There are three main categories for thermal mass flow sensors:
This section of the PCT thermal mass flow range is devoted to the bypass principle. This is common for both the meters and controllers.
A laminar flow element (LFE) is placed within the full bore flow path of the device with two main purposes. The first is to simulate laminar flow conditions such that the measuring principle operates at a Reynolds Number of less than 2000 and is therefore representative of the true flow. The second is to create a slight differential pressure such that a defined proportion of the total flow will bypass through the sensor – a capillary tube that connects upstream and downstream either side of the LFE.
Around the outside of the capillary tube there are three small windings. The first is a temperature sensor (T1), the second acts as a heater element and the third is a second temperature sensor (T2).
When no gas is flowing the conduction of heat will make T1 and T2 equal. When flow occurs, heat is stripped from the first temperature sensor but added to the second sensor. This creates a temperature difference, or delta-T, between the two sensors and it is this mechanism that forms the heart of the sensor.
The two temperature sensors actually form two legs of a Wheatstone Bridge with the change in their temperature changing their electrical resistance. The amount of heat transferred is influenced by the thermal properties of the gas as well as the amount of gas molecules passing through the sensor. It is therefore important for the gas type, or the gas mix ratio, to be known so that the calibration process can take the thermal properties into account. The remaining variation in heat transfer, or electrical resistance, is therefore only proportional to an increasing number of gas molecules i.e. the mass flow rate.
Bypass thermal mass flow sensors are manufactured using 316L stainless steel and are therefore ideal for higher pressures or where the aggressive nature of the gas might corrode other materials.
The measurement technique is not adversely effected by vacuum and so is ideal for that industry.
Lastly, the inherent design of the devices, especially when including VCR process connections, are non-particulating and hence are perfect for the Semi-Conductor and allied Industries.
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