Calibrating flow systems with changing gas mixtures

Many systems require flow calibration, verification, or validation to ensure processes are running properly. When flowing one type of pure gas or gas mixture, this is fairly straightforward. However it is a bit more challenging when flowing gas mixtures with variable compositions, such as in stack gas analysis where the relative concentrations of CO₂, O₂, and NO_x change from day to day. Here we discuss the challenges this presents and analyze common solutions.

Suboptimal solutions for changing mixtures

• The first solution is to calibrate a thermal flow meter for nitrogen or air, then apply K-factors to estimate the actual gas flow. The problem is K-factors are only valid at specific temperatures, pressures, and flow rates. Corrections therefore become increasingly inaccurate as conditions deviate from these precise conditions. Additionally, K-factors may not be available for every gas mixture.
• Another solution is using a bubble meter or piston prover to validate volumetric flow measurement. Given the volumetric flow, you can then convert to mass flow using a series of tables and calculations with viscosity and compressibility information of the gas mix at a set of temperature and pressure conditions. The downside of this method is the amount of time it takes to obtain the flow measurement and back-calculate the mass flow rates.
• A third option is to factory calibrate a mass flow meter to an estimated typical gas composition for all verifications. While this method may produce a reasonable average, results will be inaccurate when the actual gas mixture deviates from the estimation. Higher accuracy results are possible if calculating flows using the differential viscosity and compressibility of the actual gas mix (known from your analyzer) at the correct ambient pressure and temperature conditions.

Alicat COMPOSER™ to simplify gas mixtures

Alicat mass flow meters and controllers provide real-time flow readings for 98+ pure gases and gas mixtures, including up to 20 user defined gas mixtures. It is easy to create custom gas mixes using the backlit display or your choice of communication protocols, and switch between mixtures without the need for recalibration. Each mixture can include up to 5 constituent gases, with relative concentrations down to 0.01% of the total molar mix.

The device performs all correction factor calculations with no need for K-factors, measurement accuracy up to ±0.5% of reading or ±0.1% of full scale, no warm-up time, and fast response times down to 10 ms for measurement and 30 ms for control.