Flow Calibration with Changing Gas Mixtures

Multiple unit calibration setup, with three-meter portable calibration unit

Multiple unit calibration setup, including the three-meter portable calibration unit (PCU)

Flow calibration, verification and validation processes can look rather similar but in fact have differing objectives, as we outlined in a previous article. The ability of Alicat mass flow meters and mass flow controllers to provide instant flow readings and switch to different calibration gases at the touch of a button makes these processes much faster and easier. Most of our mass flow instruments now ship with up to 130 preloaded pure and mixed gas calibrations to simplify flow calibration tasks involving multiple gases.

Gas mixing flow validation setup, including mass flow meter equipped with COMPOSER

Gas mixing flow validation setup, including mass flow meter equipped with COMPOSER™

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:

  1. 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.
  2. 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.
  3. 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.
Gas Select COMPOSER gas mix composition firmware, shown on optional color display

Gas Select COMPOSER gas mix composition firmware, shown on optional color display

Alicat’s COMPOSER™ gas composition builder makes possible a faster and easier method. Included on most new Alicat mass flow meters and controllers at no additional cost, COMPOSER™ allows you to create your own custom gas mixture calibration right on the instrument. Simply select a gas constituent from the onboard Gas Select menu, and define its relative concentration to 0.01% of the total molar mix. You can do this for up to 5 constituent gases, and you can create and store 20 COMPOSER™ mixes on the instrument at a time. When it is time for flow validation simply select the mix that matches your current gas composition. If no mix matches, you can create another one in just 2 minutes or less, without ever needing a computer. Take a look at the video below to see what COMPOSER™ can do for you!

Gas Select COMPOSER™ – Accurate Mixed Gas Flow Measurement

When your gas composition changes, will your flow readings still be accurate? Alicat Scientific presents COMPOSER™, the world’s first on-the-fly mixed gas composition builder, for mass flow meters and mass flow controllers.

 

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