Calibration accuracy impacted by more than just the gas analyser

Accuracy variation in calibration gases increases the levels of uncertainty in gas measurements, which is why Signal Group manufactures gas handling and calibration equipment as well as gas analysers. Managing Director James Clements says: “The linearity check is especially important, so our Model 821S Gas Divider, for example, has been tested independently against our main competitor globally, and found to be significantly superior.”

The trial took place in North Carolina, USA, and found the slope accuracy for the Signal 821S to be just 0.25% for a NO/N2 gas mixture, whereas the competitor gas divider offered 1.6% slope accuracy. Similarly, for a SO2/N2 mixture Signal’s slope accuracy was 0.74% in comparison with the competitor’s 2.5%.

Explaining the importance of this result, James says: “Sources of uncertainty are incremental, so it makes no sense to introduce extra error unnecessarily, which is why the 821S Gas Divider is popular with both our customers and with other gas analyser manufacturers.”

The European standard EN14181 describes the quality assurance procedures for Automated Measurement Systems (AMS) installed to measure emissions to air. It says that an analyser’s linearity must be checked using five different reference concentrations, including zero. The reference concentrations should be approximately 20%, 40%, 60% and 80% of the range of two times the emission limit, and the test concentrations should be applied in a randomised sequence.

Gas dividers are normally used to create the different reference concentrations, however, it is of course also necessary to be able to validate the gas divider. To protect accuracy and reliability, the Signal Group Model 821S Gas Divider does not use mass flow controllers. A manifold block with ten identical capillaries and a precision pressure balance regulator ensure the high levels of accuracy demonstrated in the USA trial.

To check the accuracy of the 821S Gas Divider. users simply connect the zero and calibration gas to the opposite connections and repeat the test, and if the gas divider is operating correctly, the gas analyser will show exactly the same readings.

“Calibration gas bottles have a fixed ‘use-by’ date and can be costly to purchase and store, so gas dividers are ideal for conducting multi-point calibrations,” James explains. “However, purchased calibration gas itself incurs a level of uncertainty, so the 821S Gas Divider was designed to minimise increases in uncertainty, and I am grateful to the North Carolina researchers for highlighting this technical advantage.”