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
a) Sensor tolerances which are usually specified according to published
standards and manufacturers specifications.
b) Instrumentation (measurement) inaccuracies, again specified in
c) Drift in the characteristics of the sensor due to temperature cycling and
d) Possible thermal effects resulting from the installation, for 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. 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
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 which
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
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 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.
THERMAL TEMPERATURE CALIBRATION.
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 a Calibration System. The equipment required to achieve thermal calibration of temperature probes
is dependent on the desired accuracy and also 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.
1) General purpose system for testing industrial plant temperature sensors
will usually provide accuracies between 1.0C and 0.1C using comparison
2) A secondary standards system for high quality comparison and fixed point
measurements will provide accuracies generally between 0.1C and 0.01C.
3) A primary standards system uses the most advanced and precise equipment
to provide accuracies greater than 0.001C.
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
A convenient form of thermal reference is the dry block calibrator. Such units are available with various ranges spanning from -50C to +1200C
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 free-standing 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 which can be taken on
to plant 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 which 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 0C, water from 0C to 80C and oils for up to 300C. Various
molten salts and sand baths are used for temperatures in excess of 300C. 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 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. 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 which
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