Calculator
Psychrometric Trend Graph
The chart compares saturation humidity ratio and a 60% relative humidity curve across temperatures.
Example Data Table
| Temperature (°C) | Relative Humidity (%) | Pressure (kPa) | Humidity Ratio (kg/kg) | Humidity Ratio (g/kg) | Dew Point (°C) |
|---|---|---|---|---|---|
| 20.00 | 40.00 | 101.325 | 0.005783 | 5.783 | 5.99 |
| 24.00 | 50.00 | 101.325 | 0.009277 | 9.277 | 12.94 |
| 28.00 | 65.00 | 100.000 | 0.015639 | 15.639 | 20.81 |
| 32.00 | 70.00 | 99.000 | 0.021609 | 21.609 | 25.84 |
| 18.00 | 55.00 | 101.325 | 0.007033 | 7.033 | 8.82 |
Formula Used
The calculator uses standard psychrometric relationships for moist air. First, it estimates saturation vapor pressure from dry bulb temperature using the Magnus-Tetens equation.
Saturation vapor pressure: Pws = 0.61094 × exp[(17.625 × T) / (T + 243.04)]
Actual vapor pressure: Pv = RH × Pws / 100
Humidity ratio: W = 0.62198 × Pv / (P − Pv)
Where T is temperature in °C, RH is relative humidity in percent, P is total atmospheric pressure in kPa, and W is kilograms of water vapor per kilogram of dry air.
How to Use This Calculator
- Enter dry bulb temperature in degrees Celsius.
- Type the current relative humidity percentage.
- Enter local atmospheric pressure in kilopascals.
- Press the calculate button to view results.
- Review humidity ratio, dew point, and vapor pressures.
- Use the graph to compare moisture behavior visually.
- Download result or example data as CSV or PDF.
FAQs
1. What does humidity ratio measure?
Humidity ratio measures water vapor mass per dry air mass. It is usually expressed as kilograms of water per kilogram of dry air, or grams per kilogram.
2. Why is humidity ratio useful in environmental work?
It helps assess ventilation, drying, thermal comfort, air treatment, and moisture control. It is more stable than relative humidity when comparing different temperatures.
3. Does atmospheric pressure affect humidity ratio?
Yes. Lower pressure changes the relationship between vapor pressure and dry air mass. High-elevation projects should use local pressure for better estimates.
4. Is humidity ratio the same as relative humidity?
No. Relative humidity compares current moisture with saturation at the same temperature. Humidity ratio measures the actual moisture mass mixed with dry air.
5. What units are shown for the result?
The calculator reports humidity ratio in kg/kg dry air, g/kg dry air, and grains per pound. These formats support engineering and environmental review.
6. Why is dew point included?
Dew point shows the temperature where condensation begins. It helps identify moisture risk on surfaces, ducts, walls, and cooling equipment.
7. Can I use this for HVAC and indoor air studies?
Yes. It supports psychrometric review for ventilation, humidification, dehumidification, energy analysis, and comfort evaluations in buildings and controlled spaces.
8. Are these results exact for every condition?
No model is exact everywhere. These formulas are reliable for practical environmental calculations, but specialized research may require advanced psychrometric standards or software.