Calculator Inputs
Plotly Graph
The chart compares total pressure, water vapor pressure, and dry air pressure in the selected output unit.
Formula Used
Main relation: Pdry air = Ptotal − Pwater vapor
Humidity-based vapor estimate: Pwater vapor = RH × Psat / 100
Tetens saturation pressure estimate: Psat = 0.61078 × exp[(17.2694 × T) / (T + 237.3)]
Dry air moles: n = (P × V) / (R × T)
This calculator separates moist air into dry air and water vapor. The dry air portion equals the total gas pressure after removing the water vapor contribution.
How to Use This Calculator
- Enter the total mixture pressure and choose its unit.
- Select direct vapor pressure input or estimate mode.
- For estimate mode, enter temperature and relative humidity.
- Choose the output pressure unit for reporting.
- Enter sample volume to estimate dry air moles and mass.
- Press the calculate button to show the result above.
- Use the CSV or PDF buttons to save your report.
Example Data Table
| Total Pressure (kPa) | Temperature (°C) | Relative Humidity (%) | Water Vapor Pressure (kPa) | Dry Air Pressure (kPa) |
|---|---|---|---|---|
| 101.325 | 25 | 60 | 1.900 | 99.425 |
| 95.000 | 20 | 45 | 1.052 | 93.948 |
| 110.000 | 30 | 70 | 2.970 | 107.030 |
| 101.325 | Direct Input | — | 2.340 | 98.985 |
Frequently Asked Questions
1. What is partial pressure of dry air?
It is the pressure contributed by all non-water components in moist air. You get it by subtracting water vapor pressure from the total gas pressure.
2. Why does humidity reduce dry air pressure?
In a gas mixture, each component contributes part of the total pressure. When water vapor increases, it occupies a larger share, so the dry air share becomes smaller.
3. When should I use direct vapor pressure mode?
Use direct mode when your instrument, psychrometric chart, or laboratory method already gives water vapor pressure. It avoids extra estimation from temperature and humidity.
4. Is the humidity method accurate enough for routine work?
For routine chemistry, ventilation, and process checks, it is usually a practical estimate. Critical calibration or high-precision studies may need a more specialized vapor-pressure model.
5. Can I use units like atm, bar, or psi?
Yes. The calculator converts entered pressure values to a common internal unit, then shows the final answer in your selected output unit.
6. Why does the calculator also show moles and mass?
Those values help when you need more than pressure. They estimate how much dry air exists in a chosen sample volume under the entered conditions.
7. Does sample volume affect dry air pressure?
No. Dry air pressure depends on total pressure and water vapor pressure. Sample volume only affects the estimated moles, mass, and density outputs.
8. Can this calculator be used for gas collection work?
Yes. It is useful when gases are collected over water and you need to correct the measured pressure to get the dry gas or dry air pressure.