THERMAL MASS FLOW SENSOR FOR GASES

Bypass principle

Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments.

Figure A

Mass flow sensor for gases, bypass principle

In thermal instruments based on the bypass principle, only part of the gas stream flows through the sensor (as shown in figure A). The gas stream through the sensor is warmed up by two heaters (RHT1 and RHT2). Accordingly, the temperature of the tube is measured at two points (T1 and T2).

With no flow measured, the temperature differential between the two points will be zero. When the flow increases, the temperature at the first measuring point (T1) will decrease, as fluid carries away the heat (as shown in figure B). At the same time the temperature at the second measuring point (T2) will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is directly proportional to the 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 bypass to the main channel, where a flow resistance splitter takes care of proportional flow division.

The previously patented laminar flow element (flow splitter) of Bronkhorst consists of a stack of stainless steel discs with high-precision etched flow channels, having similar characteristics as the flow sensor. For higher flow rates, the discs are equipped with an additional section to filter out turbulence effects. The perfect flow division of Bronkhorst flow splitters ensures stable and calculable flow proportioning, even under varying process conditions.

Figure B

Low pressure drop applications

As a variant of the bypass principle described above, Bronkhorst designed a special construction for low pressure drop applications - the LOW-dP-FLOW series - the sensor only requires approx. 0,25...2,5 mbar. Furthermore, the larger flow channels minimize the riscs of clogging and facilitate cleaning and purging. Therefore this construction should be preferred for corrosive gas applications.