# Measuring vessel pressure scenario

I need someone to confirm or deny my understanding in the following situation.

I have an empty vessel that is in equilibrium with the atmosphere and I have a pressure gauge on the vessel that displays gauge pressure. The gauge should read 0 atm, correct? If I then close this vessel, pull vacuum on the vessel to completely evacuate it (assume this is possible), then fill it with it 2 atmospheres of nitrogen, the gauge will show 1 atm, correct?

EDIT - There is some specific volume of nitrogen that at ambient temperature will exert 2 atmospheres of pressure within my vessel. In my scenario, I mean to say I have a supply line that can deliver as much nitrogen as I need to the vessel at ambient temperature to meet this condition.

## 1 Answer

There's no such quantity as 2 atmospheres of nitrogen. If you pulled a vacuum to -1 atm, then connected the tank to a nitrogen tank of the same size at 2 atm, you'd have 2 tanks of nitrogen at 1 atm pressure (assuming that nitrogen is an ideal gas)

EDIT- from the ideal gas law: $$m \sim \frac{PV}{T}$$, assuming volume, $V > 0$, temperature, $T < \infty$. Once the vacuum is pulled, $P =0\; atm (abs), m = 0$. By adding some nitrogen such that $P = 2\; atm (abs)$, $m = 0 + \frac{2V}{T}$, then $P = 2\;atm(abs) = 1\;atm(gauge)$. I think you're ok.

• There is some volume of nitrogen that at ambient temperature will exert 2 atmospheres of pressure within my vessel. In my scenario, I mean to say I have a supply line that can deliver as much nitrogen as I need to the vessel. – J. Ari Apr 14 '17 at 19:25
• Okay, so I was not mistaken in my relationship between gauge and absolute pressure. I am having some difficulty explaining a situation with colleagues and I think we are working in different units so I needed a sanity check. thank you. – J. Ari Apr 14 '17 at 20:21