Final project report A performance assessment of domestic fridge thermometers A performance assessment of a representative range of domestic fridge thermometers was undertaken in a series of scientific experiments to ascertain the accuracy, resolution and responsiveness of the thermometers to a change in temperature. This information will be of use to those communicating to consumers about food waste and food safety, and those providing tools for sale or use by consumers. Project code: RBC821-001 Research date: April-July 2009 Date: October 2009
WRAP helps individuals, businesses and local authorities to reduce waste and recycle more, making better use of resources and helping to tackle climate change. Document reference: [RBC821-001. Report prepared by Sophie Easteal, Banbury, WRAP] Written by: R.M.George, R.D.Thorn and G.I.Hooper (Campden BRI) Front cover photography: Selection of thermometers used in this research. WRAP and Campden BRI believe the content of this report to be correct as at the date of writing. However, factors such as prices, levels of recycled content and regulatory requirements are subject to change and users of the report should check with their suppliers to confirm the current situation. In addition, care should be taken in using any of the cost information provided as it is based upon numerous project-specific assumptions (such as scale, location, tender context, etc.). The report does not claim to be exhaustive, nor does it claim to cover all relevant products and specifications available on the market. While steps have been taken to ensure accuracy, WRAP cannot accept responsibility or be held liable to any person for any loss or damage arising out of or in connection with this information being inaccurate, incomplete or misleading. It is the responsibility of the potential user of a material or product to consult with the supplier or manufacturer and ascertain whether a particular product will satisfy their specific requirements. The listing or featuring of a particular product or company does not constitute an endorsement by WRAP and WRAP cannot guarantee the performance of individual products or materials. This material is copyrighted. It may be reproduced free of charge subject to the material being accurate and not used in a misleading context. The source of the material must be identified and the copyright status acknowledged. This material must not be used to endorse or used to suggest WRAP s endorsement of a commercial product or service. For more detail, please refer to WRAP s Terms & Conditions on its web site: www.wrap.org.uk
Executive summary The objective of this study was to survey the range of fridge thermometers available to UK domestic consumers and assess the performance of a representative sample in a series of scientific experiments to ascertain their accuracy, resolution and responsiveness to a change in temperature. A wide range of domestic fridge thermometers are available to the UK consumer and this study has evaluated the performance of a representative sample. The majority are either liquid-in-glass, electronic, liquid crystal or bimetallic thermometers, although specialist devices such as infrared thermometers are also available. The accuracy of the fridge thermometers tested was generally good, with the majority of devices measuring fridge air temperatures to within ±0.5 C of the actual fridge temperature (as indicated by a calibrated, rapid response temperature measurement device). Response times of the fridge thermometers were also determined and most of the devices assessed indicated the appropriate temperature within 30 minutes, satisfying typical food safety guidance for chilled foods. It was noted in these tests that some of the fridge thermometers assessed were not designed to directly measure fridge air temperature. One type had a standard liquid-in-glass thermometer embedded into a clear liquid; this was designed to slow down the speed of response of the thermometer and be more representative of the temperature change actually experienced by the food products within the fridge. Another type used an infrared sensor to directly measure the food product/packaging temperature. Tests showed that there were no appreciable differences between the majority of fridge thermometers tested their responsiveness with either increasing temperatures (warming) or decreasing temperatures (cooling). In both heating and cooling tests, the majority of thermometers took around 20 minutes to reach the temperature indicated by the calibrated thin-wire reference temperature. The readability and usability of the thermometers varied considerably. The electronic thermometers could be read most easily and accurately as the displays showed the temperature clearly and in most cases they could be read without opening the fridge door (as a sensor is placed in the fridge and the electronic display outside the fridge). The liquid in glass thermometers were generally considered to be the most difficult to read as they often had to be looked at closely to determine the reading. The bimetallic strip thermometers generally displayed the reading clearly with a good scale. All of the thermometers had scales that spanned a much wider range than is necessary for domestic refrigerator use, this made the proportion of the scale that was of interest to the consumer (typically 0-10 C) small as a proportion of the entire range. This consequently made the thermometers more difficult to read and less precise than they could be. Repeatability between batches of a single fridge thermometer type was also assessed and it was shown that results were generally consistent. The 'Coldzone' temperature indicator was used for this test. This is a liquid crystal device used by the Food Standards Agency (FSA) as a means of promoting food safety in domestic refrigerators. These assessments showed that there was no appreciable variation between the 10 replicate devices of the indicator tested. A major advantage of this type of device is that the indication is a clear "yes/no" whether the fridge is at the appropriate food storage temperature rather than providing a precise temperature reading as the others did. The instructions provided to consumers on the packaging of fridge thermometers was often quite variable. Many fridge thermometers were supplied with no instructions, some had an indication of recommended fridge temperatures and some provided advice on the temperature ranges recommended for different categories of food. It is suggested that manufacturers of these devices should provide more detailed information to the consumer on recommended fridge temperatures; this would help in both improving food safety and reducing food waste that results from poor temperature control in domestic refrigerators. The information obtained in this study will be of use to those communicating to consumers about food waste or food safety, and those providing tools for sale or use by consumer. Table 1 summarises the results of the testing programme undertaken for this study. A performance assessment of domestic fridge thermometers 1
