Calibrate your temperature sensors on-site

A Hawco product story

Edited by the Engineeringtalk editorial team Jul 11, 2001

Companies with ISO 9000 certification must have a routine process of calibration, and the the Ametek-Jofra ATC portable reference ovens enable on-site calibrations

It's one thing to design the optimum temperature control system for a process, where the control components and tuning algorithms match the dynamics of the physical process, but it's a totally different discipline to constantly monitor it and check that it's performance is up to scratch.

In any heat process, e.g heat treatment furnaces, curing ovens, autoclaves, plastics processing, chemical, pharmaceutical, food and drink processing etc., one of the factors that significantly affect the end product quality is temperature drift.

All temperature sensors drift, or rather their output does, all electronic components in a temperature transmitter or controller have physical values that drift.

The net result quite often results in an unnoticed drift, over a period of time, of the actual process temperature whereas the indicated or measured temperature readout may appear to be well within normal limits.

This drift will have an effect on end product quality and consistency as well as having energy cost implications, so the extent of the change must be measured regularly so that plant parameters can be set up to compensate.

Money-saving tip: you don't need to scrap a perfectly good stable sensor loop just because its output has drifted a bit! What you can do instead is calibrate it! Routine, on-site calibration of the temperature sensor and its associated electronics in their normal operating ambient environmental conditions is easy with the advent of portable dry-block temperature calibrators.

Furthermore process downtime is kept to a minimum by carrying out the calibration in field.

These lightweight instruments have superceded the oil bath, sand bath and ice-point laboratory calibrators.

Companies with or seeking to obtain ISO 9000 certification must have a routine process of calibration laid down to ensure that instrumentation for controlling and measuring process temperatures are periodically calibrated to a known standard and to include all uncertainties to ensure optimum consistency of product quality.

These portable reference ovens simulate process temperatures from sub-zero (50C below ambient) up to 1200C with accuracy up to 0.04C and stability better than 0.02C.

For instance the Ametek-Jofra ATC range with built-in reference sensor enables on-site calibrations traceable to national standards and in accordance with the European Accreditation (EA) guidelines.

Other, more utilitarian instruments exist for less demanding calibration specs, together with specialist units for deep thermowells or special diameter sensors.

Software is another feature of modern dry-block calibrators; most have basic ramp-rate and dwell functions, but more recent models/versions such as the Jofra ATC-B Series can cycle the sensor plus its associated loop instrumentation through a number of set-points in its normal operating range, log the results, keep a permanent record of correction factors needed and evolving trends for individual instruments or for groups of tag numbers.

These advanced features can make the necessary task of keeping tabs on 'real' process temperatures a lot less time consuming.