How it works: High-accuracy multi-gas calibrations (Gas Select)

How it works: High-accuracy multi-gas calibrations (Gas Select)

Alicat mass flow meters and controllers can measure or control over 98 gases with NIST-traceable accuracy. In this article, we discuss how an Alicat maintains high accuracy measurements across this wide range of gases.

Alicat devices are pre-loaded with 3-dimensional gas properties data

Alicat mass flow instruments report pressure and temperature readings a thousand times every second. These readings correlate with specific data points within pre-loaded gas tables containing values from 3-dimensional gas properties data. This 3D surface charts NIST-traceable gas properties data for compressibility and viscosity against pressure and temperature across the entire usable range of the Alicat mass flow device.

As an example, imagine your device is set to flow argon. Whether flowing at 2 bar and 23°C or at 6 bar and 37°C, argon can be flowed with accuracy to ±0.6% of reading or ±0.1% of full scale.

Switching gases

Alicat's Gas Select menu

Alicat’s Gas Select menu

Each device contains pre-loaded gas tables for 98+ gases. If you want to use the device above to flow air instead of argon, you can simply select air in the Gas Select menu and it will change the refenced gas properties table.

As long as the correct gas is selected, a single device can be used to measure either argon, air, or one of the other 96+ gases across a wide range of temperatures and pressures – with no change in accuracy.

NIST-traceability

An independent calibration house used an air-calibrated Alicat device to measure a NIST-traceable 3-gas mixture of 12% CO2 + 7% O2 + 81% N2.

The total measurement error was just 0.15% of the reading at full scale, a value well within the published accuracy specifications for the mass flow meter.

Thermal devices and K-factors
Thermal mass flow controllers are also capable of flowing multiple gases, but with variable accuracy. Imagine you have a thermal flow controller calibrated to flow argon. To measure the flow of air, you must find air in the manufacturer’s table of K-factors and multiply this conversion factor against your argon-calibrated flow reading.

K-factors are accurate for specific temperature and pressure conditions. This means operating conditions deviating from those defined in the K-factor chart will result in lower accuracy flow corrections. For this reason, many manufacturers of thermal mass flow instruments provide much less stringent accuracy specifications when using K-factors.

Conclusion

With Alicat mass flow instruments, you can change between 98+ gases without any loss of measurement or control accuracy.

Discuss your process with an applications engineer