Tank Venting vs Expansion Cooling

[JBA]

TL;DR

Classic formula for P vs T: P=nRT transformed to T=P/nR indicates all tank ventings are isothermal?

In the process of developing a tank venting program and starting to investigate how to incorporate tank gas expansion cooling I realized that since: T=P/nR with R constant and the reduction of P and n directly proportional then this equation implies all tank ventings are isothermal. What am I missing?

 

[JBA]

Never mind, I just realized I used a constant temperature in determining the pressure vs mass in each mass reduction step so I ended up with the circular calculation that I was trying to avoid.