The critical portion of any thermal mass flow meter is the factory thermal mass flow meter calibration weather it may be a capillary-tube type or dispersion type. The flow rate measured by the thermal flow meter is an inferred measurement. As we already know from its operating principle that the power required maintaining this temperature differential is directly proportional to the mass flow rate. This relationship between power and mass flow rate is established during the calibration. Each sensor has slightly different heat transfer properties than that of another sensor. Hence, each instrument needs to be individually calibrated
Mostly the calibration of the thermal mass flow meter is performed using the actual gas. In a few cases, it is not possible to flow the actual gas due to safety or hazardous conditions or non-availability of that gas. In these cases, the calibration is performed using a substitute gas and it fluid correction factor loaded in the instrument with respect to actual gas thermal properties.
Measuring the signal after flowing known amount of gas through pipe to obtain the desired overheats using suitable flow measuring instrument by comparison method which has to be accredited to national standards. PCD calibrate its thermal mass flow meter using 5 points signal data throughout the range. The reason to take 5 point data is that the flow graph is nonlinear throughout the range in thermal flow principle. Once data is collected it is reviewed and analyzed. The collected data is loaded into the instrument. Hence, the flow meter is calibration and can be fixed on operating condition.
