Welcome everyone to my first installment of what I hope to be a good educational discourse on the topic of thermal mass flow meters. Many of you in the process industry are already very familiar with flow metering technologies, such as orifice plate flow meters, vortex flow meters, and magnetic flow meters, but I wanted to focus this article on the particulars of a specialized technology known as thermal mass flow measurement.
Thermal Mass Flow Measurement
The principles of thermal mass flow measurement are relatively straightforward:
- A stream of gas passes a heated element and a resistance temperature device (RTD).
- An internal calculation is made on how much power is transferred away during the convective heat transfer process.
If I speak of the Sage Prime thermal mass meter specifically, it handles this measurement in the following detailed manner:
- A heated element and an RTD are implanted in the stem of the meter, which is exposed to the cross sectional gas flow in a circular pipe.
- As the gas travels across the heated sensor, it transfers heat away from the unit.
- This heat is then made up by the meter by the addition of more power (measured in mW) to the heated element.
Figure 1: Side view of the thermal mass meter stem.
The Sage Prime thermal mass meter measures and maintains a temperature delta of 20°F between the gas and the heated element. As the gas in question strips away heat due to convection, the meter compensates by increasing the output power of the heated element and thereby maintains a 20 °F difference between the meter and the gas. This power consumption has been precisely correlated to mass flow when calibrated to the specific gas in question (see power versus flow chart in Figure 2).
Figure 2: Power versus flow calibration curve for a thermal mass meter.
Why The Heating Element Matters
This calibration curve is a fourth order (sometimes fifth order) polynomial function and is specific to the gas that is being measured. Air and natural gas are two entirely different molecular structures and have different heat transfer properties. It is vital that the thermal mass meter be calibrated to the gas you are trying to measure or your results will be invalid.
So now that we understand the measurement principle, the next question I often get is whether this is a velocity meter or a mass meter. A Coriolis meter is well known for its mass measurement properties, while a magnetic flow meter is a very good example of a velocity meter. So where does the thermal mass meter fit in?
If you recall the magnetic flow meter principle, you have a liquid (let’s say water in this example) moving through a pipe at some flow rate. The magnetic pickups in the side of the meter determine the velocity of the liquid, which is multiplied by the cross-sectional area of the pipe to determine volumetric flow rate (feet/second x cross sectional area in feet squared yields cubic feet/second).
How thermal mass differs is first it measures the amount of power consumed by the interaction of molecules moving past the heated element. If you have fewer molecules travelling past the sensor, then the power compensation will be lower and the mass flow rate will register lower. The key is that the Sage Prime thermal mass meter determines the mass flow rate in Standard Cubic Feet per Minute (SCFM) versus Actual Cubic Feet per Minute (ACFM). Unlike liquids, gases are compressible and respond to changes in pressure and temperature. That is why the conversion from ACFM to SCFM is very commonly used when trying to compare gas flow rates of two different systems.
In this case, we use standard conditions for air (National Institute of Standards and Technology defines this as 20 °C and an absolute pressure of 101.325 kPa) and can determine its density by simply looking it up. Take this number (in pounds/cubic feet) and multiply it times the standard cubic feet per minute reading of the meter and you now have the mass flow rate of your gas stream in pounds/minute (or pounds/hour if you prefer).
Thus, the thermal mass meter derives the mass flow rate based, not on velocity times a cross-sectional area, but by the heat transfer properties of the gas in question and its corresponding density and power curve.
In my next installment, I will cover the most common applications for the thermal mass flow measurement and lend some advice on which applications to avoid with this technology. Thanks for reading!
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