Meter Calibration Required
A critical portion of any thermal mass flow meter is the factory calibration. The flow rate measured by the thermal flow meter is an inferred measurement. The instrument actually measures the power required to maintain the desired overheat temperature of the heated sensor above the reference sensor. This relationship between power and mass flow rate is established during the calibration. While considerable effort is made in the design and building the sensors, each sensor has slightly different heat transfer characteristics requiring that each instrument needs to be individually calibrated.
One of the first steps is calibrating the two platinum RTDs and then characterizing the performance of the RTDs at various temperatures over the specified operating range. Because the flow meter operates on the basic principle of heat and temperature, it is necessary to perform this temperature compensation step to insure that the performance of the flow meter is independent of the gas temperature.
Thermal Flow Meter Factory Calibration
Sage Metering has developed calibration capabilities that come as close as possible to reproducing the customer’s operating conditions. The calibration is performed using the actual gas or gas mixture. In a few cases it is not possible to flow the actual gas due to safety or hazardous conditions; in these cases the calibration is performed using a surrogate gas and modeling based on thermal properties is used to determine the performance of the actual gas.
The calibration process involves placing the sensor in one of several different test sections, flowing a known amount of gas through the pipe and then measuring the signal to obtain the desired overheat. This is done at minimum 12 times over the operating range. The actual flow rate is measured using high precision turbine flow meters with appropriate pressure and temperature correction. The uncertainty of the turbine meters is 0.5% of reading. For lower flow rates a Bell Prover with an uncertainty of 0.5% of reading is used.
Data Curve Fit
Once the thermal mass flow meter calibration is complete, the data is reviewed and analyzed. A fifth order polynomial equation is used to obtain a curve fit of the data which is loaded into the instrument. The zero flow data point is very important both for the actual calibration and also when performing the Sage In-Situ Calibration Verification. The zero flow data point is a function of both the type of gas and the operating pressure. Sage obtains a zero flow data point for the sensor in air at atmospheric pressure and also for the actual gas at operating pressure using our unique zeroing chamber.