Accurate mass flow measurement of gas is difficult to obtain. The main reason is that gas is a compressible fluid. This means that the volume of a fixed mass of gas depends upon the pressure and temperature it is subject to.Consider a balloon containing one actual cubic foot of gas at room temperature (70° F) and atmospheric pressure. An increase in the room temperature causes the balloon to expand. An increase in the pressure surrounding the balloonresults in a decrease in volume. Although the volume of the balloon changes with variations in pressure and temperature, the mass of the gas inside the balloon has remained the same. This illustrates how pressure and temperatureaffect the actual volume.There are many well established methods of measuring the actual volumetricflow rate. However, the measured flow rate will vary with changes in temperatureand pressure. For virtually all industrial process operations, the user wantsto measure the mass flow rate instead of the actual flow rate. Chemical reactionswork on the basis of mass relationships of ingredients. Combustion isbased upon the mass flow rate of the air and the fuel. Gas consumption in afacility is based upon mass flow rate. To accurately measure mass flow, theactual flow rate must be adjusted to correct for any change in temperatureand pressure.Thermal mass flow technology is a method of gas flow measurement that doesnot require correction for changes in process temperature or pressure. Thermalmass flow technology also has a benefit of measurement at low velocities andgreater turndown capabilities than those obtainable with other gas flow measurement devices.
MAGNETROL (2012) Thermal Mass Flow Handbook