What is THERMODYNAMIC pressure?

Xinwen Xiao

According to the assumption for Newtonian fluid, in static state, normal stress(Pm) is EQUAL to the value of its thermodynamics pressure(Pt). In physics, they are not SAME, just happen to be equal in value in static state. While fluid moves, they are not equal any more.

My question is: what is really thermodynamic pressure? How to measure it? If you just use a tube with one end sealed and filled with mercury, which is the Toriceri(?)'s idea to measure atmosphere pressure, then what you can get is actually normal stress(Pm) because it is this normal stress that mechanically supports the mercury to prevent it to keep falling.

Ok, If you can not measure Pt, then how do you know Pm is equal to Pt in static state?

Somebody may say, enen if I can not measure, I still can calculate it by Pt=rho*R*T. Then I want to ask, if you could not measure it, how do you know you formulus is correct? How can you determine the R? When you try to build up this formulus, actually you calculate the R based on Pt, rho, and T. So actually you just recircle here if you can not measure Pt. You have to measure Pt!!! How??

Thank you.