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Thermal flow sensors are particularly well suited for the smallest measured variables in gases and liquids. The sensors consist of a heater and two temperature sensors, just a few micrometers in size.
The temperature sensors are designed as thermopiles with a sensitivity of approx. 3.6 mV/K. These elements are applied on a silicon-oxide / nitride membrane with a thickness of 200 nm, and they are protected against media influences with thin layers of silicon-oxide and silicon-nitride.
This chip is either embedded in the wall of a flow channel, or it is positioned in a so-called chamber head as a bypass solution. The minimum cross section of the channel is 600 μm x 200 μm and is appropriately adapted to the required flow rate. The flow is coupled-in via off-the-shelf connections. The maximum overpressure in the channel can be approx. 2-3 bar depending on the medium.
Due to the high sensitivity of the thermopiles, measurements can be taken with an extremely low temperature increase of the flow of a few kelvin. Through this measure the load of the medium is reduced.
The arrangement on a thin membrane generates an excellent thermal insulation and a low thermal capacity of the sensors. Thus minimal heat capacities in the micro-watt range and short response times are possible.
Also the heat-up time of the sensor after switch-on is a few milliseconds. This means that a further reduction of the mean heating capacity is possible in interval operation, thus battery operation is also feasible. Through the short response times the sensor is also outstandingly suited for dosing systems.