| Property | Value | Note |
|---|
Saturation Pressure Reference Table (-40°C to 50°C)
| Temp (°C) | Temp (°F) | Sat. Press. (kPa) | Sat. Press. (in Hg) |
|---|
Saturation pressure: the foundation of every psychrometric calculation
Saturation vapour pressure is the pressure water vapour exerts when air is fully saturated at a given temperature — the point where the air cannot hold any more moisture without condensation occurring. It depends only on temperature, not on total atmospheric pressure or elevation. This makes it a pure thermodynamic property of water, calculable from first principles using the Clausius-Clapeyron relationship and refined through empirical correlation to match measured data precisely.
Every other psychrometric property traces back to saturation pressure. Relative humidity is actual vapour pressure divided by saturation vapour pressure at the dry-bulb temperature. Dew point is the temperature at which the actual vapour pressure equals the saturation pressure (found by inverting the saturation pressure equation). Humidity ratio depends on vapour pressure, which depends on RH times saturation pressure. Understanding saturation pressure as the root calculation helps clarify why these properties relate to each other the way they do.
The ASHRAE equations: two regimes, one accurate model
The ASHRAE Fundamentals Handbook provides 2 distinct polynomial equations: one for temperatures at or above 0°C (covering liquid water saturation) and one for temperatures below 0°C (covering ice sublimation). Both are 6th-order or higher logarithmic-exponential forms fit to extensive experimental data, accurate to within 300 ppm across their respective ranges. This calculator implements both equations and automatically selects the correct one based on your input temperature.
Simplified approximations like the Magnus formula are convenient for quick mental estimates but introduce errors of 1-2% at temperature extremes — meaningful in Canadian winter calculations where outdoor design temperatures regularly span -20°C to -40°C. For professional engineering work, especially building envelope condensation analysis and commissioning verification, use the full ASHRAE equations implemented here rather than simplified approximations.
Practical application: reading the reference table
The reference table above gives saturation pressure at 5°C intervals from -40°C to 50°C, covering the full range relevant to Canadian HVAC work — from extreme prairie winter design conditions to summer cooling coil analysis. Use this table for quick lookups, or enter a specific temperature above for an exact calculation. Pair the result with the relative humidity calculator or dew point calculator to complete your psychrometric analysis.
Frequently Asked Questions
Saturation vapour pressure is the maximum partial pressure water vapour can exert at a given temperature before condensation occurs. It depends only on temperature, not total pressure. Every psychrometric calculation uses it as the foundation: RH = actual vapour pressure ÷ saturation pressure × 100. The ASHRAE Fundamentals Handbook provides precise polynomial equations far more accurate than simplified approximations, especially at Canadian winter temperature extremes. Use the relative humidity calculator to apply this directly to your conditions.
Above 0°C, the relevant phase transition is liquid water to vapour. Below 0°C, it's ice directly to vapour (sublimation) — a different process with different energy requirements and molecular bonding. ASHRAE provides 2 separate equation sets for these regimes. This matters for Canadian winter calculations, where design temperatures regularly fall below 0°C and using the wrong equation introduces meaningful errors in humidity and dew point results. This calculator automatically selects the correct equation based on your input temperature.