Ken Jefferies from Labfacility explains temperature calibration procedures and equipment, and how to choose the right technique, depending on the conflicting requirements for accuracy and ease of use.
Temperature calibration provides a means of quantifying uncertainties in temperature measurement in order to optimise sensor and/or system accuracies.
Uncertainties result from various factors including:
a) Sensor tolerances, which are usually specified according to published standards and manufacturers' specifications.
b) Instrumentation (measurement) inaccuracies, again specified in manufacturers' specifications.
c) Drift in the characteristics of the sensor due to temperature cycling and ageing.
d) Possible thermal effects resulting from the installation - for example, thermal voltages created at interconnection junctions.
A combination of such factors constitutes overall system uncertainty. Calibration procedures can be applied to sensors and instruments separately or in combination.
Calibration can be performed to approved recognised standards (national and international) or may simply 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 equipment or indirectly with traceable standards.
Thermal (temperature) calibration is achieved by elevating (or depressing) the temperature sensor to a known, controlled 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. This is a comparison process. If the sensor is connected to a measuring instrument, the 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 that are scaled for temperature or other parameters. An electrical signal, precisely generated to match that produced by the appropriate sensor at various temperatures, is applied to the instrument that is then calibrated accordingly. The sensor is effectively simulated by this means, which offers a very convenient method of checking or calibrating. A wide range of calibration simulators is available for this purpose; in many cases, the operator simply sets the desired temperature and the equivalent electrical signal is generated automatically without the need for computation. However, this 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, whilst both are held at a common, stable temperature. Alternatively, if a fixed-point cell is used, there is no comparison with a certified thermometer; fixed-point cells provide a highly accurate, known reference temperature, that of their phase conversion.
Equipment required for calibration
The equipment required to achieve thermal calibration of temperature probes is dependent on the desired accuracy and ease of use. The greater the required accuracy, the more demanding the procedure becomes and, of course, the greater the cost.
The required equipment generally falls into one of three groups:
- General-purpose system for testing industrial plant temperature sensors usually provide accuracies between 1.0 and 0.1degC using comparison techniques.
- A secondary standards system for high-quality comparison and fixed-point measurements provides accuracies generally between 0.1 and 0.01degC.
- A primary standards system uses the most advanced and precise equipment to provide accuracies greater than 0.001degC.
A typical general-purpose system comprises:
- A thermal reference (stable temperature source)
- A certified Pt100 reference probe complete with its certificate
- A precision electronic digital thermometer, bridge or DVM (digital voltmeter)
A convenient form of thermal reference is the dry block calibrator. Such units are available with various ranges spanning from -50 to +1200degC 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
Dry block calibrators provide the most convenient, portable 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. The block temperature is controlled electronically to the desired temperature. The whole assembly is housed in a freestanding case. Although the block temperature is accurately controlled, any indication provided should be used for guidance only. As with any comparison technique, a 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 that can be used for on-site calibration; and laboratory units to which industrial sensors are brought as required.
Alternative 'temperature' sources
Many laboratory furnaces and ovens are available that are specially designed for temperature calibrations. Precisely controlled, they 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 0degC, water from 0 to +80degC and oils for up to 300degC. Various molten salts and sand baths are used for temperatures in excess of 300degC.
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. Construction is such as to eliminate the possibility of element contamination, and various techniques are utilised to this end such as special sheath materials, gas filling and special coil suspension.
Precision temperature indicators are available in a wide variety of configurations and with alternative accuracy and resolution specifications. By definition, such instruments must be highly accurate and very stable. Normally the performance of the measuring instrument will be superior to that of the reference sensor to avoid compromising the system performance. As with any measuring system, such factors must be considered when specifying system components.
Fixed points are the most accurate devices available for defining a temperature scale. Fixed-point devices utilise totally pure materials enclosed in a sealed, inert environment; they are usually fragile and need to be handled with care. They work in conjunction with apparatus that 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. Since fixed-point temperatures are defined by physical laws, comparison of the test probe to a reference probe is not required.
Electrical calibration - simulators and sources
Indicators and controllers are calibrated by injecting signals that simulate thermocouples, resistance thermometers or thermistors. A simulator provides a very quick and convenient method for calibrating an instrument at many points. Very sophisticated and highly accurate laboratory instruments are available; conversely, compact and convenient portable units are available to permit on-site checking and calibration with a good level of accuracy.