It was important to assess the guidance given on the thermometer and its packaging since this is the key way in which to ensure consumers understand how to use and read their thermometer. Incorrect use could mean the thermometer is not showing the correct temperature and/or that the consumer is not aware whether the fridge is at the correct temperature. Key factors in the assessment included 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.
Tables 5-8 give an assessment of the guidance given.
Liquid in glass (LIG) thermometers
The LIG thermometers work on the principle of a bulb of liquid (usually a coloured alcohol liquid, sometime mercury), which expands or contracts along a fine bore within a glass tube upon heating or cooling.
Eight thermometers of this type were purchased for this study. Most precision LIG thermometers that are available have the temperature scale inscribed directly onto the glass column of the thermometer (these were not included in this research because they are not designed for fridge use). Less accurate versions (such as those purchased for this study have the temperature scale inscribed onto the plastic case of the device. All of the thermometers purchased for this study had the temperature scale marked on the case of the thermometer. This makes it more difficult to read and more prone to errors whilst being read.
Table 5 Liquid in glass thermometers evaluated in this study, along with their instructions for use
Thermometer
Pack instructions in italics and comments
1 Liquid in glass
"Avoid the risk of food damage by running your appliance too warm. Avoid wasting electricity by having your appliance unnecessarily cold.
Refrigerator- Food must be kept between 0 and 5°C Freezer- Food should be kept below -18°C".
This seems sensible but no guidance on where thermometer should be placed.
3 Liquid in glass (ETI) Bulb exposed to air £2.75 "Hang or place the thermometer on or as near as possible the middle shelf, towards the back of the fridge. Like food, the thermometer will take a while to reach the appliance temperature, allow one hour initially. Check that the fridge is at its correct temperature (between 3 and 5°C), if above this range, turn the refrigerator thermostat dial up to the next number, repeat until the optimum temperature is obtained".
Good comprehensive advice on use of thermometer. Interesting that a different temperature range is given here than for no. 11, which is the same manufacturer, and that the range is between 3 and 5°C, which is higher than other guidance.
4 Liquid in glass (Unbranded)
Bulb exposed to air £3.00 No instructions were supplied with this thermometer.
5 Liquid in glass (Endotherm)
Bulb immersed in liquid gel
£7.23 Positioning and temperature suggestions:
"Fridge or chiller: 1 to 5°C (40°F) Use Velcro to attach to fridge door or side of fridge.
A performance assessment of domestic fridge thermometers 30
Thermostat control: Leave the ENDOTHERM for 2 hours at coldest location, if the temperature needs adjusting, turn the thermostat to next unit, repeat if necessary.
Refrigerator control: Read temperature first thing in the morning. A progressive rise in temperature over several days indicates refrigeration failure.
Food Monitoring: Record temperature twice a day."
Good comprehensive advice on use of thermometer though following the positioning advice could mean that the coldest point in the fridge may be at the correct temperature but rest of fridge may be too warm.
6 Liquid in glass (Food Safety Direct)
Bulb exposed in air £2.49 No instructions were supplied with the thermometer but markings on case indicate that fridge should be between 0 and 5°C.
7 Liquid in glass (Foodsafe)
Bulb immersed in liquid gel
£10.00 "Hang or place the thermometer on the middle shelf, towards the back of the fridge. Like food, the thermometer will take a while to reach the appliance temperature. Check that the fridge is at the correct temperature (between 0 and 5°C), if it is above this range, turn the fridge thermostat down until the optimum temperature is obtained".
Good advice.
8 Liquid in glass (Chef Aid)
Internal (air) £1.95 Accepted temperatures should be:
Fridge. 35 to 40°F (2 to 4°C).
Useful guidance based on US regulations, but nothing about where to place thermometer.
A performance assessment of domestic fridge thermometers 31
Bimetallic strip thermometers
Bimetallic strip thermometers work on the principle that a strip or coil made from dissimilar metals, will bend or curl as the metals expand and contract at different rates with temperature.
