Thermal Mass Flow (MFM)
Thermal Mass Flow Gas (MFM) meters utilize the gas’s ability to absorb and release heat. We distinguish between two main principles. Plugs or inline meters where the entire amount of gas passes the measuring element and the bypass meters where the measuring element only measures on a controlled partial flow.
Insert the meters
The insert gauges use two interconnected sensors where one measures temperature in the gas and the other which is heated measures the energy needed to keep the temperature difference between the sensors constant. Since it is the gas molecules which transmit the heat, the output signal will be directly proportional to the gas mass throughput. With the same sensor, speeds from 25 mm / sec to 75 m / sec can be measured. The measurement ranges are calibrated with current or equivalent gas at temperature and pressure according to specification. Thermal direct mass flow meters are independent of pressure and temperature changes within specified ranges.
The meters are used on most gases. If large pipe diameters are to be measured, the sensor is supplied with several sensors distributed over the measuring rod so that the average flow across the entire pipe cross section can be calculated. The multiple-target sensors also have advanced self-monitoring so that points that differ significantly can be excluded from the average calculation.
The in-line meters are a pipe piece with the same measuring element as insert the meters. The advantage of the inline meters is that they have a well-defined diameter on the tube around the measuring point. The measuring tube is also equipped with flow directors so that no skewed velocity profiles or rotation must affect the measurement result.
These are supplied from dimension 1/4 “…. 8” NPT or flanges.
The bypass meters are mounted inline. Internally in the meter, the gas distributes itself through a bypass element which provides a pressure drop so that a fixed partial current (0 … 10 Nml / min) passes through the sensor tube. On the inlet and outlet of the sensor tube there are two coils which, in addition to heating the sensor tube, detect the temperature distribution over the tube. When the gas flows through the pipe, the heat is transported to the outlet sensor. It is then this temperature difference between the outlet sensor and the inlet sensor which is directly proportional to the mass flow rate.
The Mass Flow Meter (MFM) can also be supplied as a Mass Flow Controller (MFC). It is then equipped with a PID regulator and a gas valve so that through-flow gas can be controlled from an analog signal 4 … 20 mA or 0 … 5 vdc.
Bypass MFM and MFC are used on most clean gases. Should pollution still occur inside the sensor tube, this can easily be plugged up with cleaning needle.