# A manometer measures mechanical or thermodynamic pressure?

• A
Hi guys,
I have two questions as follows, ofcourse they may seem too basic, I have read through certain texts (engineering) but I still haven't been able to actually physically understand, so asked here
1) Whats thermodynamic pressure w.r.t physical point of view? - for a gas from thermodynamics I know its the force per unit area that an imaginary small face would experience at a point in the gas
Is this valid for the thermodynamic pressure at a point in a fluid (not necessarily gas)?
2) Which pressure does a manometer / or any pressure sensor measure? I'm know that thermodynamic pressure = mechanical pressure for in compressible flow or stokian fluid, but thats not my question. I want to know which is actually the pressure that my manometer is measuring - thermodynamic or mechanical pressure?

Andrew Mason
Homework Helper
Hi guys,
I have two questions as follows, ofcourse they may seem too basic, I have read through certain texts (engineering) but I still haven't been able to actually physically understand, so asked here
1) Whats thermodynamic pressure w.r.t physical point of view? - for a gas from thermodynamics I know its the force per unit area that an imaginary small face would experience at a point in the gas
Is this valid for the thermodynamic pressure at a point in a fluid (not necessarily gas)?
Thermodynamic pressure is defined for a substance that is in thermodynamic equilibrium. It is the force per unit area exerted by such a substance on the boundary interface with another substance (eg. the walls of a container) that mechanically contains the substance. A substance that is not in thermodynamic equilibrium (eg. a flowing substance) exerts a mechanical pressure (on the pipe walls, for example) but does not have a precisely defined thermodynamic pressure.
2) Which pressure does a manometer / or any pressure sensor measure? I'm know that thermodynamic pressure = mechanical pressure for in compressible flow or stokian fluid, but thats not my question. I want to know which is actually the pressure that my manometer is measuring - thermodynamic or mechanical pressure?
The manometer measures the difference between the mechanical pressure exerted by a substance on the container walls and the ambient (e.g. atmospheric) pressure. If the substance is in thermodynamic equilibrium (not flowing) it measures the difference between thermodynamic pressure and ambient pressure.

AM