Flow calibration with changing gas mixtures
Flow calibration, verification, and validation processes are often similar, despite their differing objectives. However, an Alicat mass flow meter or controller’s ability to provide real-time flow readings and switch between different calibration gases at the touch of a button (and without requiring recalibration) makes each of these processes much faster and easier.
Alicat mass flow instruments ship with either 98 or 130 preloaded pure and mixed gases, and users can define their own mixtures. This simplifies flow calibration tasks which involve multiple gases.
The challenge of changing mixtures
What happens when your mixed gas composition changes? Let’s take the example of analyzing stack gases for the EPA. The relative concentrations of CO2, O2, and NOx in the stack gas change from day to day. The analyzer you’re using detects today’s composition — but your flow meter does not. In order to validate the flow rates of the gas being analyzed, you have some unsatisfactory options:
1. Factory calibrate your mass flow meter to an estimated typical gas composition to use for all verifications. Although this method may produce a reasonable average, it will produce inaccurate results most of the time – whenever your actual gas mix is not identical to your estimated gas mix. However, you could achieve higher accuracy using this method by performing calculations based on the differential viscosities and compressibility of the actual gas mix (known from your analyzer) at the ambient pressure and temperature at which you’re performing the validation..
2. Calibrate your mass flow meter for nitrogen or air, then apply K-factors to estimate the actual gas flow. This method introduces inaccuracies through the K-factors, which are only valid at specific temperatures, pressures and flow rates. Outside of these precise conditions, K-factor inaccuracy increases, particularly when you do not have access to the K-factors for your specific gas mixture.
3. Use a bubble meter or piston prover to validate your volumetric flow measurement. Given the volumetric flow, you can then convert to mass flow using a series of tables and calculations which allow you to derive the overall viscosity and compressibility of the gas mix at a set of temperature and pressure conditions. However, this process is time consuming, both because of the volumetric flow measurement itself and due to the time required to back-calculate the mass flow rates.
Alicat COMPOSER to simplify gas mixtures
Alicat’s COMPOSER™ gas composition builder provides a faster and easier method to validate processes with gas mixtures. COMPOSER is included on all Alicat mass flow meters and controllers and lets you create custom gas mixes directly on the device (or via your communication protocol of choice).
Creating a new gas mix is simple: select a constituent from the onboard Gas Select menu and define its relative concentration to 0.01% of the total molar mix. Each mix can include up to 5 constituent gases, and the Alicat device can store up to 20 mixes at a time.
When you receive the day’s gas composition from your analyzer, you can choose the mix that matches the current composition, or create the appropriate mix, without ever requiring a computer.
This maximizes the accuracy of your flow calibration by taking into account the specific gas properties, with all calculations performed onboard.