Cover story I
n temperature calibration, the difference between interpolation and extrapolation is not merely technical. It is the difference between a measurement and a guess.
Interpolation works between known, verified calibration points. It is grounded in data, aligned with how temperature scales are realised, and defensible under scrutiny. Extrapolation extends beyond those points on the assumption that behaviour remains predictable where it has not been measured. Sensors, in practice, rarely make that assumption easy to justify.
INTERPOLATION, NOT ASSUMPTION
comparison point, but a single one. Between these lay a range where calibration was difficult to perform in any practical, routine way without moving to more complex and less portable equipment.
Interpolation works within known calibration points, where behaviour is measured and defensible. Extrapolation extends beyond that range, where behaviour is assumed and uncertainty increases. Platinum resistance thermometers show increasing non-linearity away from well-characterised regions. Thermocouples drift and suffer inhomogeneity under wide temperature excursions. At lower temperatures, physical effects within the sensor itself, gas behaviour, construction influences, self-heating, can all shift readings in ways that are not obvious until something goes wrong. Within a calibrated range, these behaviours are understood. Outside it, they are assumed. For many years, the lower end of the temperature range has been an area where assumption has filled the gap. Dry block calibrators have typically reached to around -90°C to -100°C. Liquid baths have operated practically to about -80°C. Below this, the next widely available reference has been liquid nitrogen at around -196°C, a useful
The Isotech Model 526 Ultra-Low Dry Block Calibrator addresses that gap directly. Extending to -115°C, it brings traceable calibration into a region that was previously difficult to reach with accessible equipment. The step may look modest on a scale spanning hundreds of degrees. In practice, it is significant. It allows users to establish calibration points where previously they had little choice but to estimate, and to build a traceability chain that no longer has to jump across an unverified region.
The importance of that becomes clear when you consider where critical processes actually operate. Ethylene has a boiling point of -103.7°C at 1 bar, a fixed physical value sitting directly in the gap between standard dry block capability and liquid nitrogen. Measurement at that temperature is not optional; it is where the process exists. Calibrating above it and extrapolating down is not a conservative approach, it is an unverified one. A similar case arises in cryopreservation and biobanking, where vitrification of biological materials depends on achieving and maintaining glass transition across a narrow window, typically -90°C to -110°C. Errors in this range are not recoverable. Regulatory frameworks increasingly require validated, traceable calibration at exactly
these temperatures, and a gap in the traceability chain there is difficult to defend. With the Model 526 in place, a calibration programme can now work upward from liquid nitrogen comparison at -196°C with genuine continuity, through the extended low-temperature range of the 526, and onward without the gaps that have historically required either assumption or highly specialised laboratory infrastructure.
And from there, the range extends further than most suppliers can follow.
Isotech equipment covers the full practical temperature calibration range in industry and the laboratory, from the deep cold of cryogenic comparison to the upper limits of industrial process measurement. The Model 547 provides stable, deep-immersion calibration for thermocouples to 1200°C. The portable Pegasus furnace reaches the same range, while the 877 Spherical Furnace extends capability to 1300°C, covering the highest temperatures encountered in industrial calibration. The Model 461 Simple Nitrogen Boiling Point Apparatus anchors the lower end at -196°C. Taken together, Isotech provides a continuous, traceable calibration capability from -196°C to 1300°C, a span that few single suppliers can match. That breadth matters, but not simply as a specification. It matters because good calibration practice depends on using the right equipment across the range, ensuring overlap between methods, and maintaining traceability at every step. A calibration programme with gaps, wherever those gaps fall, is one that relies on extrapolation to fill them. The consequence is uncertainty that cannot easily be quantified or defended.
The introduction of the Model 526 is, at one level, a practical product development. At another, it reflects a straightforward principle: that wherever measurement is possible, it should replace assumption. In temperature calibration, that commitment has a direct bearing on whether results can be trusted. Not approximately, not probably, but actually.
Isothermal Technology Limited
www.isotech.co.uk
8 April 2026 Instrumentation Monthly
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