Thermal mass flow meters are excellent devices to use when measuring compressed air flow. These devices use a hot wire anemometer method of measuring the velocity of the compressed air in the pipe.
Two probes are used, one heated and the other not, and are inserted into the compressed air pipe. The flow of air past the probes cools the heated probe, with the power applied to the heater regulated to keep a constant temperature differential. The measurement automatically adjusts for pressure, the mass flow of air going past the probe is directly proportional to the power applied to the heated probe . And once the internal piping diameter is programmed in, the output of the meter typically yields volume flow like standard cubic feet per minute. Thermal mass meters are the most commonly used style for dry compressed air measurement.
From time to time, there is trouble with thermal mass meters, especially when water inadvertently contacts the probes. This causes the meters to read full scale due to the cooling effect of the water. But there can be other issues too — such as lubricant contamination that inhibits the flow of heat from the probe. Fig. 1 shows what was found when a thermal mass meter installed 10 years ago was applied to a system operating in extreme conditions. The meter was reading about half the actual flow due to the coating of sludge on the probes.
Typically, these meters can be cleaned or sent back to the factory for recalibration. If you have meters like this, it is good to check them regularly to ensure they remain clean and accurate.