Why Choose Thermometer Calibration Points?
Calibration points can be drawn on a graph and joined together to form a calibration curve. This therefore allows the user to estimate the corrections at points where a calibration value has not been measured. More calibration points will reduce estimation errors, but each calibration point chosen will incur a cost, so the number chosen will be a trade-off between accuracy and cost.
How many calibration points?
The number of calibration points a customer will require will depend upon the application. If the thermometer is intended for general purpose use across the whole range, then at least three points should be chosen in order to arrive at a calibration curve from which interim points can be interpolated.
If the thermometer is required to keep track of a single temperature in a process, then a single point can be calibrated. It is recommended however that even if only a single point is required, that two other points be checked in order to know the variation around the point and allow interpolation.
Which points to choose?
Calibration points should either be specific to your process, such as -18°C, 0°C and 4°C for fridges and freezers, or cover around 80% of the range of your thermometer scale. A -10°C to 110°C thermometer can easily be calibrated at 0°C, 50°C and 100°C, but the top and bottom extremes of the scale are best avoided as the accuracy can wane in these areas.
It is always best to specify the temperatures you require if at all possible.
UKAS accreditation requires that an estimate of the maximum errors likely in a calibration value are displayed on the certificate. UKAS calibrations can be carried out on any of the liquid in glass thermometers we produce, but it should be considered that as the accuracy and precision of the thermometer decreases, the uncertainty of the measurement stated on the certificate will increase. Customers should therefore consider this when selecting a thermometer and we would recommend the use of a specification thermometer where low uncertainties are required.
We are often asked the question ‘When should I recalibrate my thermometer’? This can only really be answered by the user, but generally it will depend upon several factors:
What is the consequence of an incorrect reading? If you waste a batch of wine due to an incorrect temperature reading, the consequences are probably far less than if you are performing a scientific experiment to formulate a new drug. Often, the more precise the scale, the more susceptible the instrument to wandering out of calibration.
How likely is the device to go out of calibration? How often is it used and are the conditions is it used in likely to cause it to become damaged.
Has the device been subjected to any misuse or obvious damage? If the thermometer is used out on site it is perhaps more likely to become damaged or knocked than if were kept in a cupboard in the laboratory between use.
Glass is a visco-elastic material and will creep over time. New thermometers can undergo ‘secular change’, which is due to relaxation of stresses in the glass set up when it was made. For this reason it is important to check new thermometers more regularly than older ones.
Always keep your old calibration certificates to compare with the new ones and check for drift in the performance of your thermometer. If the thermometer is stable between calibrations you could consider increasing the calibration interval, conversely if it shows big changes then your instrument needs to be checked more frequently.