Basics of thermal mass flow sensors

Heat transfer between a body and the surrounding medium is dependant on the flow. This physical law is the basic of thermal mass flow sensors. The higher the flow rate the more a body is cooled down.

1. Principle

While a temperature variable resistor (RTD) is heated by an electrical current, the temperature of the heated element (RTD) can be measured. Raising the temperature results in higher resistance, consequently also the cooling of the heated element can be measured. Further more to get an extensively temperature independent measurement which allows for medium temperature and other varying mass properties, an additional temperature variable resistor is structured on the chip. This resistor measures a proportional signal to the ambient respectively to the medium temperature and is used for compensation.

See FS5 (PDF 75,0 kB) / FS5L (PDF 79,0 kB)

2. Principle

Two temperature depending resistors are symmetrical arranged, which are indirectly heated by heat transmission with a heated element in between. If there is no flow, both of the elements are heated equal. Presence of a flow results in a temperature difference of the two elements beside the heater, which is a measure for the flow direction and the mass flow. The measuring element in front of the heater in flow direction is cooled down more than the other. Therefore the heat transmission from heater to the first element is restricted, the transmission to the second element is supported by the flow.

See FS2 (PDF 81,0 kB)