Calibration of capillary tube thermal sensor type mass flow controllers is essentially a matter of temperature calibration. The capillary tube sensor is a thin wall, stainless steel tube with wire windings. The windings are made of resistance thermal detection (RTD) type wire. The resistance of this wire is a function of the wire temperature.
The sensor tube is usually enclosed in a heat insulating material and installed in a protective cover. An equal amount of heat is produced in both sensor windings either directly by a constant current source or by using a separate heater wire winding between the upstream and downstream sensor windings.
A portion of the total gas flow through the MFC is bypassed through the sensor tube. The amount of flow through the sensor is proportional to the flow through the MFC. As gas flows through the sensor tube, a resultant difference in temperature (DT) between the windings occurs. This temperature difference is directly proportional to the total mass flow through the controller.
Problems are encountered as a result of the MFC mounting position. As heat is produced inside the sensor cover, the heat rises toward the top of the cover forming temperature strata. With no gas flow through the sensor and the two sensor windings in the same stratum they will be exposed to the same temperature and they will have the same resistance. However, if the sensor (i.e., the MFC) is tilted, inlet or outlet raised or lowered, the sensor windings will find themselves in different strata, and their relative resistances will be different.
The output voltage will increase either positively or negatively, as a function of whether the upstream sensor is in a higher or lower temperature environment than the downstream sensor. Since this change in output voltage is the result of the mounting orientation and not the result of gas flow through the MFC, the gas flow rate as indicated by the output voltage will be in error.
The above discussion neglects the effects on calibration due to thermal siphoning. Free convection of the process gas at zero flow (thermal siphoning) can be a significant problem. This problem is observed in some flow controllers that are mounted vertically in a system. A zero shift is observed due to heated gas rising in the sensor tube and cooling upon exit. This sets up a real flow through the sensor and the bypass and is not an indication that the natural zero has changed. The danger here is that if this non-natural zero output is adjusted, a calibration error will result. There are a some mass flow controller models that have the sensor positioned such that thermal siphoning is reduced allowing mounting in horizontal or vertical position (Referred to as HOV) and can be used as HBD, VIU, VID. Unfortunately this arrangement does not allow them to function properly if mounted on their side (HOS) and this requires a specific HOS (Horizontal On Side) configuration to be designated for this application.
Because the proper operation of your MFC depends partially upon the MFC mounting position your MFC should be serviced in the same mounting position as it is used whenever possible.