Method
Magnus-type saturation vapour-pressure correlation for normal HVAC temperatures, followed by moist-air humidity-ratio and enthalpy relations.
Equations
pws = 0.61094 · exp(17.625 · t / (t + 243.04))
pw = φ · pws
W = 0.621945 · pwp − pw
h = 1.006 · t + W · (2501 + 1.86 · t)
Symbols and units
| Symbol | Quantity | Unit |
|---|---|---|
| Inputs | ||
| t | Dry-bulb temperature | °C |
| φ | Relative humidity | 0–1 |
| p | Atmospheric pressure | kPa |
| Results | ||
| pws | Saturation vapour pressure | kPa |
| pw | Vapour pressure | kPa |
| W | Humidity ratio | kg/kg dry air |
| W | Humidity ratio | g/kg dry air |
| h | Moist-air enthalpy | kJ/kg dry air |
Assumptions and limits
- Magnus correlation: ps=0.61094·exp(17.625·T/(T+243.04)); not an ASHRAE table reproduction.
- Normal HVAC temperature range only.
In practice
- Use the site air pressure for p when the project is well above sea level; the calculator offers the standard atmosphere as its default.
Worked example
The input values are the calculator's own verification test, run through the calculator. They show the working; they are not design values.
Inputs
| t | Dry-bulb temperature | 0 °C |
| φ | Relative humidity | 0.5 0–1 |
| p | Atmospheric pressure | 101.325 kPa |
Results
| pws | Saturation vapour pressure | 0.61094 kPa |
| pw | Vapour pressure | 0.30547 kPa |
| W | Humidity ratio | 0.00188068 kg/kg dry air |
| W | Humidity ratio | 1.88068 g/kg dry air |
| h | Moist-air enthalpy | 4.70358 kJ/kg dry air |
A preliminary check for engineer review, not an engineering approval. The project calculation to the applicable codes, authority requirements and specification governs.