Psychrometric Definitions

Introduction

Air is a mechanical mixture of gases and water vapour. Dry air (air without water vapour) is composed mainly of nitrogen (78% by volume), and oxygen (21%), the remaining 1% being made up of carbon dioxide and minute quantities of other gases. With regard to these dry air components, the composition of air is practically the same everywhere. The amount of water vapour in the air however, varies greatly between particular locations and prevailing weather conditions.

  
 
Psychrometric Chart (MOUSE ROLLOVER)
Psychrometric Chart - Common Processes
 

Since all air in the natural state contains a certain amount of water vapour, there is no such thing as "dry air". Nevertheless, the concept of dry air is useful in that it simplifies psychrometric calculations. The term "dry air" is normally only used when referring to air without water vapour, whereas the terms "air" and "moist air" refer to the natural mixture of dry air and water vapour.

Dalton's Law states that in any mechanical mixture of gases and vapours that are not combined chemically, each gas or vapour in the mixture exerts an individual partial pressure that is equal to the pressure that the gas would exert if it occupied the space alone, and the total pressure of the gaseous mixture is equal to the sum of the partial pressures exerted by the individual gases or vapours.

Air, being a mechanical mixture of gases and water vapour, obeys Dalton's law. Total barometric pressure may therefore be considered to be the sum of the pressures exerted by the dry gases and the partial pressure exerted by the water vapour.
   

Dry Bulb Temperature °Cdb

Temperature of air as measured by a standard thermometer with a dry sensing bulb. Temperature (dry bulb) is always shown on a standard psychrometric chart.

Wet Bulb Temperature °Cwb

Temperature of air as measured by a thermometer using a sensing bulb covered by a wet wick. As the thermometer is moved through the air, water will evaporate from the wick at a rate determined by the relative humidity of the surrounding air. As the water evaporates, latent heat is adsorbed from the wick which lowers the bulb temperature. The drier the air the more the thermometer cools and hence, the lower the wet-bulb reading.

At 100% RH there would no moisture evaporation from the wick (therefore no cooling effect) so the wet bulb temperature would be the same as dry bulb temperature.

Temperature (wet bulb) is always shown on a standard psychrometric chart.
   

Sling Hygrometer

A traditional instrument used to measure both dry bulb and wet bulb temperatures. The relative humidity can then be found using a psychrometric chart.

      
Traditional sling hygrometer for measuring wet & dry bulb temperatures
To ensure that the recorded wet bulb temperature is accurate, airflow over the wet bulb must be high. The wick should be clean and only distilled water is used. A battery operated aspirating hygrometer has a small fan which pulls sample air over the wet bulb.

Relative Humidity (RH)

Relative Humidity (RH) is the most commonly used psychrometric unit. Few people however, understand what it really means.

The "relative" in relative humidity expresses the relation between the amount of water vapour present and the amount that is physically possible at that temperature. In other words, relative humidity expressed as a percent, is the partial water vapour pressure in relation to it's saturation pressure.

%RH = 100% x (pw/pws) where pw is partial pressure of water vapour and pws is water vapour saturation pressure.

Relative Humidity (RH) expressed in percent, is the ratio of the actual partial pressure exerted by the water vapour in a volume of air, to the maximum partial pressure that would be exerted by the water vapour if the air were saturated, at that temperature.

Relative Humidity is always shown on a standard psychrometric chart (percentage saturation).
   

Absolute Humidity (AH)

As the partial pressure exerted by each gas also represents the volume of each gas present (proportionally), then it follows that as the partial pressure exerted by water vapour increases, so the volume of water vapour present also increases.

Absolute Humidity is the mass of water present in unit volume of moist air at a given temperature and pressure and is measured in grams per cubic metre of air (g/m³).

Note that the term absolute humidity is often mistaken for mixing ratio or humidity ratio.

Absolute humidity is not normally shown on a standard psychrometric chart.

Water Activity (aw)

An expression of the relative avalability of water in a particular food or substance. Pure water has a water activity of exactly 1.00

The water activity of a substance is expressed as one-hundredth of it’s relative humidity.

Mixing Ratio or Humidity Ratio (w)

Humidity Ratio defines the mass of water in the volume occupied by 1 kg of dry air. In other words it is the amount of water required to be evaporated into 1 kg of dry air in order to achieve a specific condition.

Humidity Ratio is often expressed in grams of water per kilogram of dry air (g/kg).

Note that there can be a significant difference between Mixing/Humidity Ratio and Absolute Humidity.

Air conditioning engineers find Mixing/Humidity Ratio far more practical than Absolute Humidity. Unlike Absolute Humidity, Mixing/Humidity Ratio does not change with temperature (unless cooled below the dewpoint).

Note that the term "Moisture Content" is often used instead of "Mixing or Humidity Ratio". This is now generally accepted within the HVAC industry. It is also used throughout this web site.

Moisture Content is normally shown on a standard psychrometric chart (kg/kg of dry air).

Specific Humidity (SH)

The ratio of water vapour to air (dry air plus the water vapour) in a particular volume of air. Specific humidity is related to mixing ratio (and vice versa).

This is not often used in psychrometric calculations.

Dewpoint (°Cdp)

As the mass of water vapour that air can hold is dependant on the air temperature (and pressure), if the air is cooled, the amount of water it can hold is reduced and the relative humidity subsequently rises. If air is cooled to the point where it becomes 100% saturated (pws), then any further cooling will cause moisture to deposit out of the air in the form of condensation or dew. The temperature at which this occurs is the dew point of the air.

The dewpoint is the temperature below which water vapour in the air will start to condense.

Even though dewpoint is expressed as a temperature, it is determined by the amount of water vapour in the air at a given pressure. Dewpoint is always less than or equal to the actual air temperature and, at normal atmospheric pressure, must always be between -273°C and 100°C.

Dewpoint is not shown separately on a psychrometric chart as it relates directly to dry bulb or wet bulb temperatures.

Sensible Heat

May be defined as heat which when applied to (or removed from) a substance, causes only a change in temperature. Sensible heat is measured in joules per kilogram (J/kg).

Latent Heat

May be defined as heat which when applied to (or removed from) a substance, produces a change in state without any change to temperature. Latent heat is measured in joules per kilogram (J/kg).

Enthalpy of humid gas (ht)

Measure of the total energy in a humid gas. In other words, it is the sensible heat + the latent heat. Enthalpy is a useful concept in air conditioning where it is important to know how much of the "stored" energy will be consumed or released when the temperature or moisture content is raised or lowered. Enthalpy is measured in Joules per kilogram (J/kg) or more commonly kJ/kg.

The reference point for enthalpy is completely dry air at a temperature of 0°C (0°C, 0%RH = 0 J/kg).

Specific enthalpy is normally shown on a standard psychrometric chart (kJ/kg).

Note that Joule is the SI unit of Energy.

The rate of energy transfer is known as Power which is measured in J/s or watt (1 kJ/s = 1 kW).
   

Specific volume (v)

The specific volume is the volume of unit mass of dry air at a given temperature normally expressed as m³/kg and is also shown in the psychrometric chart.

Note: that the specific volume is a measure of dry air, since the addition of any associated water vapour has no effect on the volume (Dalton’s Law).

Standard Air

It is common practice to express air quantities by volume flow (m³/s) rather than by mass flow (kg/s).

However, since the volume of any given mass of air varies with temperature and humidity (pressure), an air standard has been established so that an air quantity expressed as a volume flow rate represents a known equivalent mass flow rate.

    
 
Standard Air
Standard Air is defined as air having a density of 1.2 kg/m³ or a specific volume of 0.833 m³/kg.
   
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