The Food Safety Direct Quick Guide to Thermometers

What is a thermometer?

A thermometer is a device which measures temperature or temperature gradient, using a variety of different principles. The word thermometer is derived from two smaller word fragments: thermo from the Greek for heat and meter from Greek, meaning to measure.

Digital hand-held thermometers

Digital thermometers use a sensor, the part that "senses" the temperature, the information from which it converts into an electronic display of the temperature being measured.
These are thermometers that have a probe (sensor) separate from the main body of the instrument. In other words the sensor is connected to the thermometer by a cable. These can be further subdivided into thermometers that have fixed probes and those that have interchangeable probes. The advantage of the latter is that you can use probes designed for different purposes.

Our more expensive thermometers have a vast array of compatible probes available. For example there are probes for measuring liquids, frozen food, between pack temperatures, long probes for hot oil, surface temperature probes etc. There are 3 types of probe available on our website, thermomcouples, thermistor and platinum resistance. See below for more information.

See our digital hand-held thermometer range

Digital pocket thermometers

As with the section above, these thermometers use a sensor, to "sense" the temperature, the information from which is converted into an electronic display.
The main difference from the digital hand-held thermometers (above) is that these thermometers have an integral probe. In other words the sensor is permanently attached to, and part of the instrument. Many of the thermometers in this group have folding probes that retract into the body of the thermometer.

The advantage of these thermometers is that they can be operated using one hand rather than having to hold the instrument in one hand and the sensor or probe in the other. The disadvantage is that the probes are not generally interchangeable, although there are exceptions.

See our digital pocket thermometer range

Infrared thermometers

This type of thermometer measures temperature using infrared emitted from an object's surface. They are also called non-contact thermometers to describe the device's ability to measure temperature from a distance. Some have lasers to help aim the thermometer.

These thermometers basically consist of a lens to focus the infrared energy on to a detector, which converts the energy to an electrical signal which is displayed as a temperature. This allows temperature to be measured from a distance without the thermometer coming into contact with the food being measured.

Infrared thermometers can be used to serve a wide variety of temperature monitoring functions. For example they are ideal for checking oven temperatures or chilled cabinet temperatures.

Specifications of infrared thermometers include the temperature accuracy (usually plus or minus a degree or two). The field of view or distance to spot ratio (D:S) which measures the diameter of the temperature measurement area compared to the distance between the thermometer and the food being tested. For example, to measure a target area one inch wide from a distance of 6 inches you would need a sensor with a D:S of 6:1 or greater.

Emissivity is the natural level of infra red radiation from the surface of any material. Emissivity is measured on a scale from 0.1 to 1.0, where 1.0 represents the radiation from a black body. Some of our thermometer's sensors have fixed whilst others have an adjustable emissivity setting. If fixed, you would not get accurate readings from shiny surfaces (because most sensors are calibrated for non-shiny surfaces).

See our infrared thermometer range

Spirit-filled thermometers

As the temperature rises a spirit in the tube expands and as it does it rises up the tube. The temperature is read off a scale at the side of the tube.

You will find examples of spirit-filled thermometers in our fridge/freezer, oven, food and drink, room, bath, garden, workplace and healthcare sections.

Dial thermometers

They use a bi-metallic strip wrapped into a coil, with one end of the coil fixed to the housing of the thermometer whilst the other end drives the pointer.

The strip consists of two strips of different metals, usually steel and copper, which expand at different rates as they are heated. The metal with the higher expansion is on the outer side of the coil. When the strip is heated it causes the metal to bend one way and the other way when cooled. Although our dial thermometers are available in different casing, plastic, stainless steel etc the technology behind them is the same.

As with our spirit-filled thermometers, you will find examples of dial thermometers in many sections. They are popular for use in ovens, fridges and as room thermometers.

Digital fridge thermometers

A sensor is placed in the fridge and the electronic display outside the fridge. A very thin wire connects the sensor and the display and passes through under the fridge's door seal.

See our fridge feezer thermometer range

Thermometer Probes

Choosing a sensor best suited to your needs for measurement range, accuracy and response time. Some accuracy levels are set by legislation or by

The sensor must also match the instrument. Our standard probes use three basic sensor types:-

1. Thermocouples

Essentially these are two wires of dissimilar metals joined together at the measurement tip (the hot junction) and joined at the other end to the input terminals of the instrument (the cold junction).

The two main thermocouple types are:

Type K Thermocouple

General purpose thermocouple with a wide measurement range and a fast response to temperature changes.

Type T thermocouple

Particularly suited to low and sub-zero temperatures such as those found in autoclaves and health applications. Also has a fast response to temperature changes and high accuracy, useful for food applications.

2. Thermistor (PST)

Used for precision measurements using a semi-conductor sensor. Provides high accuracy over a narrow temperature range and good performance over long lead runs. Suitable for food and cold store applications.

3. Platinum Resistance Thermometers

The electrical resistance of these sensors changes with temperature and this is measured to give the most accurate temperature measurements. We supply PT100 sensors.