There is an old saying about a man with two watches who never knows what time it is. One wonders if this applies to having two flow meters?
Fig. 1 shows the comparison of two thermal mass flow meters measuring the same compressed air stream. We can see there is a disparity between the readings, one showing much lower than the other.
This picture was taken during a transition between an old and new flow meter. The upper unit had serviced the plant for 15 years and had become inaccurate, due to corrosion and lubricant contamination. The new unit is a hot tap design that is easily removed for cleaning without having to take down the plant.
The replacement was prompted by an air audit — this audit discovered that two 1,000-cfm rated compressors appeared to be only producing 1,500 cfm at full load. Something wasn’t right. So, after much testing and some expensive compressor servicing, the “problem” turned out to the flow meter, not the compressors!
It is always best to ensure that the flow meters you used are tested and calibrated regularly, to keep your system running correctly and to save money from potential future problems.
