Industrial humidity glossary

Absolute Humidity

Absolute humidity, or water-vapour density, is the mass of water vapour divided by the total volume occupied by a moist-air sample.

Adsorption

Adsorption is an increase in the concentration of a substance at an interface caused by surface forces.

Air Change Rate

Air change rate is a defined volumetric airflow divided by the volume of a space, commonly reported as air changes per hour.

Air Infiltration

Air infiltration is the unintended flow of outdoor air into a building, room or enclosure through cracks, leakage paths and normal use of exterior openings.

Materials, corrosion and risk

Atmospheric Corrosivity

Atmospheric corrosivity is the capacity of a local atmospheric environment to cause corrosion of metals, classified or estimated using a defined method and material basis.

Atmospheric Pressure

Atmospheric pressure is the total pressure exerted by the surrounding atmosphere at a stated location and time.

Installation, validation and maintenance

Condensate Drainage

Condensate drainage is the collection and removal of liquid water produced by a cooling-and-dehumidifying surface or related equipment.

Condensation

Condensation is the formation of liquid water when water vapour reaches a condition at which the vapour phase can no longer be maintained.

Control Deadband

A control deadband is a defined range of the controlled variable or error within which the controller makes no new corrective action or does not switch state.

Controlled Environment Agriculture

Controlled environment agriculture is crop production in greenhouses or other structures that use horticultural and engineering methods to control selected growing conditions.

Defrost

Defrost is an operating action or cycle that removes frost from a refrigeration evaporator so that airflow and heat transfer can continue.

Desiccant Dehumidification

Desiccant dehumidification removes water vapour from air by transferring it to a solid or liquid sorbent with a lower water-vapour pressure.

Desiccant Regeneration

Desiccant regeneration is the process of releasing collected water from a desiccant so that its moisture-removal capacity is restored.

Desiccant Rotor

A desiccant rotor is a slowly rotating, air-permeable matrix that carries desiccant between process-air and regeneration-air sectors.

Dew Point

Dew point is the temperature at which moist air reaches saturation when cooled at constant pressure without changing its water-vapour content.

Dry-bulb Temperature

Dry-bulb temperature is the air temperature indicated by a suitable temperature sensor whose sensing surface is dry and protected from avoidable radiation and moisture effects.

Energy and electrical enclosures

Electrical Enclosure Moisture Control

Electrical enclosure moisture control is the coordinated control of vapour entry, internal moisture, air dew point and surface temperature to keep enclosure conditions within the equipment's defined environmental limits.

Entering-air Condition

Entering-air condition is the stated thermodynamic and flow state of the air at a defined equipment or process inlet.

Frost Point

Frost point is the temperature at which water vapour in a gas is in equilibrium with a plane surface of ice at the stated pressure.

Humidistat

A humidistat is a humidity-responsive controller that switches or modulates equipment according to a humidity input and control setting.

Humidity Ratio

Humidity ratio is the mass of water vapour in a moist-air sample divided by the mass of dry air in that sample.

Humidity Setpoint

A humidity setpoint is the desired value of the humidity variable that a controller is instructed to maintain.

Hygrometer

A hygrometer is an instrument that measures a defined humidity quantity in a gas, such as RH, dew or frost point, water amount fraction or water mass ratio.

Latent Load

Latent load is the rate of heat associated with a moisture phase change or a change in the water-vapour content of an air stream or space.

Leaf Temperature

Leaf temperature is the temperature of a leaf surface or tissue at a stated measurement location and time.

Measurement Uncertainty

Measurement uncertainty is a non-negative parameter that characterises the dispersion of quantity values attributed to a measurand from the information used.

Moist-air Enthalpy

Moist-air enthalpy is the combined specific enthalpy of the dry-air and water-vapour components, normally reported per unit mass of dry air in psychrometrics.

Moisture Generation Rate

Moisture generation rate is the mass of water released as vapour by a source within a defined time interval.

Moisture Load

Moisture load is the rate at which water vapour must be removed from a defined space or process to maintain the selected humidity condition.

Plant Transpiration

Plant transpiration is the vaporisation of water within plant tissues and the release of that water vapour to the surrounding air, mainly through stomata.

Process Air

Process air is the airstream that passes through a dehumidifier to have water vapour removed for use in a space or industrial process.

Psychrometric Chart

A psychrometric chart is a pressure-specific graphical representation that relates the thermodynamic properties of moist air.

Rated Dehumidification Capacity

Rated dehumidification capacity is the water-removal rate assigned to a dehumidifier under a named test method and specified operating conditions.

Refrigerant Dehumidification

Refrigerant dehumidification uses a refrigeration cycle to cool air below its dew point, condense water on an evaporator and drain the condensate.

Regeneration Air

Regeneration air is the airstream used to restore a desiccant and carry the released water vapour away from the dehumidifier.

Regeneration Energy

Regeneration energy is the energy input used to restore a desiccant's moisture-removal capacity during a stated regeneration or reactivation process.

Regeneration Temperature

Regeneration temperature is the stated temperature of the regeneration stream at a defined point, commonly the air entering a heat-regenerated desiccant sector.

Relative Humidity

Relative humidity is the ratio of the actual water-vapour pressure in moist air to the saturation water-vapour pressure corresponding to the air temperature, expressed as a fraction or percentage.

Return Air

Return air is air removed from a space and routed back toward an air-handling, heating, cooling or ventilation system for recirculation or exhaust.

Saturation

Saturation is the equilibrium moist-air state in which the water-vapour partial pressure equals the saturation pressure for the stated temperature and condensed phase.

Saturation Vapour Pressure

Saturation vapour pressure is the equilibrium pressure of water vapour over liquid water or ice at a specified temperature.

Sensible Heat Load

Sensible heat load is the rate of heat addition or removal associated with changing or maintaining dry-bulb temperature without counting moisture phase change.

Sensible Heat Ratio

Sensible heat ratio is the sensible heat-transfer rate divided by the sum of sensible and latent heat-transfer rates for the same process.

Sensor Drift

Sensor drift is a continuous or incremental change in a sensor's indication over time caused by changes in the metrological properties of the measuring instrument.

Specific Humidity

Specific humidity is the mass of water vapour divided by the total mass of the moist-air sample.

Supply Air

Supply air is the air delivered by a ventilation, heating, cooling or air-conditioning system to a space.

Surface Temperature

Surface temperature is the temperature at the boundary of a material, component or item of equipment at a specified location and time.

Thermal Bridge

A thermal bridge is a local low-thermal-resistance path that allows heat to flow more readily between two surfaces than through the surrounding construction.

Vapour Pressure Deficit

Vapour pressure deficit is the difference between saturation water-vapour pressure at a stated temperature and the actual water-vapour pressure of the air.

Ventilation Air

Ventilation air is the outdoor air intentionally introduced to a space to meet a defined ventilation requirement.

Materials, corrosion and risk

Water Activity

Water activity is the ratio of the water-vapour pressure above a material at equilibrium to the vapour pressure above pure water at the same temperature.

Water-vapour Partial Pressure

Water-vapour partial pressure is the pressure contribution assigned to water vapour within a moist-air gas mixture.

Wet-bulb Temperature

Thermodynamic wet-bulb temperature is the temperature at which water evaporating into a moist-air sample can bring it to saturation adiabatically at the same total pressure.