Psychrometric charts
Psychrometric charts are complex graphs that can be used to assess the physical and thermodynamic properties of gas-vapour mixtures at a constant pressure. They are often used to assess the properties of moist air. This can be useful in the design of heating, ventilation and air-conditioning systems for buildings, and psychrometric charts often include a zone in the middle that represents the range of conditions that people find comfortable under different circumstances (such as summer and winter).
Typically, the properties represented on psychrometric charts are:
- Dry-bulb temperature: A measure of air temperature recorded by a thermometer exposed to the air but shielded from radiation and moisture.
- Wet-bulb temperature: The temperature recorder by a thermometer that has its bulb wrapped in cloth and moistened with distilled water. The rate of evaporation from the wet bulb, and so the temperature it records varies depending on the humidity of the air it is exposed to.
- Relative humidity: The ratio of the actual vapour pressure relative to the vapour pressure of saturated air at the same temperature, expressed as a percentage.
- Specific volume: The volume of a unit weight of dry air.
- Dew point temperature: The highest temperature at which water vapour will condense.
- Humidity ratio: The dry-basis moisture content of air expressed as the weight of water vapour per unit weight of dry air.
- Enthalpy: The energy content of air.
The state of moist air can be determined from any two of these properties, from which all other properties can then be determined.
Atmospheric pressure varies with altitude and so a number of psychrometric charts are available for different atmospheric pressures, however, for altitudes of less than 600m the sea-level psychrometric chart is often considered adequate.
[edit] Find out more
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
HAB is a bridge design concept which incorporates an integrated hydraulic system in order to carry more weight.
ICE publish a discussion paper looking at the role of the engineer in creating inclusive cities.
A PQP describes the activities, standards, tools and processes necessary to achieve quality in a project's delivery.
How Lidl has been actively working to reinforce their brand through sustainability.
Association of British Insurers describe full-scale cladding tests as 'utterly inadequate'.
This article examines the changing policy commitments and evolving definitions of the zero carbon home.
Researchers believe they may have created a 'game-changing' new form of concrete using graphene.
Grouting refers to the injection of materials into a soil or rock formation to change its physical characteristics.
Part of Designing Buildings Wiki, BREEAM Wiki will advance knowledge sharing for the BRE family of sustainability tools.
Iron, glass and wood underfoot
From the decorative to the utilitarian, and from the photographed to the forgotten.
New BRE book considers the progression from project-based knowledge creation to whole-life urban knowledge management.
This CIOB article explores the concept of value in building design and construction.