Absolute Humidity
Ratio of the mass of water vapour in the air to the total volume of the air (g/m3).

Absolute Zero
Complete absence of energy and atomic movement defined as 0.00K, -273.15°C, or -459.67°F.

Absorption
The process of transferring matter or energy into another body.

Acceleration
The rate of change of velocity of an object when subject to a vibration stimulus.

Accelerometer
Transducer typically used to convert mechanical vibration movements into an equivalent electrical signal so that a vibration meter may measure it.

Acclimatisation
The ability of a body to adapt to its thermal environment.

Accuracy
The numerical difference between an instrument reading and the true value of the quantity being measured. Often expressed as limits that will not be exceeded when the instrument is used within it’s stated operational conditions, i.e. ±5°C.

Acoustic calibrator
A battery-powered device which emits a known sound pressure level at a known frequency, when mechanically connected to a compatible microphone, for the purpose of simple field-checking the accuracy of a sound measuring instrument.

Action level
A sound level limit at which certain duties have to be carried out according to local regulations in a country.

Actual Pressure
The atmospheric pressure at the level of measurement.

ACGIH
American Conference of Government Industrial Hygienists

Adsorption
The collection of vapours from the atmosphere by condensation on the surface of a porous solid with a high surface area (e.g charcoal). The adsorbed material can be recovered by thermal or solvent desorption.

Advection
The horizontal transport of heat or cold, in the atmosphere, or the oceans. When a warm air mass passes over a cold land or sea surface it causes the air to cool and the water vapour held within it to condense.

Air Temperature
Typically expressed in °C, it is the temperature of the ambient air.

Air Velocity
The magnitude and direction of air flow, typically expressed in metres per second.

Ambient
The surrounding environment atmosphere.

Ambient Monitoring
The monitoring of air quality outside buildings, being representative of the air that the general (or identified) population is breathing.

Ambient Noise
Encompassing sound, at a given location, usually made up from the sounds from many sources, both distant and close-by.

Ambient Pressure
The pressure of the ambient air sometimes referred to as barometric pressure.

Ambient Temperature – ta
The average temperature of the air surrounding an instrument/sensor or experienced by the operative during the work activity. This parameter forms one of the fundamental input values for the calculation of various derived values related to heat stress.

Analogue-To-Digital Converter
A device or circuit which outputs a digital number corresponding to an analogue signal input at that level.

Anechoic room
A room whose boundary surfaces absorbs substantially all the incident sound energy thereby affording non-reverberant conditions.

Anemometer
An instrument for measuring the speed of wind or air.

Aneroid Capsule
A thin semi-evacuated metal capsule, which when used in a barometer deforms with changes in pressure. One end being fixed and the other moves a pen on a chart or a hand on a dial. Aneroid means without air.

ANSI
The American National Standards Institute

ASCII
American Standard Code for Information Interchange; a seven or eight bit code used to represent alphanumeric characters.

Area, Effective Radiating (Ar)
The surface area of a body, that exchanges radiant energy with a radiant source, measured in m2.

Area, Wetted (Aw)
The area of skin covered in sweat, expressed in m2.

Asphyxia
Suffocation from a lack of oxygen. Chemical asphyxia is produced by a substance which combines with the blood’s haemoglobin to reduce its capacity to transport oxygen. An example of a chemical asphyxiant is carbon monoxide.

Aspirated
Forced air flow over a sensor.

Atmometer
The generic name for instruments used to measure the rate of evaporation of water into the atmosphere.

Audible Sound
Sound of such character as to excite the sensation of hearing.

Audiometer
An instrument for the measurement of certain characteristics of hearing, particularly, the hearing threshold level and hearing loss.

‘A’ weighting
One of the standard frequency response correction curve as specified in international standards. Used to simulate the human hearing capability.