Table 1 Summary of the results of the testing programme. Therm. number Image Operating principle (manufacturer) 1 Liquid in glass (Brannan) 2 Liquid in glass (Brannan) 3 Liquid in glass (ETI) 4 Liquid in glass (Unbranded) 5 Liquid in glass (Endotherm) Therm. price Accuracy (based on testing programme) 2.50 Measured temperature to within ±0.5 C of the reference temperature 2.50 Measured temperature to within ±1 C of the reference temperature 2.75 Measured temperature to within ±0.5 C of the reference temperature 3.00 Measured temperature to within ±1 C of the reference temperature 7.23 Measured temperature to within ±0.5 C of the reference temperature. Large thermal lag, took appreciably longer than the others to reflect changes in temperature (heating and cooling) Scale Readability Comments -40 to +30 C 1 C divisions 70 divisions in total -40 to +30 C 1 C divisions 70 divisions in total -40 to +25 C 1 C divisions 65 divisions in total -30 to +30 C 1 C divisions 60 divisions in total -40 to +40 C 1 C divisions 80 divisions in total 1 C error possible if scale is not viewed straight (parallax error) 1 C error possible if scale is not viewed straight (parallax error) 1 C error possible if scale is not viewed straight (parallax error) 0.5 C error possible if scale is not viewed straight (parallax error) 2 C error possible if scale is not viewed straight (parallax error) On-pack guidance suggests fridge should be set to 3-5 C No instructions were supplied Large potential error in reading A performance assessment of domestic fridge thermometers 2
Therm. number Image Operating principle (manufacturer) 6 Liquid in glass (Food Safety Direct) Therm. price Accuracy (based on testing programme) 2.49 Measured temperature to within ±0.5 C of the reference temperature Scale Readability Comments -30 to +50 C 10 C divisions 8 division in total 2-3 C error (parallax error and estimation of scale) Large potential error in reading 7 Liquid in glass (Foodsafe) 10.00 Measured temperature to within ±0.5 C of the reference temperature. Large thermal lag, took appreciably longer than the others to reflect changes in temperature (heating and cooling) -40 to +40 C 1 C divisions 80 divisions in total 2 C error possible if scale is not viewed straight (parallax error) Large potential error in reading 8 Liquid in glass (Chef Aid) 1.95 Measured temperature to within ±0.5 C of the reference temperature -30 to +30 C 1 C divisions 60 divisions in total 1 C error possible if scale is not viewed straight (parallax error) 9 Bimetallic 3.50 Measured temperature to within ±1 C of the reference temperature -30 to +30 C 1 C divisions 60 divisions in total 0.5 C error possible if scale is not viewed straight (parallax error) No instructions were supplied 10 Bimetallic (Food safety Direct) 2.49 Measured temperature to within ±1 C of the reference temperature -30 to +30 C 10 C divisions 6 divisions in total 3-4 C error (parallax error and estimation of scale) Large potential error in reading A performance assessment of domestic fridge thermometers 3
Therm. number Image Operating principle (manufacturer) Therm. price Accuracy (based on testing programme) 11 Bimetallic (ETI) 1.80 Measured temperature to within ±0.5 C of the reference temperature Scale Readability Comments -30 to +30 C 1 C divisions 60 divisions in total 1 C error possible if scale is not viewed straight (parallax error) 12 Electronic (Multi Thermo) 13 Electronic (Digitron) 11.50 Measured temperature to within ±0.5 C of the reference temperature 26.75 Measured temperature to within ±0.5 C of the reference temperature 14 Electronic (ETI) 6.80 Unable to measure temperature to within ±1 C of the reference temperature -50 to +150 C 0.1 C resolution -30 to +40 C 1 C resolution -9.9 to +49.9 C 0.1 C resolution 0.1 C (possible error when temperature is rounded to 1 decimal place). 1 C (possible error when temperature is rounded to 0 decimal place). 0.1 C (possible error when temperature is rounded to 1 decimal place). Unable to measure temperature to within ±1 C of the reference temperature 15 Electronic (ETI) 14.00 Measured temperature to within ±0.5 C of the reference temperature -39.9 to +69.9 C 0.1 C resolution 0.1 C (possible error when temperature is rounded to 1 decimal place). A performance assessment of domestic fridge thermometers 4
Therm. number Image Operating principle (manufacturer) Therm. price Accuracy (based on testing programme) 16 Electronic (ETI) 7.50 Measured temperature to within ±0.5 C of the reference temperature Scale Readability Comments -39.9 to +69.9 C 0.1 C resolution 0.1 C (possible error when temperature is rounded to 1 decimal place). 17 Infrared surface (Fluke) 57.00 Unable to measure temperature to within ±1 C of the reference temperature -30 to +200 C 0.1 C resolution. LED highlights above 60 C and below 4 C 0.1 C (possible error when temperature is rounded to 1 decimal place). Unable to measure temperature to within ±1 C of the reference temperature 18 Liquid crystal (Coldzone) 1.58 Coldzone indicators changed from showing OK when the environment temperature was in the range 1 C to 2 C to a blank indication when the environment temperature was above 3 C to 4 C NA Specification says that "OK" is displayed at temperatures less than +5 C Risk of fridge being set at too low a temperature A performance assessment of domestic fridge thermometers 5
