Patent ID: 8667830

Claim:
A method of determining a flow rate of gas through a gas flow controller into a processing chamber, the method comprising: establishing a flow of gas through the gas flow controller at a constant rate; fixing a known volume upstream of the gas flow controller by interrupting the gas flow upstream of the known volume; performing a process in the processing chamber and during the process: measuring at a first time, a first pressure in the volume; measuring at a second time after the first time, a second pressure in the volume; determining a rate of drop in pressure from the first pressure and the second pressure; determining a fixed flow rate based upon the rate of pressure drop and the known volume; and reestablishing the gas flow into the volume in such a way that the flow through the gas flow controller is not perturbed beyond a certain level wherein the reestablishment of the gas flow into the known volume is done with a controlled flow rate, such that the rate of rise in pressure inside the known volume is sufficiently low so that the gas flow controller is not perturbed beyond a certain level.