EKS-HUM-010 / Moist-air state and psychrometric properties

Saturation Vapour Pressure

饱和水蒸气压

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

Aliases: saturation vapor pressure, saturated water-vapour pressure

QUANTITY, SYMBOL AND UNIT

Quantity / objectSaturation vapour pressure

Symbolp_ws

SIPapascal

SIkPakilopascal

Definition

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

It depends primarily on temperature and on whether the reference condensed phase is liquid water or ice. Psychrometric calculations use it to relate actual vapour pressure to RH and to solve dew- or frost-point temperature.

Relations and distinctions

Saturation and actual vapour pressure

p_ws(T) is the equilibrium limit at the specified temperature; p_w is the actual vapour partial pressure in the air sample.

Water and ice branches

Below the triple-point region, saturation over ice and over supercooled liquid water are not the same relation.

Engineering meaning

Use the phase-appropriate saturation relation when calculating dew point, frost point or RH.
Keep the correlation's temperature range and pressure model with any software implementation.

Common misconceptions

MisconceptionSaturation vapour pressure is a fixed property of air.

Accurate explanationIt is principally a function of temperature and the reference water or ice phase, not a fixed value for an air sample.

Application boundaries

Do not extend a correlation beyond its published temperature range.
At elevated total pressures, distinguish pure-water saturation pressure from the saturation vapour pressure in real moist air.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, saturation-pressure equations and perfect-gas relations; lines 458–503 and 599–603
Supports: Saturation pressure is a function of temperature; The liquid-water and ice branches are different; Saturation pressure of pure water differs slightly from saturation vapour pressure in moist air at elevated total pressure

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