Contents 1.0 Introduction... 7 2.0 Methodology for fridge thermometer assessment... 8 3.0 Results of the fridge thermometer assessment... 9 3.1 Survey of the range and types of fridge thermometers available to UK consumers... 9 3.1.1 Conclusion from the survey of the range and types of fridge thermometers available to UK consumers... 14 3.2 Results of the assessment to measure fridge thermometer performance... 15 3.2.1 Accuracy of the fridge thermometers... 15 3.2.2 Conclusion from the tests to measure accuracy... 20 3.2.3 Resolution, readability and clarity of the fridge thermometers... 21 3.2.4 Conclusion from the tests to assess resolution, readability and clarity of the fridge thermometers... 23 3.2.5 Response times of the fridge thermometers to changing fridge temperatures 23 3.2.6 Conclusion of the tests to assess the response times of the fridge thermometers to changing fridge temperatures... 27 3.2.7 Repeatability of selected fridge thermometers... 28 3.3 An assessment of the guidance given to the consumer... 29 3.3.1 Conclusions from the assessment of the guidance given to the consumer... 34 4.0 Conclusions from this study... 35 5.0 References... 36 Acknowledgements Campden BRI gratefully acknowledge the financial support and technical guidance provided by WRAP for this work. A performance assessment of domestic fridge thermometers 6
1.0 Introduction Advice from the Food Standards Agency (FSA) states that ensuring a fridge is at the correct temperature (typically between 0-5 o C) is essential to prevent the growth of potentially harmful bacteria in foodstuffs (Richmond, 1991; FoodSense, 1994). Fridge temperatures also have a key role in minimising food spoilage and waste (FSA, 2009). For these reasons it is advisable to use a fridge thermometer to check fridge temperatures. A study by the Food Refrigeration and Process Engineering Research Centre (FRPERC) reported that over 60% of UK consumers did not know the appropriate temperature for their fridge to store food safely. It also revealed that over two-thirds of consumers surveyed did not have a fridge thermometer and, of those that did, the fridge temperature was checked less than once a month (FRPERC, 2005). More recent research suggests that the number of consumers knowing the temperature that their fridges should be set to has increased to almost 80% (WRAP, 2009), but this research and additional research by the FSA 1 reveals that the majority still do not have a reliable method for determining at what temperature their fridge is actually operating. Previous surveys of UK domestic fridge temperatures (James and Evans, 1992) showed that fridge temperatures ranged from 0 o C to 12 o C, with an average of around 6 o C. This study suggested that the most appropriate way of determining fridge temperatures would be to use a fridge thermometer. However, there is some uncertainty on the performance and reliability of thermometers sold for domestic use. Generally, they are relatively low cost devices and there is often little guidance to the consumer on their appropriate use for effective monitoring of fridge temperature. There are also many different types of fridge thermometers currently available, including: electronic digital thermometers; liquid-in-glass thermometers (mainly spirit-filled, sometimes mercury-filled); mechanical thermometers (e.g. bimetallic dial types); liquid crystal device (LCD) thermometers; and infrared thermometers. Each of these devices are also available in different formats, for example: thermometers that measure fridge air temperature; thermometers that are encased in a food simulant material to mimic temperature similar to that of the food stored in the fridge; thermometers that are designed to probe the food; thermometers that indicate maximum and minimum temperatures; thermometers that have a scale for direct reading of temperature; and thermometers that have generic scale markings, e.g. 'safe', 'chilled', 'acceptable' and 'OK'. The objective of this study was to survey the range of fridge thermometers available to UK domestic consumers and assess the performance of a representative sample of these thermometers in a series of scientific tests to ascertain the accuracy, resolution and responsiveness of the thermometers to a change in temperature. 1 http://www.food.gov.uk/multimedia/pdfs/publicattitudestofood.pdf A performance assessment of domestic fridge thermometers 7
2.0 Methodology for fridge thermometer assessment The assessment of the fridge thermometers was done in three stages: 1. A survey of the range and types of fridge thermometers available to UK consumers was conducted and examples were purchased for the evaluation phase of the study. The assessment encompassed fridge thermometers from across the range available to consumers, including replicate samples of one device to assess the repeatability of the device. 2. An assessment programme was undertaken to measure fridge thermometer performance, including: accuracy of temperature measurement; resolution, readability and clarity of each fridge thermometer; response times of each thermometer to a change in fridge temperature; and consistency of fridge thermometer performance. 3. An assessment was made of the guidance given to the consumer on the packaging, on how to use the fridge thermometer. Key factors included assessing the presence and accuracy of: advice on reading the temperature display, e.g. how to read the thermometer accurately and consistently; advice on interpreting the reading given on the fridge thermometer, e.g. what does the temperature mean? advice on where to place the thermometer in the fridge; advice on what temperature the fridge should be set to; advice on what the consumer should do if the thermometer indicates that the fridge temperature is outside the recommended temperature range; and the clarity of the advice given to the consumer. Thermometer 18 (detailed in Table 2) is not designed to provide a quantitative indication of fridge temperature and so could not be compared directly with the other fridge thermometers in this study. It is therefore excluded from the majority of the tests. See section 3.2.7 for details of results for this thermometer. A performance assessment of domestic fridge thermometers 8
