EKS-HUM-012 / Moist-air state and psychrometric properties

Psychrometric Chart

湿空气线图

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

Aliases: moist-air chart

QUANTITY, SYMBOL AND UNIT

Quantity / objectGraphical representation of moist-air properties

UnitNot applicable

Definition

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

A state point is located from two independent properties, after which other properties can be read or estimated. Typical chart families include dry-bulb temperature, thermodynamic wet bulb, RH, humidity ratio, enthalpy and specific volume.

Relations and distinctions

Chart and calculator

A chart gives a visual relationship and approximate reading; a calculator applies equations numerically under its stated model.

One chart and another pressure

A chart prepared for sea-level pressure cannot be assumed accurate at a substantially different altitude or barometric pressure.

Engineering meaning

Use the chart to visualise heating, cooling, humidification, dehumidification and mixing paths.
Select the chart matching the project pressure or use equations with the actual pressure.

Common misconceptions

MisconceptionOne psychrometric chart applies unchanged at every altitude.

Accurate explanationHumidity ratio, enthalpy and specific volume coordinates depend on total pressure, so charts have a pressure or altitude basis.

Application boundaries

Manual chart readings have plotting and interpolation uncertainty.
A chart state point does not include moisture generation, leakage, transient surfaces, equipment curves or control behaviour.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, psychrometric charts and Figure 2 description; lines 798–835
Supports: A psychrometric chart graphically relates moist-air properties; Each chart has a stated pressure or altitude basis; The chart includes dry bulb, wet bulb, RH, humidity ratio, enthalpy and specific-volume families

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