a perfect vacuum because small amount of particles  like Hydrogen,neutrinos,photons,or any other quantum particles  exists in every point in space.
 
 

In short
1.perfect vacuum or absolute vacuum is a state of a space in which the complete absence of matter is maintained.
2.It is a hypothetical concept

Absolute pleasure and Gauge pressure measurement

Absolute pressure measurement

When pressure measurements are done with reference to perfect vacuum  , it is called absolute pressure or absolute scale for pressure measurements.The zero reference point is the perfect vacuum state or absolute vacuum or absolute zero . No negative values in this scale.Absolute vacuum is the zero reference   0mmHg.In this scale atmospheric pressure is (760mmHg  or 1 atm

Absolute pressure and Gauage pressure

Absolute pressure and gauge pressure

Gauage pressure

In this scale of pressure  measurements,  atmospheric pressure is taken as zero reference ,ie Atmospheric pressure is taken as 0 in any units (0 mmHg ,0 psi ,0 bar etc).

From the above concepts ,it is evident that absolute  pressure exceeds gauge pressure  numerically by the amount of atmospheric pressure .Or we can write it as 

Absolute pressure  =  Gauge pressure  +  Atmospheric pressure 

In instrumentation and measurement systems , mostly the gauge scale is used .But there are some conditions where absolute pressure measurement has been employed . And the instruments are properly tagged to indicate the scale of pressure measurements .For example a pressure gauge marked it’s unit as ‘psia’ means its measurement is in absolute scale  with pressure unit in psi .The a after psi stands for absolute .For gauge scale it is like ‘psi g’.The  g represents gauge scale