3.0 Results of the fridge thermometer assessment 3.1 Survey of the range and types of fridge thermometers available to UK consumers Most domestic home-ware or cooking equipment suppliers sell fridge and freezer thermometers. They are also often advertised in the national press and in various magazines, catalogues and websites. A survey of the types available via these sources suggested that there are five major types available. These are liquid-in-glass thermometers, electronic thermometers, bimetallic thermometers, infrared thermometers and liquid crystal thermometers. Within each category there was a reasonable variation in price, operating principle, size and readabilty. However, within each type of fridge thermometer available, there was little difference between different makes of thermometer other than cosmetic changes to reflect branding and packaging. From the desk-based review of literature and web-based sources, a total of 18 different domestic fridge thermometers were purchased for this study, representing the range currently available to UK consumers. They were purchased from a variety of sources, including mail order catalogues, internet suppliers, magazine advertisements and in store. These included eight versions of liquid-in-glass fridge thermometers, three versions of bimetallic fridge thermometers, five versions of electronic fridge thermometers, one version of a liquid crystal fridge thermometer and one version of an infrared fridge thermometer. Full details of the thermometers chosen for the assessment are given below in Table 2, which summarises the fridge thermometers and their major features. Images of the thermometers are given in Figures 1-17 below. Table 2 Domestic fridge thermometers evaluated in this study. Thermometer number Operating principle (manufacturer) Location of sensor part of thermometer Thermometer price 1 * Liquid in glass (Brannan) Bulb exposed to air 2.50 2 * Liquid in glass (Brannan) Bulb exposed to air 2.50 3 Liquid in glass (ETI) Bulb exposed to air 2.75 4 Liquid in glass (Unbranded) Bulb exposed to air 3.00 5 Liquid in glass (Endotherm) Bulb immersed in liquid gel 7.23 6 Liquid in glass Bulb exposed in air 2.49 (Food Safety Direct) 7 Liquid in glass (Foodsafe) Bulb immersed in liquid gel 10.00 8 Liquid in glass (Chef Aid) Internal (air) 1.95 9 Bimetallic Inside case 3.50 10 Bimetallic (Food safety Direct) Inside case 2.49 11 Bimetallic (ETI) Inside case 1.80 12 Electronic (Multi Thermo) Penetration probe 11.50 13 Electronic (Digitron) Inside case 26.75 14 Electronic (ETI) Inside case 6.80 15 Electronic (ETI) Outside case 14.00 16 Electronic (ETI) Outside case 7.50 17 Infrared surface (Fluke) Food temperature 57.00 18 Liquid crystal (Coldzone) Air temperature 1.58 * Thermometers 1 and 2 were identical. There were both included to test consistency. A performance assessment of domestic fridge thermometers 9
Figure 1 Thermometer 1 and 2. Figure 2 Thermometer 3. Figure 3 Thermometer 4. Figure 4 Thermometer 5. Figure 5 Thermometer 6. A performance assessment of domestic fridge thermometers 10
Figure 6 Thermometer 7. Figure 7 Thermometer 8. Figure 8 Thermometer 9. Figure 9 Thermometer 10. A performance assessment of domestic fridge thermometers 11
Figure 10 Thermometer 11. Figure 11 Thermometer 12. Figure 12 Thermometer 13. A performance assessment of domestic fridge thermometers 12
Figure 13 Thermometer 14. Figure 14 Thermometer 15.. Figure 15 Thermometer 16. A performance assessment of domestic fridge thermometers 13
Figure 16 Thermometer 17. Figure 17 Thermometer 18. 3.1.1 Conclusion from the survey of the range and types of fridge thermometers available to UK consumers There appears to be a good range of fridge thermometers available to the consumer and they are available widely from a variety of sources, such as stores, magazines, national and local press advertisements and numerous website sources. The prices of fridge thermometers also covers a wide range, from less than 2.00 for the simple LCD indicator type to over 50 for the infrared types. The wide variety of fridge thermometers available can accommodate different consumer budgets and preferences for design and appearance. However, such a wide range could cause some consumer confusion particularly given that people will not necessarily know which fridge thermometer would give the most meaningful information of the temperature of their fridge. Although five different types of fridge thermometer were chosen for this assessment, the most common fridge thermometers available to the consumer were either the liquid-in-glass, bimetallic or electronic types. Consequently, more examples of these thermometers were chosen for the testing programme than the other types. A performance assessment of domestic fridge thermometers 14
3.2 Results of the assessment to measure fridge thermometer performance An assessment was undertaken to assess the performance of the domestic fridge thermometers in a range of tests designed to simulate actual conditions of use. Four tests were used to assess performance: accuracy of temperature measurement, as compared with UKAS-calibrated thermometers; resolution, readability and clarity of each fridge thermometer; response times of each thermometer to changing fridge temperatures; and repeatability of fridge thermometer performance. The fridge thermometers selected for this work were compared with standard reference thermometers which had been previously calibrated to a national standard (United Kingdom Accreditation Service, UKAS). Campden BRI operate a thermometer calibration service which is approved to UKAS standards (UKAS Testing Laboratory No. 0407). 3.2.1 Accuracy of the fridge thermometers For this test, each of the fridge thermometers were placed in a controlled-temperature cold room environment at a nominal 'chilled' temperature of between +2 and +5 C. The air temperature immediately surrounding each of the thermometers under test was monitored using thin bare-wire thermocouples (type K, nickel-chromium) attached to a temperature datalogger (Squirrel datalogger, Grant Instruments Ltd). Prior to the test, the thermocouples and datalogger had been previously calibrated against a nationally traceable reference temperature instrument. Measurements of each fridge thermometer were taken at 2 minute intervals over a period of c. 30 minutes at the controlled temperature. For the infrared thermometer, the device was used to measure the temperature of a container of water, which had been allowed to equilibrate to the temperature of the cold room. Table 3 shows the results of the 'calibration' on the fridge thermometers under test. The stable temperature achieved by the calibrated thin wire thermocouple was +3.7 C and the table shows the temperature displayed by each test thermometer and the difference between the measurements of the test thermometer and the calibrated reference thermometer. A performance assessment of domestic fridge thermometers 15
