Now, what happens when your mixed gas composition changes? Let’s say you’re analyzing stack gases for the EPA, and the relative concentrations of CO2, O2 and NOx in the stack gas changes from day to day. Your analyzer knows what today’s composition is, but your flow meter does not. When it comes time to validate the flow rates of the gas being analyzed, you have some unsatisfactory options:
Calibrate your mass flow meter at the factory to an estimated typical gas composition for all verifications. This will produce inaccurate results most of the time, except when your actual mix happens to reflect the ideal composition used as the basis of verification. Achieving higher accuracy by this method requires complex calculations of the differential viscosities and compressibilities of the actual gas mix at the actual validation pressure and temperature.
Calibrate your mass flow meter for nitrogen or air as a default, and then apply K-factors to estimate the actual gas flow. This method again introduces inaccuracies through the K-factors, which are valid for certain temperatures, pressures and flow rates. Outside of this sweet spot, K-factors inaccuracy increases. K-factors that do not match your specific gas mixture introduce even greater inaccuracy.
Conduct validation with volumetric flow measurement using a bubble meter or piston prover. Conversion to mass flow then requires the use of complex tables and calculations that derive total gas mix viscosity and compressibility for the temperature and pressure conditions used for validation. This process can take a long time for the volumetric flow measurement itself, and then much greater time for the back-calculation of mass flow rates.
Keywords: Calibration Gases
