#Temperature measurement

Liquid thermometer

Heating causes thermometric fluid (e.g. mercury or alcohol) to expand.

Increased volume is measured as height of a column calibrated to temperature.

Pros: Cheap, no power required, reliable, no calibration.
Cons: Slow, invasive, hazardous if broken.

Bourdon gauge thermometer

Thermal expansion of fluid pushes a spiral metal tube which unwinds and moves a pointer on a calibrated scale.

Pros: Cheap, no power required, less invasive.
Cons: Inaccurate, slow.

Bimetallic strip thermometer

Two bonded metal strips (e.g. copper and steel) expand at different rates, causing them to bend. The degree of curvature is calibrated to temperature.

Pros: Cheap, no power required.
Cons: Slow, inaccurate.

Differing rates of thermal expansion between the two metals (orange and grey) cause the strip to bend as temperature changes.

Liquid crystal thermometer

Liquid crystals change colour with temperature. The colour is compared with a calibrated scale.

Pros: Cheap, no power required
Cons: Limited temperature range, inaccurate, subjective.

Infrared thermometer

Measures electromagnetic radiation emitted by the body, which is within the infrared spectrum.

Higher temperature → higher frequency radiation.

Pros: Non-invasive, sensitive.
Cons: Less accurate; incorrectly assumes the patient behaves as a blackbody (that all energy is emitted as thermal energy, where the frequency is solely dependent on temperature).

Thermocouple

Relies on the Seebeck effect: a voltage develops at the junction of two different metals and varies predictably with temperature

Pros: Rapid and reliable over a broad range of temperatures that far exceed what can be found in the human body.
Cons: Expensive, requires recalibration.

This is a common thermocouple configuration using chromel and alumel metal alloys. The voltage detected by the voltmeter (right) correlates with temperature.

Thermopile

Multiple thermocouples connected in series to increase sensitivity.

Pros: Greater sensitivity.
Cons: More complex and costly.

Resistance thermometer

Commonly made from platinum (platinum resistance thermometer).

Electrical resistance in the wire increases with temperature; measured using a Wheatstone bridge and calibrated to temperature.

Pros: Accurate, sensitive, minimal calibration drift.
Cons: Occasional recalibration required.

Resistance increases linearly with temperature in resistance thermometers.

Thermistor

Electrical resistance of a semiconductor decreases with increasing temperature; measured using a Wheatstone bridge.

Often used for temperature measurement and calculation of cardiac output (via thermodilution) in pulmonary artery catheters.

Pros: Cheap, reliable, fast.
Cons: Calibration drift; damaged by sterilisation heat and so cannot be reused.

Resistance in thermistors is inversely related to temperature.

Fibre-optic thermometry

Gallium–arsenic crystal at fibre tip absorbs light. The absorption spectrum varies with temperature.

An illuminator transmits light to the crystal and the absorption spectrum is analysed by spectometry.

Pros: Accurate in high electromagnetic interference environments (e.g. MRI).
Cons: Expensive.

Fibre-optic thermometer.

By Lady Mann CC-BY-SA 3.0