Table 3 Results of the constant temperature test on domestic fridge thermometers. Thermometer number Thermometer type Temperature measured by test thermometer Difference between test thermometer and reference of +3.7 C 1 LIG 4.2 C 0.46 C 2 LIG 4.35 C 0.61 C 3 LIG 3.7 C -0.04 C 4 LIG 2.8 C -0.94 C 5 LIG 4 C 0.26 C 6 LIG 3.8 C 0.06 C 7 LIG 3.65 C -0.09 C 8 LIG 3.95 C 0.21 C 9 Bimetallic 4.45 C 0.71 C 10 Bimetallic 3.05 C -0.69 C 11 Bimetallic 4.15 C 0.41 C 12 Electronic 4.07 C 0.33 C 13 Electronic 3.8 C 0.1 C 14 Electronic 5.12 C 1.38 C 15 Electronic 3.8 C 0.06 C 16 Electronic 3.92 C 0.18 C 17 Infra red 2.54 C -1.20 C 18* Liquid crystal - - * results for this thermometer are shown below in Section 3.2.7. The results in Table 3 show that the fridge thermometers tested were relatively accurate, with 11 of the 17 thermometers in this test measuring temperature to within ±0.5 C of the reference temperature and 15 of the 17 thermometers measuring temperature to within ±1 C of the reference temperature. There were also no obvious differences in performance between the different classes of fridge thermometer; the liquid-in-glass, bimetallic and electronic thermometers all performed similarly. The two fridge thermometers that performed less well in this test were Thermometers 14 and 17. The reasons for this were thought to be that Thermometer 14 (electronic type) had the electronic temperature sensing element embedded within a robust plastic enclosure that may have resulted in poor thermal heat transfer. Thermometer 17 (infrared type) measures temperatures at the surface of a food or food package and, consequently, the results from this device could not be directly compared with the air temperature measured by the calibrated reference temperature probe. The following figures (Figures 18 to 21) show how each of the fridge thermometers under test respond in comparison to the actual cold room (fridge) temperature, as measured by the calibrated thin wire thermocouple. This reflects the ability of the fridge thermometers to accurately measure the air temperature within the fridge. The calibrated thin-wire thermocouple will respond accurately and rapidly to changes in fridge temperature, including the slight temperature fluctuation of fridge temperature that occurs during normal operation of the fridge. The ability of the fridge thermometers to follow the indicated calibrated thermocouple temperature gives an indication of accuracy and speed of response of the fridge thermometer. A performance assessment of domestic fridge thermometers 16
Figure 18 shows the measurements recorded by the liquid in glass thermometers (Thermometers 1 to 8). The temperature in the controlled-temperature room, as measured by the calibrated thin-wire thermocouple, was cycled between 2 C and 5 C. This changing temperature influenced the measurements provided by the fridge thermometers under test, but due to the much larger dimensions of the fridge thermometers, they responded much more slowly to changes in temperature than the calibrated thermocouple. There is consequently an inherent time lag between the temperature measurements and a slight difference in the actual temperatures measured between the calibrated thermocouple and the fridge thermometers. Figure 18 Temperatures measured with the liquid-in-glass fridge thermometers. 5.5 5 Thermometer temperature ( C) 4.5 4 3.5 3 2.5 Reference Thermometer 1 Thermometer 2 Thermometer 3 Thermometer 4 Thermometer 5 Thermometer 6 Thermometer 7 Thermometer 8 2 0 10 20 30 40 Time (minutes) of test A performance assessment of domestic fridge thermometers 17
Figure 19 shows the measurements recorded by the bimetallic strip fridge thermometers (Thermometers 9, 10 and 11). The temperature in the controlled-temperature room, as measured by the calibrated thin-wire thermocouple, again cycled between 2 C and 5 C. The bimetallic thermometers used in this test were much larger and bulkier than the calibrated thin wire thermocouple and consequently they responded much more slowly to changes in temperature than the calibrated thermocouple. As shown for the previous class of fridge thermometers, there is an inherent time lag between the temperature measurements and a slight difference in the actual temperatures measured between the calibrated thermocouple and the fridge thermometers. Figure 19 Temperatures measured with the bimetallic fridge thermometers. 5.5 Thermometer temperature ( C) 5 4.5 4 3.5 3 2.5 Refrerence Thermometer 9 Thermometer 10 Thermometer 11 2 0 10 20 30 40 Time (minutes) of test A performance assessment of domestic fridge thermometers 18
Figure 20 below shows the measurements recorded by the electronic fridge thermometers (Thermometers 12 to 16). As before, the temperature in the controlled-temperature room was cycled between 2 C and 5 C. This changing temperature influenced the measurements provided by the fridge thermometers under tests, but due to the much larger dimensions of the fridge thermometers, they responded much more slowly to changes in temperature. As indicated for the previous class of fridge thermometers, there is an inherent time lag between the temperature measurements and a slight difference in the actual temperatures measured between the calibrated thermocouple and the fridge thermometers. This was particularly apparent for fridge thermometer 14 which had the electronic temperature sensing element embedded within a robust plastic enclosure that seemed to result in a slower and less accurate measurement of fridge temperature, possibly as a result of poor thermal heat transfer. Figure 20 Temperatures measured with the electronic fridge thermometers. 5.5 Thermometer temperature ( C) 5 4.5 4 3.5 3 2.5 2 0 10 20 30 40 Reference Thermometer 12 Thermometer 13 Thermometer 14 Thermometer 15 Thermometer 16 Time (minutes) of test A performance assessment of domestic fridge thermometers 19
