The main flow-limiting component in a mass flow controller is the control valve. The inlet pressure applied to the controller acts to push gas through the controller. If the controller is feeding into a vacuum, this vacuum aids the gas flow by drawing 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 5 PSIG backpressure 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 varies in size depending on the flow rate, density, viscosity, and pressure. The MFC control circuit determines the positioning of the valve to maintain the desired gas flow during operation. The amount of valve positioning change is limited, and the maximum and minimum openings are the result of mechanical adjustments made to the valve.
These mechanical adjustments depend on the differential pressure the controller is expected to experience in normal operation, the maximum and minimum flow rates of the controller and the density of the gas that will actually be flowing through it. If a device is properly configured for a low-density gas, like Hydrogen, the valve will be adjusted to the opening required to give the expected full-scale flow of Hydrogen at 100% set point and at the pressure differential specified. It will then be set to give the optimum control movement for that gas to give as much control as possible at the lower range of the flow scale.
If a gas of significantly different density, like Nitrogen, is then flowed through the device the flow indicated would be accurate because the gas Conversion Factor (CF) of Hydrogen to Nitrogen is 1.01. Nitrogen is approximately 14 times denser than Hydrogen; it is very probable the expected full-scale flow could not be achieved because the valve opening was set for Hydrogen. As the set point was increased the valve would reach the maximum allowable opening, as set for Hydrogen, and then hold there. It literally cannot open any more. Even though the set point was increased, the valve would no longer move, and the flow of Nitrogen would remain constant. The flow would then return to normal control as the set point was decreased and the valve opening caught up with the Nitrogen flow. This plateau of Nitrogen flow will usually show up at the 60-75% set point for most models properly setup for Hydrogen.
Many users attempt to use N2 and the corresponding CF to check the incoming calibration of all flow controllers, regardless of the designated gas. This is much easier and cheaper than trying to maintain the systems and gases required duplicating a professional calibration facility. In order to make sure the above problem, among others, is not encountered these device must be initially calibrated so the corresponding flow of Nitrogen can be achieved in the device. This requires adjusting the valve for the full scale N2 flow required to test the device. This means the valve is not optimized for the designated gas but for the test gas. Optimal flow control, especially at the lower flow ranges, will be significantly compromised. In this case we have configured the flow controller to give the test results we want at the expense of compromising the operation of the device in the process.
Although the pressure-operating window of a properly configured device can be relatively large, it is not unlimited, and it is determined in many cases by the model of the device, the gas used, and the expertise and patience of the technician performing the setup. Needless to say the most important factor is the accurate determination of the ACTUAL pressure conditions that the device will experience and communicating that information to the calibration lab.