This is part 1 of a 2 part blog on Thermal Mass Flow Sensor. We will describe the desired characteristics of a thermal mass sensor in Part 1 and Part 2 will discuss the operation of the 300 Series flow sensor (Patent #6,125,695) from Teledyne Hastings.
A thermal mass flow meter consists of the following:
Electronic Circuit Card
A cutaway is shown in the image on the right.
In a typical thermal mass meter, gas enters the flow meter via the upstream port which is attached to the process with a fitting (VCR, Swagelok…). Most of the gas will move through the bypass shunt; however, a certain fraction will flow through the thermal mass flow sensor. Note that the shunt is selected such that amount of gas moving through the flow sensor is approximately the same at full scale flow. The gas then exits the flow meter via the downstream port.
Ideally, the thermal mass flow sensor would exhibit the following characteristics: first, it would be linear. What we mean by linear is that the sensor’s electronic output should be directly proportional to the flow rate moving through the sensor throughout its range. Linearity of the flow sensor leads to the second desired characteristic: accuracy. An accurate flow sensor can give the users the benefit of better gas flow measurement, control, and understanding of their system parameters.
Before we move on with our desired characteristic list, we need to discuss a little about how linearity can factor into calibration. Typically, a thermal mass flow meter is calibrated in nitrogen (or in the case of very large flows, it may be calibrated in air). The output of the flow meter can then be scaled for use in other process gases. (In other words, the flow meter technician can calibrate a flow meter for use in a corrosive process gas like silane (SiH4) – without having to use silane). A linear flow sensor will retain its linear behavior as the gas is switched from the calibration gas (N2) to the process gas.
Our next desired characteristic is fast response. Ideally, the flow sensor would respond instantaneously to a change in the flow rate. Aside from the obvious benefit of instant real-time vision of the flow in a process, fast response becomes critical when the flow meter is coupled with a proportional control valve to create a thermal mass flow controller. Finally, we would like the thermal mass flow sensor to have a low pressure drop. A low differential pressure drop across the flow meter is ideal for leak detection and gas sampling applications.
Teledyne Hastings' Thermal Mass Flow Sensors are used worldwide. For more information on on Best Practices for Mass Flow Controllers and Mass Flow Meters download our whitepaper.
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In our next blog, we will describe the sensor that is core to the Teledyne 300 Series of mass flow meters. We will also look at how the 300 Series thermal mass flow sensor addresses each of the desired characteristics described above.