Physics

Measurement of humidityJoel Green

ian Dyer

Abstracthumidity is a measure of the amount of water vapour present in a gas.

it can be expressed in a number of ways. Absolute humidity is the mass

of water vapour present in a given volume of gas, and is expressed as

mg/ml or g/m3. Relative humidity is the ratio of the mass of water vapour

present in a volume of gas to the maximum amount of water vapour the

gas could hold at a specific temperature, and is expressed as a percent-

age. The dew point is the temperature at which the gas is fully saturated

and condensation begins to occur. hygrometers are instruments which

measure humidity; they can measure absolute humidity, relative humidity

or dew point. They range from simple mechanical hygrometers, through

psychrometers, to more complex electrical and optical instruments. The

importance of humidity for anaesthetists lies in the comfort of the thea-

tre environment, safety with regard to static electricity, adverse effects of

dry medical gas supplies and heat loss from the body. Other examples

of when measurement and control of humidity are important in medicine

include sterilization of equipment and neonatal incubators.

Keywords absolute humidity; dew point; hygrometer; relative humidity;

water vapour

Definitions

Humidity is a measure of the amount of water vapour present in a gas. It can be expressed in several ways.

Absolute humidityThis is the mass of water vapour present in a given volume of gas, expressed as mg/ml or g/m3.

Absolute humidity = Mass of water vapour/Volume

Relative humidityThis is the ratio of the mass of water vapour present in a vol-ume of gas relative to the maximum amount of water vapour

Joel Green, MBBCH, FRCA, is Specialist Registrar in Anaesthetics at the

University Hospital of Wales, Cardiff (UK). Conflicts of interest: none

declared.

Ian Dyer, FRCA, is Consultant Anaesthetist at the Princess of Wales

Hospital, Bridgend (UK). His interests include regional and orthopaedic

anaesthesia and medical education. Conflicts of interest: none

declared.

ANAEsThEsiA AND iNTENsiVE cARE MEDiciNE 10:1 4

the gas could hold at a specific temperature. It is expressed as a percentage:

Relative humidity = (Mass vapour present/mass when saturated) × 100

It may also be calculated using partial pressures:

Relative humidity = Water vapour pressure/Saturated vapour pressure

Dew pointAs a gas is cooled it reaches a temperature at which it can hold no more water vapour and is fully saturated. Any further cooling causes condensation of the water vapour into liquid form. The temperature at which this occurs is called the dew point.

Importance for anaesthetists

Operating theatre relative humidity is usually between 40% and 60%. Humidity levels below 35% cause dry eyes, throat and skin, and excessive thirst. Evaporation is more rapid at low humidity, increasing heat loss via sweat and body fluids. Above 50% rela-tive humidity, static build-up is minimized. High humidity levels are uncomfortable.

Breathing dry gas supplied by cylinders or pipelines can cause drying, keratinization and ulceration of the mucosal membranes, and reduce ciliary action. Mucus secretions become more vis-cous and may plug small airways leading to atelectasis. Increas-ing the humidity of inspired gases can reduce these problems. Circle breathing systems achieve this by allowing rebreathing of expired gas. In addition, the use of soda lime to remove carbon dioxide from the circle liberates heat and water as part of the chemical reaction. A heat and moisture exchange filter causes moisture from expired gas to condense within the filter. Some condensation will evaporate when drier inspired gas passes through the filter, raising inspired humidity. In intensive care settings, hot water bath humidifiers and nebulizers are used to increase humidity.

Measurement of humidity

Devices which measure humidity are called hygrometers. They can measure absolute humidity, relative humidity or dew point temperature.

Mechanical or hair hygrometersThis is an old type of hygrometer consisting of a mechanical strain gauge attached to a tensioned length of material (usu-ally hair, nylon or cellulose), which expands or contracts in proportion to the relative humidity (Figure 1a). Its response curve is non-linear, which must be compensated for within the mechanism. It is simple and inexpensive, but disadvan-tages include the tendency for calibration to drift, ‘setting’ of the material if it remains at a constant humidity for prolonged periods of time, slow response times and unsuitability for low temperature use.

