THERMAL MASS FLOW MEASUREMENT
Thermal Mass Flow Meters and Controllers make use of the heat conductivity of fluids (gases or liquids) to determine mass flow.
Mass flow sensor for gases, by-pass principle
As shown in figure A a part of the gas stream flows through the sensor, and is warmed up by heaters RHT1 and RHT2. Consequently the measured temperatures T1 and T2 drift apart, as shown in figure B. The temperature difference is directly proportional to mass flow. Electrically, temperatures T1 and T2 are in fact temperature dependent resistors RHT1 and RHT2.
In figure A it is shown how the signals measured in the sensor are amplified to electric signals. The sensor is mounted as a by-pass to the main channel, where a patented flow resistance splitter takes care of proportional flow division, also under varying process conditions. This laminar flow element consists of a stack of stainless steel disc with high-precision etched flow channels, having similar characteristics as the flow sensor.
Mass flow sensor for gases, inline principle
Mass Flow Meters with inline sensor (no by-pass) consist of a straight flow channel, into which two stainless steel probes protrude; a heater probe and a temperature sensor probe. A constant temperature difference (ΔT) is created between the two probes and the energy required to maintain this ΔT is proportional to the mass flow rate. Based on this concept, mass flow can be measured with low pressure drop, mainly caused by the gas fittings and the mesh screens which are incorporated for flow conditioning (see figure C). Compared to traditional thermal MFMs and MFCs with by-pass, the construction of the direct measuring CTA principle (Constant Temperature Anemometry) is less sensitive to humidity and contamination.