Figure 21 below shows the measurements recorded by the infrared fridge thermometers (Thermometer 17). The temperature in the controlled-temperature room was again cycled between 2 C and 5 C. This changing temperature influenced the measurements provided by the infrared thermometer under test. This thermometer measured the temperature of a small container of water in the fridge and due to the much larger dimensions of this container compared to the thin wire thermocouple, this thermometer responded much more slowly to changes in temperature. Figure 21 Temperatures measured with the infrared fridge thermometer. 5.5 Thermometer temperature ( C) 5 4.5 4 3.5 3 2.5 2 0 10 20 30 40 Time (minutes) of test Reference Thermometer 17 3.2.2 Conclusion from the tests to measure accuracy This test showed that all of the fridge thermometers evaluated showed some variation from the calibrated thin wire thermocouple. None of the fridge thermometers measured the full extent of temperature change in the fridge as indicated by the reference device. This was not unexpected, as all the fridge thermometers had a larger thermal mass than the thin-wire reference temperature probe. However, no particular type of thermometer stood out as being significantly better or worse than another (N.B. the infrared fridge thermometer does not directly measure fridge air temperature and so cannot be directly compared with the calibrated reference temperature probe). All types of fridge thermometer also showed some degree of thermal lag, i.e. they responded to changes in temperature more slowly than the calibrated thin-wire reference temperature probe. Again, this was expected of the devices as they have larger thermal mass than the thin-wire reference probe and take longer to respond to changes in temperature. Indeed, all of the fridge thermometers had some degree of built-in thermal lag - this would prevent the devices from responding too rapidly to changes in temperature that were not representative of the temperature within the fridge e.g. on opening the fridge door. A large thermal lag is a key feature of some of the fridge thermometers under test: thermometers 5 and 7 have a liquid-in-glass thermometer embedded into a silicon oil, which has the effect of slowing down the temperature response. The intention of this type of fridge thermometer is not to indicate fridge air temperature, but to indicate the temperature likely to be experienced by the food within the fridge. The results of this test suggest that it is not possible to recommend any one particular fridge thermometer, or even one type of fridge thermometer, over another. These tests highlight the importance of consumers keeping any thermometer in the fridge for at least 30 minutes before taking a reading. The thermometers are not A performance assessment of domestic fridge thermometers 20
designed to measure rapidly changing temperatures given this is not, in reality, a requirement of these devices under normal consumer use. Testing the accuracy of the thermometers was important, as the accuracy of a device can be described as how closely the reading on the device corresponds to the measurement scale. This would have to be determined by averaging the readings of the device over a period of time in a stable temperature environment. The results show that all of the thermometers performed reasonably well. 3.2.3 Resolution, readability and clarity of the fridge thermometers In this test, an assessment of the factors which influence how the consumer may read the fridge thermometer was made. The resolution of the fridge thermometer indicates the temperature range and smallest scale divisions that the thermometer can read. The readability is the ability to distinguish between individual increments on the temperature scale, allowing the consumer to measure temperature to e.g. 1 C, 0.5 C or 0.1 C. The clarity of the fridge thermometer is a subjective assessment of the ease of reading the thermometer, e.g. is the scale clear or does the thermometer need to be taken out of the fridge to be read? For this test, an assessment was made by Robin Thorn (Head of Thermometer Calibration Services, Campden BRI). Table 4 indicates the resolution and an assessment of readability of each of the fridge thermometer types under test. The thermometer scales were assessed and the divisions noted. On some of the thermometers the scales were very clear and it was possible to estimate a half scale division point between the scale divisions, this is why some of the resolutions are greater than the divisions on the thermometer. Table 4 An assessment of thermometer resolution, readability and clarity. Thermometer number Thermometer type Scale (range, scale divisions) 1 LIG -40 to +30 C 1 C divisions 70 divisions in total 2 LIG -40 to +30 C 1 C divisions 70 divisions in total 3 LIG -40 to +25 C 1 C divisions 65 divisions in total 4 LIG -30 to +30 C 1 C divisions 60 divisions in total 5 LIG -40 to +40 C 1 C divisions 80 divisions in total 6 LIG -30 to +50 C 10 C divisions 8 division in total 7 LIG -40 to +40 C 1 C divisions 80 divisions in total Description Refrigeration thermometer - LIG thermometer encapsulated in silicon oil Colour scales indicated for -18 C to 0 C, 0 C to 5 C and 5 C to 63 C ranges Foodsafe Fridge thermometer Possible error associated with reading scale (maximum resolvable resolution) 1 C error possible if scale is not viewed straight (parallax error) 1 C error possible if scale is not viewed straight (parallax error) 1 C error possible if scale is not viewed straight (parallax error) 0.5 C error possible if scale is not viewed straight (parallax error) 2 C error possible if scale is not viewed straight (parallax error) 2-3 C error (parallax error and estimation of scale) 2 C error possible if scale is not viewed straight (parallax error) A performance assessment of domestic fridge thermometers 21
