I am trying to develop a basic model for calculating internal room pressure. I am using the vent opening equation. q = Cd A ((2 *delta P )/p)^0.5 If I have a set the vent 2500 mm2; A = 2.5e-3 m^2 Cd = 0.61 Internal pressure to be 102,135 and the external pressure to be 102,140; p = 1.21205 (at 20 C) delta P = 5 Pa. volume = 55 m^3 q = Cd A ((2 *delta P )/p)^0.5 so q = 0.004308549 m^3/s or 0.258512945 m^3/ min If I wanted to calculate the next internal pressure, the mass entering the room is 0.313331 kg, Converting this back into a pressure value, P = m * R *T / V P = 480 Pa. So the new internal pressure is 102135 + 480 = 101715 Pa So the next pressure difference is 475, which causes a larger air flow and a larger difference pressure the next time. It starts to spiral out of control. Any ideas?