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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.

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.

tmfpricipleillustrationsmall.jpg

figure A

thermisch_principe

figure B

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.

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 (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.

CTA Mass Flow Sensor

figure C

for liquids

Liquid mass flow meters of the LIQUI-FLOW® series are built around a stainless steel tube without any moving parts or obstructions. The heater/sensor assembly is arranged around the tube and, by following the anemometric principle, a constant difference in temperature (ΔT) is created and the energy required to maintain the ΔT is dependent on the mass flow rate. Due to the benefits of the unique patented sensor, the fluid will be warmed to a maximum of 5°C, thereby making the LIQUI-FLOW® Series suitable for fluids with low boiling points.