Thermometer number Thermometer type Scale (range, scale divisions) 8 LIG -30 to +30 C 1 C divisions 60 divisions in total 9 Bimetallic -30 to +30 C 1 C divisions 60 divisions in total 10 Bimetallic -30 to +30 C 10 C divisions 6 divisions in total 11 Bimetallic -30 to +30 C 1 C divisions 60 divisions in total 12 Electronic -50 to +150 C 0.1 C resolution 13 Electronic -30 to +40 C 1 C resolution 14 Electronic -9.9 to +49.9 C 0.1 C resolution 15 Electronic -39.9 to +69.9 C 0.1 C resolution 16 Electronic -39.9 to +69.9 C 0.1 C resolution 17 Infrared -30 to +200 C 0.1 C resolution. LED highlights above 60 C and below 4 C Description Colour scales for - 15 C to 0 C, 0 C to 8 C and 8 C to 30 C ranges Needle vibrated with any movement Food storage/ refrigeration Probe type temperature sensor. Temperature can be read with fridge door shut, temperature updates slowly. Fridge thermometer. Temperature sensor within body of instrument. Temperature can be read with fridge door shut Temperature can be read with fridge door shut Non-contact food safety thermometer 18 Liquid crystal Specification says that "OK" is displayed at temperatures less than +5 C Possible error associated with reading scale (maximum resolvable resolution) 1 C error possible if scale is not viewed straight (parallax error) 0.5 C error possible if scale is not viewed straight (parallax error) 3-4 C error (parallax error and estimation of scale) 1 C error possible if scale is not viewed straight (parallax error) 0.1 C (possible error when temperature is rounded to 1 decimal place). 1 C (possible error when temperature is rounded to 0 decimal place). 0.1 C (possible error when temperature is rounded to 1 decimal place). 0.1 C (possible error when temperature is rounded to 1 decimal place). 0.1 C (possible error when temperature is rounded to 1 decimal place). 0.1 C (possible error when temperature is rounded to 1 decimal place). n/a A performance assessment of domestic fridge thermometers 22
3.2.4 Conclusion from the tests to assess resolution, readability and clarity of the fridge thermometers As can be seen from Table 4, the readability and usability of the thermometers varied. The electronic thermometers could be read most easily and accurately as the displays showed the temperature clearly and in most cases they could be read without opening the fridge door. The liquid in glass thermometers were generally considered to be the most difficult to read as they often had to be looked at closely to determine the reading. This may lead to inaccuracies as a consumer may have to remove the thermometer from the fridge to read it, which in turn may affect the displayed reading. However the slow speed of response of most of the liquid in glass thermometers means that this is unlikely to be a issue unless the user takes more than 30 seconds to read the thermometer. The bimetallic strip thermometers generally displayed the reading clearly with a good scale. The two thermometers sold by Food Safety Direct, (Thermometers 6 and 10) were generally considered to be the most difficult to read as the scales were very widely spaced and not clear. The same was the case with Thermometers 5 and 7. Given fridge temperatures should fall between 0-5 o C, an error of 2-3 o C is significant and could lead to an incorrect temperature being set. All of the thermometers had scales that spanned a much wider range than was necessary for domestic refrigerator use, this made the proportion of the scale that is of interest to the consumer (typically 0-10 C) small as a proportion of the entire range, this consequently made the thermometers more difficult to read and less precise than the ideal. 3.2.5 Response times of the fridge thermometers to changing fridge temperatures The response time of the fridge thermometers to react to step changes in fridge air temperature was determined through two tests. Firstly, a warming-up test was conducted. The fridge thermometers were allowed to equilibrate to a fridge temperature of 5 C for a period of at least 24 hours and then brought into a controlled temperature environment of 18 C (thought to be a typical kitchen ambient temperature). Secondly, a cooling-down test was conducted where the fridge thermometers were equilibrated to a room temperature of 18 C and then taken into a fridge environment of 5 C. Measurements from each test fridge thermometer were taken at 5 minute intervals throughout the test to establish the rate of response of each fridge thermometer to the step change in temperature. Results from the fridge thermometers under test were compared with a calibrated thin-wire thermocouple. Figures 22 to 25 below show the results of the warming-up tests for the different categories of fridge thermometer evaluated in these tests. The figures show the temperatures measured by each device in the chilled environment and the 'step change' as the devices were taken into the room temperature regime. The results indicated that, as expected, the thin wire thermocouple responded to changes in temperature more rapidly than any of the fridge thermometers under test, but all the thermometers generally responded well. Two of the liquid in glass thermometers (Thermometers 5 and 7) have the sensor part of the thermometer encapsulated within a liquid gel in order to slow down the speed of response. They are marketed as thermometers which indicate the fridge temperature as it might be experienced by the food and not the fridge air temperature and therefore took a longer time to respond to changes in temperature. Some of the electronic thermometers responded very slowly to the changes in temperature, this is due to a number of factors, including the positioning of the sensing element. On one device (Thermometer 13), the slow response was due to the slow rate at which the display unit was updated by the sensors. A performance assessment of domestic fridge thermometers 23
Figure 22 Response of liquid-in-glass fridge thermometers to changing fridge temperatures (warming). Thermometer temperature ( C) 25 20 15 10 5 0 0 20 40 60 80 Time of test (minutes) Reference Thermometer 1 Thermometer 2 Thermometer 3 Thermometer 4 Thermometer 5 Thermometer 6 Thermometer 7 Thermometer 8 Figure 23 Response of bimetallic fridge thermometers to changing fridge temperatures (warming). 25 Thermometer temperature ( C) 20 15 10 5 Reference Thermometer 9 Thermometer 10 Thermometer 11 0 0 20 40 60 80 Time of test (minutes) A performance assessment of domestic fridge thermometers 24
Figure 24 Response of electronic fridge thermometers to changing fridge temperatures (warming). 25 Thermometer temperature ( C) 20 15 10 5 0 0 20 40 60 80 Reference Thermometer 12 Thermometer 13 Thermometer 14 Thermometer 15 Thermometer 16 Time of test (minutes) Figure 25 Response of infrared fridge thermometer to changing fridge temperatures (warming). 25 Thermometer temperature ( C) 20 15 10 5 Reference Thermometer 17 0 0 20 40 60 80 Time of test (minutes) A performance assessment of domestic fridge thermometers 25
