Calibration and Achievable Accuracy of Gas Sensors
Summary
The measured gas concentration is only as accurate as the gas sensor can be calibrated. The traceability of the measurement signal of each gas sensor to an accredited standard is ensured during calibration using certified calibration gases.
Often the absolut accuracy of available calibration gases is the limiting factor for the achievable accuracy of a gassensor, together with the sum of all uncertainties of meteorological measurement parameters (pressure, temperature, humidity).
Calibration process
In series production, each sensor is calibrated at least to the zero point and the full-scale range (FSR) using certified calibration gase mixtures. The accuracy of the available calibration gas mixtures is typically 1 – 2 %v. For this reason, a measuring accuracy of 1 – 2 %v of the value at full scale is also normally stated in the data sheet for gosense gas sensors.
For additional calibration points or for very accurate galibration gas mixtures, we are producing calibration gas mixtures in our gas laboratory using volumetric methods in accordance with DIN 51898–2. With this method a mixing accuracy of 0.5 – 1.0 %v of the original calibration gas is achievable.
These measurements are repeated at various temperature and pressure points of the gas.
Calibration Gas Mixtures
The basis of any calibration procedure and its accuracy is the specific calibration gas mixture used. Upon request, this can be certified by METAS [4] and/or traced back to a standard using a manufacturer’s certificate of analysis. Manufacturers can produce calibration gas mixtures with an accuracy of 0.1 to 10 % of the gas concentration, depending on the gas and concentration. The exact concentration is stated in a measurement certificate with an analytical accuracy (test gases Class 1) of ≥ 0.5 % to 2 % [2].
Calibration of the Linearization Curve
The optical infrared absorption of NDIR gas sensors does not behave linearly with respect to gas concentration. Ideally, it follows the Lamber-Beer Law of absorption, yielding in a logarithmic relation between volume concentration and optical absorption. In practice, it is not logarithmic but something else.
For this reason, a calibration curve is measured for each sensor during production. This is defined at several gas concentrations between the zero point and the FSR point and is typically mixed with the FSR gas mixture using a gas mixing method during the measurement. fastLine sensors are calibrated using the volumetric method specified in DIN 51898–2 [3], which allows mixtures of two gases to be produced with a mixing accuracy of 0.5 to 1.0 %v.
The calclation from the non-linear absorption signal to a linear volume conenctration we call „linearization“.
Correction for pressure and temperature
Infrared absorption measurements of gases depend directly on temperature T and pressure P. These variations with P and T can be calculated with physical accuracy and are compensated for by the gas sensor. During calibration, temperature cycles are also used to measure and correct the systematic drift of the sensor components.
Our meteorologica measurements, such as pressure, are traceable to the Swiss national standards held by METAS.
References
[1] DIN 55350–13: „Concepts in quality and statistics; concepts relating to the accuracy of methods of determination and of results of determination“
[2] ISO 5725–1 : „Accuracy (trueness and precision) of measurement methods and results – Part 1: General principles and definitions.“
[3] „Limit of detection A closer look at the IUPAC definition“, Anal. Chem. 1983, 55, 7, 712A–724A, http://dx.doi.org/10.1021/ac00258a001