Table 6 Bimetallic strip thermometers evaluated in this study, along with their instructions for use.
Thermometer number
Operating principle (manufacturer)
Location of sensor part of thermometer
Thermometer price
Pack instructions
9 Bimetallic (Unbranded)
Inside case £3.50 No instructions were supplied with this thermometer
10 Bimetallic (Food
safety Direct)
Inside case £2.49 No instructions were supplied the thermometer but markings on case indicate that fridge should be between 0 and 5°C
11 Bimetallic (ETI) Inside case £1.80 "Hang or place the thermometer on the middle shelf, towards the back of the fridge. Like food, the thermometer will take a while to reach the appliance temperature, allow one hour initially. Check that the fridge is at the correct temperature (between 0 and 5°C, in the green zone), if it is above this range, turn the fridge thermostat down until the optimum temperature is obtained".
Good advice and visual indicator is useful.
A performance assessment of domestic fridge thermometers 32
Electronic thermometers
Electronic thermometers can operate on a variety of principles to sense and display the temperature of an environment. The most common method of operation for low cost devices is the use of a thermistor, a small sensing element that changes electrical resistance in response to temperature. Electronic devices can be very stable and accurate but some of the models tested are very slow to respond to changes in temperature.
Table 7 Electronic thermometers evaluated in this study, along with their instructions for use.
Thermometer number
Operating principle (manufacturer)
Location of sensor part of thermometer
Thermometer price
Pack instructions
12 Electronic (Multi
Thermo)
Penetration probe £11.50 Instructions on use were given, but did not contain any guidelines for the temperatures that a fridge should be operated at, or how the thermometer should be used.
13 Electronic (Digitron) Inside case £26.75 Instructions on use were given, but did not contain any guidelines for the temperatures that a fridge should be operated at, or how the thermometer should be used. The user can set alarms if the temperature is outside a preset limit.
14 Electronic (ETI) Inside case £6.80 Instructions on use were given, but did not contain any guidelines for the temperatures that a fridge should be operated at or how the thermometer should be used.
15 Electronic (ETI) Outside case £14.00 Instructions on use were given, but did not contain any guidelines for the temperatures that a fridge should be operated at, or how the thermometer should be used.
16 Electronic (ETI) Outside case £7.50 Instructions on use were given, but did not contain any guidelines for the temperatures that a fridge should be operated at, or how the thermometer should be used.
17 Infrared surface
(Fluke)
Food temperature £57.00 (Paraphrased from instructions given). The thermometer is designed to check that chilled foods are kept chilled and hot foods are kept hot. To assist in this check, there is an indicator that lights when the foods are below 4°C and above 60°C.
A performance assessment of domestic fridge thermometers 33
A performance assessment of domestic fridge thermometers 34
Thermo-chromic indicators
Thermo-chromic indicators use liquid crystal displays to indicate temperature, or to provide a warning message when temperatures are above or below a particular temperature threshold.
Table 8 Thermo-chromic indicators evaluated in this study, along with their instructions for use.
Thermometer number
Operating principle (manufacturer)
Location of sensor part of thermometer
Thermometer price
Pack instructions
18 Liquid crystal
(Coldzone)
Air temperature £1.58 each Printed on the indicator “If the centre of the circle is green your chiller temperature is set correctly. If the centre of the circle is Black your chiller temperature is set too high!”
Clear instructions
3.3.1 Conclusions from the assessment of the guidance given to the consumer
The instructions enclosed with the thermometers ranged from some comprehensive examples to ten of the thermometers that had no indications of how best to use them.
Only eight of the thermometers tested had instructions that gave an indication of what temperature a fridge should be operating at, and these showed a variation in the temperatures specified. Four of the thermometers were supplied with instructions that specified the position where the thermometer should be placed; however, we are not aware of any general pattern as to where the coldest point in the fridge is normally located. There may be limited benefit from using a fridge thermometer to measure the coldest location of the fridge as the warmest location is likely to have more of an impact on food safety.