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It is important to distinguish between bypass and immersion flowmeters.
This type of flow meter is primarily intended for low to very low flow rates of clean gases. At the entrance of the meter an LFE (laminar flow element) is present, which ensures that laminar flow behavior occurs in the meter. This creates a constant ratio of gas through the sensor and bypass. The sensor consists of a tube containing two temperature-sensitive resistors that give off heat. A temperature difference occurs between the two resistors, which is proportional to the gas flow through the sensor.
An immersion flowmeter has two thermal sensors. These sensors are in the tube, where the gas runs through. One of the sensors is heated and the other one measures the temperature of the heated sensor. The flowing gas cools the heated sensor. The higher the flow, the stronger the cooling. Industrial thermal flow meters are available in two types:
With this type a constant amount of energy is supplied to the sensor, with which the sensor is heated up. At 0-flow or low gas speeds, the sensor becomes relatively warm. The advantage is that this technique is relatively simple.
Constant temperature anemometry:
In this type, the temperature difference between the reference and heated sensor is kept at a constant level. When the cooling of the flowing medium is higher, more energy is added. This technique is more complicated but has a higher accuracy and shorter response time.