Calibration of contact thermometers
Contact thermometers are calibrated by inserting them into an enclosure or environment at a uniform steady temperature, of suitable size and with an appropriate access for the thermometers.
The enclosure is either:
- a fixed point cell whose temperature is known
- a bath or chamber whose temperature is measured using standard thermometers
Some disadvantages of fixed point calibration are:
- Only a few thermometers can be measured while the phase change (melting or freezing) lasts
- Several fixed points are needed to cover a given range.
- The method is time-consuming and therefore expensive.
A simpler, more versatile, and cheaper alternative is to set up a temperature controlled environment, usually a stirred liquid (bath, a furnace or a metal block ‘calibrator’), into which the thermometers are inserted with two or more calibrated standard thermometers or thermocouples.
The bath or furnace is set to control at a series of steady temperatures covering the range of the calibration, which can be chosen at will.
calibration bath (Fluke Calibration)
At each stable point, measurements are made of all the thermometers in a suitable scanning procedure, the temperature is determined from the standards, and the calibration is thereby achieved.
The thermal environments must be:
- uniform in temperature over the critical volume
- stable (or only slowly drifting) during the measurement period
Uniformity and stability must be checked before use. The conditions are tested during calibration if more than one standard thermometer is used.
Calibration by comparison allows a higher throughput of thermometers and is amenable to greater automation than using fixed points. It is therefore the preferred method for secondary calibration laboratories and factories.
Examples of temperature calibration apparatus
Stirred liquid baths, such as this one from Isothermal Technology Ltd, provide the most stable and uniform calibration environments in the range from about − 100 °C up to 500 °C, using alcohol or acetone, water, various silicone oils, and molten salts. They should be deep enough, typically 450 mm, to ensure proper immersion depths for the probes and standards which will be inserted, and the control and measurement procedure can be conveniently programmed. At higher temperatures air-circulating fluidised alumina baths (sand baths) or tube furnaces are used.
The metal block calibrator is a portable apparatus consisting of a cylindrical block, usually of an aluminium alloy about 150 mm long and 30 mm in diameter. This is drilled with holes as required for the probes to be inserted along almost the complete length. The block sits in an insulated heating unit so that its temperature can be controlled from about 30 °C up to 450 °C or higher.
When the temperature is steady the calibrations of the test probes are established by comparing their readings with readings of standard probes which are also inserted. This is simple, convenient and inexpensive compared with the use of liquid baths, but the accuracy is limited mainly by the uniformity and stability of the temperature in the block and by immersion effects (heat conduction) in the test and standard probes.
Guidance on using and testing these calibrators is available from www.euramet.org under Documents and Publications.
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