Temperature and Humidity

Describe the measurement of temperature and humidity

Temperature is average kinetic energy of molecules within a material, and is measured in degrees. It is distinct from heat, which describes the transfer of thermal energy from one body to another body, and is measured Joules. The two are related by the specific heat capacity, which describes how much energy (J) must be applied to a body to raise its temperature by 1°K, without a change in state.

Humidity may be either absolute or relative:

  • Absolute Humidity is the mass of water vapour in a volume of air
  • Relative Humidity measures the percentage saturation of air at current temperature, or more formally:

Measurement of Temperature

Temperature is measured by a number of methods:

Liquid Expansion Thermometry

This is used in mercury thermometers. These consist of:

  • A graduated evacuated capillary of negligible volume, attached to
  • A mercury reservoir, of much greater volume, separated by
  • A constriction ring
    Prevents travel of mercury up the capillary by gravity.


  • When heated, the kinetic energy of the mercury increases and it expands, forcing it up the capillary
    As the thermal expansion coefficient for all liquids is very small, the capillary must be of a very small volume to create a usable device.
  • The speed that this occurs is related to the time-constant of the system
    This is typically 30 seconds. Measurement therefore takes ~4 time-constants, or 2 minutes.


  • Easy to use
  • Accurate
  • Reusable
  • Sterilisable
  • Cheap


  • Slow response
    Only accurate once it has reached thermal equilibrium.
  • Glass can break
    May cause release of mercury or alcohol.
  • Inaccurate at:
    • Low temperatures with mercury
      Freezes at -38.8°C.
    • High temperatures with alcohol
      Boils at 78.5°C.


Electrical methods include:

  • Resistance thermometer
    Platinum wire increases electrical resistance with increasing temperature.
    • Therefore the voltage drop across the wire will correspond to the temperature of the wire
    • Change in resistance is linear across the temperature range
    • However, these are expensive.
  • Thermistor
    Metal (e.g. SiO2) semiconductor which changes its resistance in a predictably non-linear fashion (run-away exponent) with temperature.
    • Can be manufactured so that change is linear over the clinical range
    • Much cheaper than wire resistance methods
    • The degree of voltage drop is usually very small, however this can be amplified using a Wheatstone bridge
    • e.g. Pulmonary artery catheter, oesophageal probe
  • Thermocouple
    At the junction of two dissimilar metals, a potential difference will be produced proportional to their temperature. This is known as the Seebeck effect.
    • Non-linear (wash in exponent)
    • Degrade over time

Measurement of Humidity

Humidity can be measured by a number of methods:

  • Hair Hygrometer
    Hair (actual hair) changes elasticity depending on the humidity of air. Changes in elasticity can be related to changes in humidity.

  • Wet and Dry Bulb
    This system measures both temperature and relative humidity.

    • Two thermometers are used
      • One is wrapped in a wick, which is attached to a water reservoir
        This is the wet thermometer.
      • The dry thermometer gives a measurement of surrounding air temperature
    • The wet thermometer is cooled due to evaporative cooling from the wick
      High energy water molecules become vapour, leaving only low energy molecules behind.
    • The temperature difference between the thermometers is a function of:
      • Latent heat of vapourisation of water
      • How much evaporative cooling is occurring
        This is function of humidity.
        • At 100% relative humidity, no evaporative cooling will take place and the temperatures will be equal
        • As humidity decreases, evaporative cooling will cool the wet thermometer, and the temperature difference allows humidity to be determined


  1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
  2. Alfred Anaesthetic Department Primary Exam Tutorial Series
Last updated 2021-04-22

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