Learn about the vital importance of gas sensor calibration in this article.
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Calibration is an essential part of any measurement. Compliance with most certification standards and ensuring the continued performance of industrial equipment relies on regular calibrations and the measurement information being suitably logged for full traceability.1
The performance of a sensor is usually evaluated by a number of different characteristics, including sensitivity, range/dynamic range, accuracy, linearity, repeatability, noise, resolution, bandwidth, response time, reach, the transfer function and any errors.
Calibration quality is an essential part of maintaining the accuracy of a sensor and evaluating its performance by comparing the measured reading to a known value of a reference standard. A calibration measurement helps minimize the uncertainty in a measurement as the ‘drift’ in the measured value versus ‘true’ value can be characterized.
Gas sensors are used in a number of different industries for quality control and also frequently as safety devices.2 For safety devices, calibration is a crucial part of any risk management and safety procedures, as inaccurate measurements could lead to fatal levels of toxic gases being underdetected and not triggering any related alarm systems.
Calibration Standards
Each measurement device will have its own specific calibration procedures and how frequently calibration measurements need to be performed will also depend on the sensor type. Handheld devices and sensors exposed to very harsh or variable environmental conditions may need more frequent calibrations as the changing conditions often result in larger amounts of drift in the measured values.3
Calibration measurements are normally performed by measuring a reference standard under similar conditions to the operational conditions. A reference standard is a material that is considered suitably well-characterized, and its properties can be considered as a comparator for an unknown compound.
However, there are different grades of reference materials and choosing an appropriate and suitable reference material for a calibration measurement can be a challenging task in itself.
All measurement values trace back to the definitions agreed upon by the International System of Units (SI) for values such as meters, kilograms, seconds and moles.4 There was a change in 2019 to the definitions of the base units5, but these remain the values to which all others are referenced. Sometimes, reference standards are thought of as a layered pyramid, and the SI values are what make the top of the pyramid represent the ‘true’ value.
A truly traceable primary calibration standard can be traced back to the SI standards, where each measurement in the chain of measurements has an evaluation of the measurement uncertainty. In practice, primary reference standards are evaluated by highly certified and specialized calibration and metrology laboratories, like the National Institute of Standards and Technology.
Standard or secondary references or lower grade reference standards are a lower rung down the pyramid from traceable primary calibration standards. A secondary calibration standard will be referenced to a primary calibration standard rather than directly to the SI values.
Other references, like working or field standards or in-process standards, are still useful but represent the bottom rungs of the reference materials pyramids.
The reason for still wanting ‘inferior’ reference standards is that primary standards are often a question of cost and convenience. Minimizing uncertainties often requires highly precise and accurate equipment and proper and comprehensive calibration is costly to perform, demanding stringent environmental conditions and different measurement checks.
The key consideration when deciding what level of reference standard is appropriate for the calibration of a gas sensor is knowing what level of accuracy is really needed and what kind of measurement ranges will be used. Safety applications, trace gas detection, or very stringent quality control measurements will all demand better-quality reference material.
Procedures
The type of reference material, as well as its grade, is also important for sensor calibration. The reference material is ideally as close in composition to the ‘real’ measurement as possible and ideally, a calibration will be carried out using a number of different conditions so the precision, accuracy and reproducibility can all be evaluated. While matrix effects are not as strongly perturbing for gas sensors, it is important if mixtures are to be measured that the calibrations also cover conditions involving gas blends.
There are a number of different companies and national laboratories that offer a comprehensive range of reference materials. Most gas sensors will be used to detect a gas concentration, so a number of gas standards are used at different concentrations to evaluate the accuracy of the measurements over the full measurement range. Ideally, the calibration concentration range will be greater than the expected ‘real’ measurement range.
The consequences of not calibrating gas sensors can be very serious. Most safety and quality assurance standards require routine calibration for compliance. Without regular calibration, it is also impossible to know if a sensor is performing correctly or not, and so if the sensor is responsible for some kind of process control system, inaccurate readings may mean significant amounts of production time and resources are wasted. Regular calibrations also make it easy to identify if maintenance of the sensor is required.
While performing high-quality calibration measurements is a complex topic with many details to consider, measurements without calibrations are essentially meaningless as there is no way to verify the accuracy or quality of the data captured.
References and Further Reading
Martins, A. B., Farinha, J. T., & Cardoso, A. M. (2020). Calibration and certification of industrial sensors – a global review. WSEAS Transactions on Systems and Control, 15, 394–416. https://doi.org/10.37394/23203.2020.15.41
Hunter, G. W., Akbar, S., Bhansali, S., Daniele, M., Erb, P. D., Johnson, K., … Vander Wal, R. L. (2020). A Critical Review of Solid State Gas Sensors. Journal of The Electrochemical Society, 167(3), 037570. https://doi.org/10.1149/1945-7111/ab729c
Johnston, J. D., Magnusson, B. M., Eggett, D., Mumford, K., Collingwood, S. C., & Bernhardt, S. A. (2014). Sensor drift and predicted calibration intervals of handheld temperature and relative humidity meters under residential field-use conditions. Journal of Environmental Health, 77(3), 22–28. https://www.jstor.org/stable/26330117
Moldover, M. R., & Ripple, D. C. (2003). Comment on “General principles for the definition of the base units in the SI.” Metrologia, 40(5). https://doi.org/10.1088/0026-1394/40/5/L01
Stock, M., Davis, R., De Mirandés, E., & Milton, M. J. T. (2019). The revision of the SI - The result of three decades of progress in metrology. Metrologia, 56, 022001. https://doi.org/10.1088/1681-7575/ab28a8
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