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Physics

Methods of measuring humidity

a Hair hygrometer. b Wet and dry bulb psychrometer. c Regnault’s hygrometer. d Chilled mirror hygrometer. e Bulk polymer hygrometer. f Optical absorption

hygrometer. LED, light-emitting diode; UV, ultraviolet; IR, infrared

a b

e f

c d

Hair

Wet temperature

Dry temperature

Muslin wickWater

∆T

Air LED

Detector

Mirror

°CSilver test tube

Ether

Gas

UV/IR

source

Sensor

Polymer

Electrodes

Figure 1

Wet and dry bulb psychrometerA psychrometer is a simple device which uses the tempera-ture difference between two thermometers to calculate relative humidity (Figure 1b). One thermometer has its bulb wrapped in a muslin wick dipped in distilled water and measures ‘wet tem-perature’. The other measures ‘dry temperature’. As air passes over the two thermometers water evaporates from the wick. This causes one thermometer to read lower than the other, the tem-perature difference being the wet bulb depression. The rate of evaporation depends on ambient humidity, the wet bulb depres-sion is inversely proportional to the humidity. Relative humidity is obtained from a psychrometric chart using the dry temperature and the wet bulb depression. They are inaccurate at low humidity levels or if the wick becomes clogged, since maximum cooling of the wet bulb may not be achieved. The wet bulb depression falls to zero at 100% humidity, so at high humidity levels platinum- wire thermometers connected via a differential amplifier provide the best accuracy. Wet and dry bulb psychrometers should not be used in enclosed spaces as evaporation from the wet bulb will increase the humidity.

Optical condensation hygrometersRegnault’s hygrometer: a thermometer measures the tempera-ture of ether within a silver test tube (Figure 1c). Air is bubbled through the ether causing it to evaporate and cool owing to the latent heat of vaporization. When condensation is observed on the surface of the test tube, the temperature of the ether is noted. This is the dew point temperature, which together with ambient temperature, allows absolute humidity to be read from a graph.

Chilled mirror hygrometer: more complex instruments detect surface condensation on a cooled mirror (Figure 1d). A light-emitting diode shines on the mirror surface. The amount of light reflected is measured by a photodetector. Condensation scatters

ANAEsThEsiA AND iNTENsiVE cARE MEDiciNE 10:1 46

the light so less reaches the detector. A feedback mechanism keeps the mirror at the dew point temperature, which is mea-sured electronically by a thermometer embedded in the mirror. These devices are one of the most accurate measures of humid-ity, and are widely used as a calibration standard. However, they are expensive, complex and require skilled maintenance. Accur-acy is limited by that of the thermometer. Contamination of the mirror surface will also cause errors. This is minimized by peri-odically heating the mirror to above dew point temperature and recalibrating, either manually or as part of an automated cycle.

Bulk polymer hygrometersThese consist of a hygroscopic polymer layer sandwiched between a metal electrode base plate and an upper water-perme-able electrode (Figure 1e). Absorption of water by the polymer layer, in proportion to the ambient relative humidity, changes the electrical characteristics of the sensor. These hygrometers mea-sure either resistance or capacitance changes depending on the polymer used. They are cheap, compact and accurate and have a fast response time. Resistance sensors are resistant to most con-taminants, whereas the capacitance sensors are not.

Optical absorption hygrometersElectromagnetic radiation passing through a gas or vapour pro-duces a characteristic absorption spectrum specific to that gas. Beer’s law states that the amount of radiation absorbed by a gas is proportional to the concentration of the gas and to the path length of the radiation beam. This principle can be used to measure the concentration of water vapour present, that is, the humidity. Radiation in either the infrared spectrum or the ultra-violet spectrum is used. The gas to be analysed passes through a sample chamber. It is illuminated by two different wavelengths of light – one having maximal absorption by water vapour and the other having minimal absorption to act as a reference. Reflecting

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Physics

mirrors may be used to increase the radiation beam path length without increasing the size of the instrument.

The relative humidity is derived from the absorption ratio. This method minimizes errors due to absorption of radiation by other gases. These instruments are accurate, have rapid response times and are unaffected by contaminants. However, they are complex and expensive and require periodic calibration.

Medical applications

EnvironmentSimple mechanical hygrometers are suitable for monitoring ambi-ent humidity in theatre.

IncubatorsIncubators for premature babies require accurate measure-ment and control of humidity. Bulk polymer sensors are most suitable.

ANAEsThEsiA AND iNTENsiVE cARE MEDiciNE 10:1 4

SterilizersSterilizers use a mixture of chemicals, pressure, heat and steam. Measurement and control of the environment is crucial to ensure efficient sterilization. Bulk polymer resistance sensors are used as they are the most resistant to corrosion.

Medical gas suppliesPipeline and cylinder gas must be dry, as moisture can act as a focus for bacterial growth. Precise humidity measurement and control is required and so chilled mirror hygrometers are used. ◆

FuRthER READIng

Davis PD, Kenny GNc. Basic physics and measurement in anaesthesia,

5th edn. London: Butterworth heinemann, 2003.

Wiederhold P. Water vapor measurement: methods and

instrumentation. New york: Marcel Dekker, 1997.

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