Chapter 3.4 - Mass Flow Measurement and Control

Abstract

Two general approaches—pressure based and thermal based—can be commonly found in modern instrumentation for continuous mass flow measurement of gases into vacuum systems. Flow measurements may be made by measuring pressures upstream and downstream of a laminar flow element, an orifice plate, a Venturi element, or a molecular flow element. Also, simple mechanical devices known as rotameters can serve as convenient volumetric flow indicators. This chapter provides an overview of thermal mass flow controller technology. A thermal mass flow meter (MFM) is often combined with a control valve and electronics to form an instrument known as a mass flow controller (MFC). The sensor, associated electronics, and bypass form a mass flow meter. To make the instrument a mass flow controller, a control valve is placed downstream from the meter and a control circuit is added to the electronics. Control valves used in typical MFCs allow continuous adjustment of the conductance as opposed to the on/off action of a shutoff valve. The resolution of conductance is much better than 0.1% of the open conductance to obtain the desired control range of the MFC.