Valve sequencing can cause many of the systematic problems encountered with flow control. A common scenario is as follows: during a manufacturing process, a pneumatic on/off valve external to the MFC is closed while a non-zero set point is applied to the MFC. The MFC internal control valve will open to its maximum in an effort to maintain the desired flow through the MFC. The system is then pumped-out until a specified vacuum is reached and the process is then restarted by opening the pneumatic on/off valve. Extreme overshoot with possible oscillation and long settling times can result. This is due to the change in system pressure and gas composition within the flow controller itself. Further, with the set point left “On”, the control valve will respond by driving full open, creating more internal heat. All of these factors work to change the thermal equilibrium and therefore change the natural zero of the MFC. As the process is restarted, the thermal equilibrium of the sensor is upset once again and the oscillation and overshoot mentioned above may occur.
Another effect is changing process gas (supply gas) temperature. These effects are seen more on a day-to-day, or season to season, rather than a run-to-run basis and are often mistaken for long-term drift of the MFC. To understand the mechanism that accounts for this calibration change, we must remember that the sensor element is actually two components, the sensor tube and the MFC bypass. The ratio of the flow passing through this combination is dependent upon their hydraulic diameters providing that the Reynolds’ Numbers in each are compatible and maintain a consistent ratio. The Reynolds’ Number is a function of velocity, diameter (or hydraulic diameter), density, and viscosity. Density is extremely temperature dependent for most gases used. This means that if the density of the gas passing through the tube varies disproportionately to the density of gas passing through the bypass, the bypass ratio and therefore the calibration will change. This will happen if the supply gas temperature changes. With up to 10 degree swings in temperature, the bulk average temperature of the gas passing through the sensor tube remains constant (the capillary tube is a very efficient heat exchanger), but the gas passing through the bypass retains the variation in temperature and thus density, creating the density change that effects the bypass ratio and therefore calibration. Many MFC manufacturers publish the effect of gas temperature on the accuracy of their device.
Another common effect occurs when there are multiple MFCs on a common gas line or manifold. If each line is not equipped with independent regulation, then pressure interruptions can occur. A typical situation is when one or more MFCs are controlling simultaneously, while another MFC opens, the gas line pressure drops. The other operating MFC valves on that line will open to accommodate the pressure starvation, then close, and then open (a condition called ringing) causing unsteady flow performance. In some cases, the gas pressure drops so low that some MFCs are unable to reach the desired flow. If multiple MFCs will share a gas line, it is advised to install a regulator on each line. The main trunk pressure can also be elevated, for example, to 90 psig, in order to accommodate multiple lines. This way, line “noise”, pressure drops, droop and sag will buffered out.
Process gas composition and quality, especially the presence of moisture, has an effect on both calibration and performance of an MFC. With varying compositions, the heat capacity of the gas changes and the calibration will be affected. Moisture can have the same effect, or worse, it can cause acids such as HCl to form in the flow controller. These acids can degrade seals and create leaks or have a more insidious effect of combining with other compounds to form clogs inside the sensor tube or bypass. This can cause very large changes in the MFC operation. (See the tutorial on Contamination)