The major flow limiting component in a mass flow controller is the control valve. The inlet pressure applied to the controller acts to push gas molecules through the controller. If the controller is feeding into a vacuum, this vacuum aids the gas flow by pulling gas molecules through the controller.

Therefore, with an inlet pressure of 20 PSIG, (Pounds per Square Inch Gauge) and the controller outlet feeding into a 20 millitorr vacuum, essentially -14.7 PSIG, the pressure differential would be approximately 35 PSID, (Pounds per Square Inch Differential)

This differential pressure is computed by subtracting the outlet pressure, actually a vacuum in this case, from the inlet pressure as follows:

20 - (-14.7) = 34.7 PSID

This same differential pressure, and thus the approximate same gas flow, can be achieved by applying 35 PSIG to the inlet with the outlet feeding into an atmospheric chamber at 0 PSIG. This approximation ignores any calibration errors due to feeding into a vacuum.

If the inlet pressure is 20 PSIG and the controller is feeding a system with a back pressure of 5 PSIG, the differential pressure would be 15 PSID, i.e.,

20 - (+5) = 15 PSID

The five pound back pressure will try to reduce the gas flow through the controller. This 15 PSID pressure will force considerably less gas through a given orifice than would the 35 PSID pressure.

The MFC valve is a variable orifice and the orifice size changes to accommodate different flow rates, set points and pressures. The MFC control circuit determines the positioning of the valve during operation. The amount of valve positioning change is limited and the maximum and minimum openings are the result of a mechanical adjustment made to the valve.

The mechanical adjustment required depends on the differential pressure the controller is subjected to, the maximum and minimum flow rates of the controller and the density of the gas actually flowing through it.

Because the proper operation of your MFC depends partially upon the differential pressure applied to it, your service organization should use that same differential pressure when they service your MFC.