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Temperature measurement: non-electrical techniques

Created: 2/6/2005

Liquid expansion

  • Mercury thermometer
  • Alcohol thermometer

Dial thermometers

  • Bimetallic strip thermometer
  • Bourdon gauge thermometer


  • Liquid crystal
  • Infrared

CliniTemp moving line temperature indicator strips can be read accurately and continuously to fractions of a degree over the entire temperature range of the thermometer with no gaps in temperature.The moving line temperature technology has also been shown to reliably reflect body temperature when placed on the foreheads of anaesthetised patients within the wide range of ambient temperatures found in the OR and Recovery.

Mercury thermometer

A glass bulb filled with mercury is connected to a narrow evacuated glass capillary tube. As the mercury is warmed, it expands and rises up the capillary tube into a vacuum. (Also known as a Torricellian vacuum, which is actually mercury vapour at its saturated vapour pressure). The temperature corresponds to a point on a calibrated temperature scale, measured by the height of the fluid column.
A mercury-in-glass thermometer is effective in the range -39°C to ~250°C. If a thermometer is required for lower temperatures, the alcohol-in-glass thermometer (utilising the same principle) can be used in the range -117°C to 78°C.


  • Simple
  • Cheap
  • Re-usable
  • No power supply needed


  • Slow response time
  • Easily broken
  • Mercury toxicity
  • Risk of cross-infection (Salmonella and Clostridium)
  • Withdrawn from NHS use


Of or pertaining to Torricelli, an Italian philosopher and mathematician, who, in 1643, discovered that the rise of a liquid in a tube, as in the barometer, is due to atmospheric pressure.

1. Bimetallic strip

Two dissimilar metals (different coefficients of expansion) fixed together in a coil. Differing expansion in each metal causes the coil to wind/unwind, as the temperature changes. This causes a pointer to move across a dial.
The Bimetallic strip is a mechanical temperature sensor element. It converts temperature to a mechanical displacement. This displacement may be coupled to a switch for simple on-off function, to a needle of an indicator or to a position detector for electronic output.
By far the most common application of the bimetallic strip is as a thermostat switch used for temperature and energy control.
Bi-metallic strip


2. Bourdon gauge

Patented by French watchmaker Eugène Bourdon in 1849.

Bourdon Gauge

This is a hollow spiral metal tube, based on the Bourdon pressure gauge. The tube terminates at one end in a temperature-sensing bulb containing a volatile liquid or vapour. This expands as the temperature rises, causing the tube to unwind, which in turn moves a pointer.


  • Cheap
  • Robust
  • Easily read


  • Poor accuracy
  • Prone to calibration errors


A plastic strip impregnated with heat-sensitive chemical dots, which, when exposed to an increase in temperature, melt, resulting in a change of colour. They provide a faster reaction time than mercury, taking 1 minute for an oral reading and 3 minutes for an axillary reading. Tempadots are disposable and have the advantage that they reduce the risk of cross-infection.

Liquid crystals

Reflectivity changes with temperature


The frequency of infrared energy emitted by an object changes with temperature and can be measured. Infrared tympanic thermometers (ITTs) measure the frequency of infrared light emitted by the tympanic membrane. The operator needs to ensure that the external auditory canal is free from wax prior to use. The use of ITTs is replacing more traditional thermometry such as mercury-in-glass. Besides obvious health concerns over the use of mercury, the increase in the use of tympanic thermometers is related to their speed and relatively non-invasive method of operation.

The Exergen Temporal Scanner provides a totally non-invasive, quick, easy and reproducible method of core temperature measurement. By gently stroking the Temporal Scanner across the forehead, it will provide a reading in approximately 3 seconds without the discomfort associated with oral, rectal, or tympanic thermometers.

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