Calibration Very often, it has not.
Platinum resistance thermometers can provide highly accurate measurements—but only when adequately immersed. Insufficient immersion leads to stem conduction, where heat travels along the sensor sheath, pulling the measured temperature away from the true value of the medium. Surface probes are notoriously unreliable. Measuring true surface temperature is inherently difficult due to the complex thermal interaction between the sensor, the surface, and the surrounding environment. Accurate temperature measurement doesn’t just happen. It relies on careful attention to immersion depth, contact quality, and thermal stability. And we must always remind ourselves—the sensor is measuring its own temperature, not necessarily the temperature we think it is.
WHEN TO CALIBRATE, WHEN TO REPLACE In many industrial environments, especially where low-cost thermocouples are used, it may even be better not to calibrate.
Thermocouples are prone to drift, but more critically, they suffer from inhomogeneity; this is where different sections of the wire behave differently due to age, stress, or thermal factors. Calibrating one section doesn’t guarantee that the rest of the wire will perform similarly. For common thermocouples like Type K or Type T, it’s often more effective to replace rather than calibrate. The cost of calibration can exceed the cost of a new sensor, and the confidence gained can be limited at best.
procedures validated? Were the staff trained? Were the environmental conditions controlled? An accredited calibration, such as one performed under UKAS to ISO/IEC 17025, provides confidence for all these factors. It confirms that the lab is technically competent, that its results are traceable, and that the uncertainties are properly stated and independently assessed. In short, it provides confidence that the measurement can be defended. If you were ever required to defend a measurement in court—or under regulatory scrutiny—you would want the assurance that comes from following best practice. That is where UKAS accreditation matters. It not only confirms traceability, but also that best practice has been followed. A traceable certificate might look adequate on paper, but it is the accredited certificate that gives you confidence—and lets you sleep at night.
TEMPERATURE IS NOT ALWAYS WHAT YOU THINK IT IS
It is easy to assume that a thermometer tells you the temperature of the object you are measuring. But what it tells you is the temperature of its own sensing element, and this will only match the real value if thermal equilibrium has been achieved.
Instrumentation Monthly September 2025
ON-SITE VS LABORATORY CALIBRATION There is no single right answer to the question of where calibration should be performed. Each option has its place.
Laboratory calibration—especially under accreditation—offers the lowest uncertainties and most controlled conditions. It’s essential for reference thermometers and high accuracy applications. On-site calibration, however, offers its own advantages. It can be quicker, reduces handling risks, avoids the need to ship sensors off-site, and allows checks to be made in the instrument’s actual working environment. That said, field temperature calibration is often more challenging than calibration in the
laboratory, with larger baths having deeper immersion and superior temperature uniformity.
Both on-site and laboratory calibration have their place. There is no single best option, it really depends on the application and the level of uncertainty required.
CHOOSING THE RIGHT EQUIPMENT Temperature calibration equipment (and required budget) varies widely, from fixed-point cells and precision bridges to dry blocks and handheld testers. The challenge is to select equipment to suit your application, not necessarily the most expensive, or having the widest temperature range, or ‘on paper’ performance; factors to consider:
What accuracy is genuinely needed?
How many sensors to calibrate at a time? Is the equipment big enough? Will there be loading errors?
What are the lengths of the sensors? Can they be adequately immersed?
What about the clearance around the sensors to avoid error from air gaps?
How to provide traceability, is UKAS (ISO 17025) required?
It is about choosing the right equipment for the particular application and taking into account immersion depth, temperature uniformity, and avoiding the trap of being misled by display resolution. Just because a calibrator shows a value to three decimal places does not mean that level of accuracy or uncertainty can be achieved in practice. What matters is not what the display shows, but what can be reliably measured.
IN THE END, IT IS ABOUT CONFIDENCE Calibration is not just an exercise. It is about confidence, confidence that your measurements are meaningful, your processes are under control, and your data will stand up if questioned.
That confidence comes from understanding the measurement process, choosing the right equipment and the correct methods and procedures. No matter what the area, getting ‘temperature right’ remains fundamental, and it’s rarely as simple as you might think.
If you are unsure or have doubts about the best calibration approach for your application, seek advice. With temperature calibration, small details often make a big difference. Speaking with someone experienced in measurement and uncertainty can help you avoid costly errors and ensure confidence in your results.
Isothermal Technology Ltd, (Isotech)
www.isotech.co.uk
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