A Guide to Types of Thermometers Apr. 29, 2015


We can feel how hot or cold something is. However, sometimes things are just too hot or cold for us to feel safely. At other times we need to know exactly how hot or cold something is. When we need to measure temperature (t°) correctly we need to use an instrument called a thermometer. This measures t° in degrees Celsius (sometimes called centigrade) or degrees Fahrenheit. The measurement is always indirect, that is it is a measurement not of t° itself but of the effect of t° on some physical property of a material used in the thermometer. In response to changes in t°, for example, solids, liquids and gases expand and contract; the pressure of a gas that is kept at a constant volume changes; and the electrical resistance of a metal or alloy increases and decreases.

Most thermometers have scales marked off in degrees, the units in which t° is measured. The most common scales are Fahrenheit, Celsius and Kelvin.

Different types of thermometers are intended to be used to measure different types of environments for t° settings. Choosing the right thermometer for a task is essential for getting accurate readings on the items you are measuring.

There is a variety of thermometers available, each with a specific use. Understanding how different thermometers work will help you to find a thermometer that will be most suitable for your needs. This will help to ensure that the results you provide are credible so that you can trust the information you are using to make decisions.

Types of Thermometers

Liquid-in-glass / Traditional / Spirit-filled Thermometers

The most common, often the cheapest and most popular type of thermometer. Traditional thermometers are designed for at home use for measuring the t° of the air or water in both °F or °C. These should not require much maintenance, but they may have a varied accuracy, range and resolution. They consist of a narrow, sealed glass tube with a bulb filled with mercury, alcohol (usually dyed red, to make it easily visible), or some other liquid at its lower end.

As the t° rises, the liquid expands and rises in the tube. A drop in t° causes the liquid to contract, so that its level in the tube falls.

Dual or Multiple Sensor Thermometers

These thermometers are designed for professional use as they allow the simultaneous monitoring of two or more environments over both short and long periods of time. They are ideal for applications such as indoor and outdoor t° comparisons, the monitoring of storage areas, and classroom maintenance. The main reader of this thermometer can be mounted to a desk or wall and the probes can be inserted into a variety of environments for readings.

  • Typically consist of main display unit and external t° sensors
  • Each external sensor reading is displayed on the main unit
  • The main unit can typically be wall or desk mounted
  • Traceable accuracy is recommended
  • Variable measurement range, resolution and accuracy

Irreversible Temperature Labels

Temperature indicating labels are self-adhesive t° proofing foils. They are the ideal solution to monitor and verify t° specific operations such as sterilisation or climate conditions during storage and transit. They are also suitable for use where t° values should not be exceeded, such as engine or bearing monitoring. They are a relatively cheap, yet effective way to achieve permanent records of accuracy.

The labels are produced in a variety of t° points and sizes and permanently darken when exposed at or above the indicated t° point. The labels are self-adhesive and resist oil, water, and steam, which allows them to be placed on any smooth surface quickly and easily.

  • Each segment has both °C and °F values
  • Each label is oil and water resistant
  • The colour change is irreversible even after the object being measured cools down
  • After use, labels can be removed
  • Variable measurement range, resolution and accuracy

Fixed and Wired Probe Thermometers

Probe thermometers are designed to deliver instant and accurate t° readings of foods, liquids and semi-solid samples via its fixed probe, which will be suitable for both immersion and penetration. They are ideal for use in the catering trade for hygiene testing, retail outlets, and laboratories and they will vary in size and focus on providing solid, accurate readings to ensure user safety.

  • Very easy to use and highly portable
  • Probes will vary in size, material, and manoeuvrability
  • Wired probe thermometers are similar to fixed probe thermometers, the difference being that the probe is attached to the thermometer via a wire.
  • This maximises utility without compromising accuracy
  • Traceable accuracy is recommended if used in a professional environment
  • Variable measurement range, resolution and accuracy

Thermocouple / K-type Thermometers

These thermometers are designed to deal with extreme t°s and are normally found in laboratories and industry. They cater for applications requiring high precision, and can facilitate a range of interchangeable plug in probes for air, liquid, penetration and surface t° measurements. The specific t° range will vary by brand but they work to provide a non-slip grip that is manoeuvrable for use in a variety of environments without compromising accuracy. They're often the best choice of a thermometer if you have several applications in one professional environment.

  • They offer wide t° measurement ranges, high accuracy, and fast measurement response times
  • Non-slip function buttons and impact-resistant casing make these thermometers ideal for use in harsh working environments
  • Probes will vary in size, material, and manoeuvrability
  • Traceable accuracy is recommended if used in a professional environment
  • Variable measurement range, resolution and accuracy

Data Logging Thermometers

These provide t° results in real time that can be downloaded to a computer to note the t° history of an environment to check for variances. This can be used to provide graphical data and accurate readings that are suitable for professional use.

  • The recorded measurements can be attached to emails or reports, and is great for fault finding or traceability in controlled environments
  • A user may select the logging rate, start time, alarm limits, and logging mode
  • The supplied analysis software provides a graphical display of data, showcasing the total readings, a zoom facility for individual readings, and a visual indication of alarm levels throughout the logging period
  • Traceable accuracy is recommended if used in a professional environment
  • Variable measurement range, resolution and accuracy

Infrared (IR) Thermometers

An infrared (IR) thermometer, measures the infrared (heat) radiation given off by an object to determine its t° and can measure very high or low surface t°s without the need to make contact with the item in question, which is ideal for spot checking t° settings within an environment. The pyroelectric thermometer contains a type of crystal that produces an electrical signal whose strength is proportional to the infrared radiation to which it is exposed.

  • Important factors to consider before measuring are laser spot-size and the emissivity value of the surface being measured
  • The error in the accuracy of a measured value can be as much as 30% of the reading just based on emissivity
  • Traceable accuracy is recommended if used in a professional environment
  • Variable measurement range, resolution and accuracy

Thermal Imaging Cameras

A thermal imaging camera allows immediate identification of hot and cold areas via thermal imagery. They are ideal for detecting hidden problems, surveying buildings to find moisture and leaks, identifying energy loss and poor insulation, electrical faults, and detecting plumbing issues.

  • Some cameras facilitate the download of imagery to PCs or Laptop computers.
  • The recorded data can then be attached to emails or reports, and is great for fault finding or traceability in controlled environments
  • Variable image display resolution
  • Some models include a correction for emissivity and reflected t°
  • Variable measurement range, resolution and accuracy

Liquid-crystal Thermometers

These contain liquid crystals that change color over a certain range of t°s. Various mixtures of liquid crystals are used, each mixture in a separate container and each becoming light in color at a specific t°. In digital types, each container is covered with a stencil of a number that corresponds to the t° at which the mixture in the container becomes light in color. Fever thermometers designed to be placed on the forehead and most indoor digital thermometers are of this type.

Gas Thermometers

The most common type in industry, the constant-volume gas thermometer determines t° by measuring the pressure needed to keep the volume of a gas constant as the t° varies. The pressure of a gas confined to a constant volume varies with t° in a known manner; if the pressure of the gas is measured, its t° can be easily calculated.

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