1. The problem statement, all variables and given/known data This is a experimental problem that I am trying to model: I have an air source with constant pressure P connected with a small chamber (cylinder). The chamber has initial internal pressure P_0 and as time t goes, the pressure in the chamber raises to P. So, find the step response of P_chamber(t). 2. Relevant equations Thats a great question itself, hehe. These are the ones I tried: P+ρU^2+ρgh = const PV=mRT dm/dt = ρU A 3. The attempt at a solution P_chamber = (m(t)RT)/V m varies over time, V is constant, R is constant and T is a problem. (don't know if I can keep it constant, but I did) I assume the tank is very large compared with the chamber, so U for the tank is 0. I also neglected the hydrostatic term as ρ and h are very small. So, P=P_0+P_chamber+ρU^2 well, I'm stuck at this point. Can't reach a diff eq. Experimentally the step response looks like P_chamber(t) = P(1-e^(-t/B)) ; where B is a time constant So it should be a first order diff. eq. Thanks in advance!