Within the PST group, we have a network of ISO17025 accredited laboratories for the calibration of humidity, temperature and dew point instrumentation. We are proud to say that we can offer some of the lowest measurement uncertainties of any commercial laboratory in northern Europe. The PST group has a global presence with multiple locations in Europe, Asia and America.
Calibration is the only way to prove instrument performance. By comparing an instrument to a known reference at several stable conditions we can assess what error exists. Over time we can assess the instruments drift and repeatability. Once errors are known they can be corrected for or adjusted out.
ISO17025 calibration is regularly audited to ensure compliance to strict standards. All ISO17025 laboratories have defined measurement uncertainties and regularly perform inter-comparisons with other laboratories to ensure performance.
Calibration is essential for compliance within many industries (FDA, GAMP, ISO 9001) and should be considered by anyone that wishes to make meaningful measurements.
When selecting the number of points to calibrate an instrument at, you must consider the relationship between the output of the sensor and what this represents in terms of a measurement. For example, in a platinum resistance thermometer, the relationship between resistance output of the sensor and what this relates to in terms of temperature is linear, meaning only a couple of calibration points would generally be required to get an idea of how the sensor responds to changes in temperature. However, for capacitive relative humidity sensors, the relationship between capacitance of the sensor and relative humidity is non-linear (graph 1), meaning calibration at more points are recommended – we would usually suggest at least three – one at each end of the measurement range, and one in the middle.
An instrument need not be calibrated over its entire operating range, only the range that it will be used at, because a change in temperature can influence the way a relative humidity sensor responds, it is recommended that %rh calibration is done at various temperature set points to cover the range that the sensor will be used at. For example, if a sensor is to be used to measure over a range of 10%rh to 90%rh at 10°C to 50°C, you might want to calibrate at 5%rh, 50%rh, & 95%rh, each at 5°C, 30°C, and 55°C.
Graph 1: The very slightly non-linear response of a capacitive relative humidity sensor means calibration is required at multiple points covering the working range to give the best idea of the characteristics of the sensor.
Improving precision is the key to mitigating risks and reducing costs. However, measurement uncertainty is a parameter that is often overlooked. It is an important aspect of measurement that can have both direct and indirect effects on product or service quality, costs, decisions, and risks.
Measurement uncertainty is a calculation that defines the range in which the “true value” is expected to lie. It is calculated by tabulating the known sources of error in terms of probability distribution. The result is typically displayed as a value at 95% confidence, meaning we are 95% sure that a measurement will fall inside the uncertainty. For example, if a calibration is done at 20%rh, with an uncertainty of ±0.6%rh, and 95% confidence, we can be 95% sure that the true value of the calibration is in the range of 19.4%rh to 20.6%rh.
Uncertainly is a far better quantification of the quality of a measurement compared to accuracy, uncertainty considers details and history of the specific instrument and how it is used.
The need for increased accuracy is not as important as the need for high quality measurements and a good knowledge of measurement uncertainty. If a highly accurate device is not used correctly, results can be meaningless.
At PST, we have a network of ISO17025 laboratories that can work together to deliver high quality instrument calibrations. To maximize our efficiency and keep lead times to a minimum any product returned to Michell UK for a UKAS relative humidity calibration may now be calibrated at Michell, Ely or Rotronic, Crawley, both laboratories offer the same standard uncertainties, and both are fully UKAS accredited and maintain full traceability to the National Physical Laboratory in Teddington, GB.
You can find out about all of our accredited laboratory locations on our dedicated calibration labs page