Accurate temperature calibration

Paul Boughton

Temperature calibration provides a means of quantifying uncertainties in temperature measurement in order to optimise sensor and/or system accuracies.

Uncertainties result from various factorsincluding: sensor toleranceswhich are usually specified according to published standards and manufacturers specifications; instrumentation (measurement) inaccuraciesagain specified in manufacturers specifications; drift in the characteristics of the sensor due to temperature cycling and ageing; possible thermal effects resulting from the installationfor example thermal voltages created at interconnection junctions.

A combination of such factors will constitute overall system uncertainty. Calibration procedures can be applied to sensors and instruments separately or in combination. They can be performed to recognised standards (national and international) or may constitute checking procedures on an ‘in-house’ basis.

Temperature calibration has many facets. It can be carried out thermally in the case of probes or electrically (simulated) in the case of instruments and it can be performed directly with certified equipmentor indirectly with traceable standards.

Thermal (temperature) calibration is achieved by elevating (or depressing) the temperature sensor to a knowncontrolled temperature and measuring the corresponding change in its associated electrical parameter (voltage or resistance).

The accurately measured parameter is compared with that of a certified reference probe; the absolute difference represents a calibration error. If the sensor is connected to a measuring instrumentthe sensor and instrument combination can be effectively calibrated by this technique.

Absolute temperatures are provided by fixed point apparatus and comparison measurements are not used in that case.

Electrical calibration is used for measuring and control instruments which are scaled for temperature or other parameters.

An electrical signalprecisely generated to match that produced by the appropriate sensor at various temperatures is applied to the instrument which is then calibrated accordingly. The sensor is effectively simulated by this means which offers a very convenient method of checking or calibration.

A wide range of calibration ‘simulators’ is available for this purpose; in many casesthe operator simply sets the desired temperature and the equivalent electrical signal is generated automatically without the need for computation. Howeverthis approach is not applicable to sensor calibration for which various thermal techniques are used.

Essentially the test probe reading is compared with that of a certified reference probe while both are held at a commonstable temperature. Alternativelyif a fixed point cell is usedthere is no comparison with a certified thermometer; fixed point cells provide a highly accurateknown reference temperature – that of their phase conversion. The equipment required to achieve thermal calibration of temperature probes is dependent on the desired accuracy and also ease of use. The required equipment generally falls into one of three groups:

  • General purpose system for testing industrial plant temperature sensors will usually provide accuracies between 1.0?C and 0.1?C using comparison techniques.

  • A secondary standards system for high quality comparison and fixed point measurements will provide accuracies generally between 0.1?C and 0.01?C.

  • A primary standards system uses the most advanced and precise equipment to provide accuracies greater than 0.001?C.

A typical general-purpose system comprises a thermal reference (stable temperature source)a certified PT100 reference probe complete with its certificate (Fig. 1)a precision electronic digital thermometer (Fig. 2)bridge or digital voltmeterand a convenient form of thermal reference is the dry block calibrator. Such units are available with various ranges spanning from -50?C to 1200?C and have wells to accept various test and reference probe diameters.

Alternative temperature sources for comparison techniques include precisely controlled ovens and furnaces and stirred liquid baths.

Dry block calibrators provide the most convenientportable facilities for checking industrial probes and they usually achieve reasonably rapid heating and cooling. The units consist of a specially designed heated block within which is located an insert having wells for the probes. Although the block temperature is accurately controlledany indication provided should be used for guidance only. As with any comparison techniquea certified sensor and indicator should be used to measure the block temperature and used as a reference for the test probe. Two types of unit are available; portable units which can be taken on to plant for on-site calibration and laboratory units to which industrial sensors are brought as required.

Many laboratory furnaces and ovens are available which are designed for temperature calibrations. Precisely controlledthey feature isothermal or defined thermal gradient environments for probes.

Stirred liquid baths provide superior thermal environments for probe immersion since no air gaps exist between the probe and medium.

Thermal coupling is therefore much better than the alternatives described and stirring results in very even heat distribution throughout the liquid. Alcohols are used for temperatures below 0?Cwater from 0?C to 80?C and oils for up to 300?C. Various molten salts and sand baths are used for temperatures in excess of 300?C.

A reference standard platinum resistance thermometer is a specially constructed assembly using a close tolerance Pt100 sensing resistor or a specially wound platinum element with a choice of Ro values.

Precision temperature indicators are available in a wide variety of configurations and with alternative accuracy and resolution specifications.

Fixed points are the most accurate devices for defining a temperature scale. These utilise totally pure materials enclosed in a sealedinert environment. They work in conjunction with apparatus which surrounds them and provides the operational conditions required for melting and freezing to obtain the reference plateaux. The housings incorporate isothermal blocks with wells into which the probes are placed.

Indicators and controllers are calibrated by injecting signals which simulate thermocouplesresistance thermometers or thermistors. A simulator provides a very quick and convenient method for calibrating an instrument at many points.

Ken Jefferies is with Labfacility LtdBognor


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