Because the sensing technique of the flow controller is thermal, changing temperatures of the environment influences the operation of the MFC. If the temperature in which the MFC operates changes significantly, the zero output of the device may seem to track these changes. This will result in a calibration shift and may also show up as a repeatability problem. (See the tutorial on external corrosion in another section.)
RF and Grounding
Because of the construction of the sensor element (fine wires wrapped around a capillary tube), it can act as a very effective antenna. When exposed to high levels of Radio Frequency Radiation, the output of the sensor can be affected, especially on some earlier devices that were not adequately protected. The sensor has an inherently low raw signal to noise ratio, so when this interference is received it is amplified along with the flow signal. This can create a control band around the desired set point that is not as precise as desired and can create controller oscillation that renders the device useless.
In many equipment installations it is easier to just tie the signal ground and the case ground together. This can result in a ground loop, especially on tools with RF power sources installed, causing severe control problems with the flow controller. This problem is not easily duplicated at the test bench in our calibration lab and has resulted in many misunderstandings with customers over the years. Proper grounding and isolation of the signal and common ground can greatly reduce the effect of this problem