ETI's recommendations for temperature measurement depend on the number of units held by an organisation, the time available to check on instruments and monitor performance, the importance that accurate measurement plays in the performance of the business, combined with the manner in which measurement is monitored.
For a small business, perhaps small bakery or delicatessen, it is recommended that two thermometers be held. To comply with the requirements of BS EN 13486:2002 – Temperature recorders and thermometers for the transport, storage and distribution of chilled, frozen, deep-frozen/quick-frozen food and ice cream – Periodic Verification, at least one instrument should be checked and certified by a UKAS accredited laboratory once a year. (When returned from certification the other instrument should be compared with the certified one as soon as possible) Keeping an annual record of results will enable determination of how stable is the instrument over a long period and provide more confidence.
Comparison can be undertaken by placing the two (or more) probes, with instruments, close together in different temperatures. A glass with a mixture of crushed ice and a little water will provide a temperature of near to 0°C to within ±0.5°. This therefore enables comparison against a known temperature, and is a good indication of a thermometer’s performance.
Boiling water can provide a temperature of approximately 100°C but great care must be taken to avoid scolding and it must be realised that the temperature will drop very quickly when taken from the heat source. A container of warm/hot water can provide a comparison of instruments at a temperature between zero and 100° but this temperature cannot be specifically proved.
The two instruments should read the same ±1.5°C (or whatever tolerance difference your Quality System may determine). This tolerance allows for the fact that both instruments may be slightly out of adjustment and if one instrument is reading 0.7°C high, and the other 0.7°C low, there will be an apparent difference of 1.4°C. If an ETI Reference Thermometer (222-005) is used, this comes with a UKAS Certificate and is specified as being accurate to within ±0.2°C. The certificate will detail the correct accuracy and this is an excellent instrument to have as a company’s reference standard.
There are various alternative methods of checking a thermometer, and one of the easiest is to use an ETI Comparator (814-132); an instruction leaflet is available – combined with a certified reference thermometer.
Systems can be checked by immersion in a stirred water bath (822-900) or even a mini calibration bath (822-950) and comparing the temperature against another thermometer that you know is accurate i.e. has a current UKAS certificate. An ETI Reference Thermometer (222-005) is ideal.
Where an instrument has a socket for a probe, Calibrated Test Caps are available. These may be used on a regular basis to monitor the performance of the instrument. Test caps are available for Thermocouple, Thermistor and PT100 instruments.
ETI also manufacture thermometer Checkers, which act as probes at three different temperatures, and are used in the same way as test caps. However, a checker will only check the accuracy of the instrument and very often it is the probe that is faulty. To ensure confidence in a measurement, the combined system of instrument and probe should be calibrated.
Because of the frequency of damage inflicted on the probe, ETI recommend that a spare probe for each type of instrument be kept, to cover for instances of malfunction.
ETI Dry-wells are available to measure system accuracies and can check temperature systems at temperatures of between –10°C and 300°C. The added use of a Reference Thermometer (222-005) would provide added confidence.
Whichever method is used, confidence that a reference thermometer is reading correctly and is traceable to National Standards requires a UKAS certificate, and whilst an individual’s organisation will determine how often calibration is made, the manufacturer will recommend that an annual UKAS certificate is obtained.