Figures 26 to 29 below show the results of the cooling-down tests for the different categories of fridge thermometer evaluated in these tests. The results indicated that, as expected, the thin wire thermocouple responded to changes in temperature more rapidly than any of the fridge thermometers under test. Again, because two of the liquid in glass thermometers (Thermometers 5 and 7) have the sensor part of the thermometer encapsulated within a liquid gel in order to slow down the speed of response, these thermometers took appreciably longer (particularly Thermometer 5) than the thin-wire reference temperature probe to cool to fridge temperature. Figure 26 Response of liquid-in-glass fridge thermometers to changing fridge temperatures (cooling). Thermometer temperature ( C) 25 20 15 10 5 0 0 10 20 30 40 Time of test (minutes) Reference Thermometer 1 Thermometer 2 Thermometer 3 Thermometer 4 Thermometer 5 Thermometer 6 Thermometer 7 Thermometer 8 Figure 27 Response of bimetallic fridge thermometers to changing fridge temperatures (cooling). Thermometer temperature ( C) 25 20 15 10 5 0 0 10 20 30 40 Reference Thermometer 9 Thermometer 11 Thermometer 12 Time of test (minutes) A performance assessment of domestic fridge thermometers 26
Figure 28 Response of electronic fridge thermometers to changing fridge temperatures (cooling). 25 Thermometer temperature ( C) 20 15 10 5 0 0 10 20 30 40 Time of test (minutes) Reference Thermometer 12 Thermometer 13 Thermometer 14 Thermometer 15 Thermometer 16 Figure 29 Response of infrared fridge thermometer to changing fridge temperatures (cooling). 25 Thermometer temperature ( C) 20 15 10 5 Reference Thermometer 17 0 0 10 20 30 40 Time of test (minutes) 3.2.6 Conclusion of the tests to assess the response times of the fridge thermometers to changing fridge temperatures The temperature response trials were carried out to determine how quickly the thermometers responded to a step change in temperature, whilst it is unlikely that consumers will carry out such a test (if the thermometer is left in the fridge) it was an important test, as it gives an indication as to how fast the thermometers will respond to a more gradual change in temperature such as placing a large hot item of food in the fridge (which could warm up the fridge contents). The project team were looking to see whether there were any really slow or really fast thermometers - the former may have implications on food poisoning if response times were of the order of a couple of hours and the latter may have implications if they responded so quickly and changed as soon as the fridge door was opened. A performance assessment of domestic fridge thermometers 27
The results of these tests showed that there were no appreciable differences between the majority of fridge thermometers tested in changing with either increasing temperatures (warming) or decreasing temperatures (cooling). In both heating and cooling tests, the majority of thermometers took around 20 minutes to reach the temperature indicated by the calibrated thin-wire reference temperature. The thin wire thermocouple used to monitor the ambient temperature around the thermometers was considered to change temperature within 10-15 seconds and any apparent lag in response was caused by intervals between measurements. A difference was seen with Thermometers 5 and 7 which took appreciably longer than the others to reflect changes in temperature (heating and cooling). As mentioned previously, these thermometers had the active temperature sensing element (a liquid-in-glass thermometer) embedded in a silicon oil to deliberately suppress the speed of temperature response. These fridge thermometers are sold as indicators of food temperatures in the fridge rather than fridge air temperature indicators and therefore are designed to respond more slowly to temperature change. The infrared fridge thermometer was the most rapidly responding thermometer in these tests, both in heating and cooling. The results of this fridge thermometer, which measured the temperature at the surface of a small container of water, responded as quickly as the thin-wire reference probe. In order to provide the consumer with a meaningful indication of fridge temperature, it is suggested that the most useful fridge thermometer would fully respond to changes in temperature within 30 minutes of the temperature change. This would correspond with the recommended guidelines for food handlers in the preparation of foods and the maximum time at room temperature: 30 minutes (critical limit 2 hours) (Food Hygiene Handbook, 2005). This timescale would be useful in indicating significant temperature changes within the fridge that may cause problems in food safety and would avoid reflecting minor changes. Significant temperature change may occur if, for example, the fridge door was left open for a period of time or if a large, warm food item was introduced into the fridge which then affected the overall fridge temperature. The results of this test on the fridge thermometers suggested that the majority of fridge thermometers complied with this requirement; those fridge thermometers (Thermometers 5 and 7) that did not are deliberately designed to respond differently. 3.2.7 Repeatability of selected fridge thermometers The results presented earlier in this report showed that fridge thermometers of the same type could show a slight difference in performance (e.g. in the accuracy tests). For example, Thermometers 1, 2 and 3 are very similar in design and construction, but gave temperature measurements that were 0.5 C different in the fridge environment. Consequently, an assessment of the repeatability between different samples of the same thermometer was carried out. In this test, Thermometer 18 (liquid crystal thermometer, 'Coldzone') was used (Figure 30) to ascertain the variability between different samples of a single type of thermometer. This thermometer does not have a numerical display, but if the fridge temperature is below +5 C, then the liquid crystal display indicates an "OK" message to the consumer. These fridge thermometers therefore, do not provide a quantitative indication of fridge temperature and so could not be compared directly with the other fridge thermometers in this study. This particular thermometer is used by the FSA as a means of promoting the importance of correct fridge temperatures in guaranteeing food safety. A performance assessment of domestic fridge thermometers 28