Calculator Inputs
Formula Used
W = 0.62198 × Pv / (P - Pv)
h = 1.006T + W(2501 + 1.86T)
Pv = (RH / 100) × Pws(T)
Pv = Pws(Tdp)
Pv = WP / (0.62198 + W)
In these equations, T is dry-bulb temperature in °C, W is humidity ratio in kg/kg dry air, Pv is actual vapor pressure in kPa, P is total pressure in kPa, and Pws is saturation vapor pressure.
How to Use This Calculator
- Enter the dry-bulb temperature.
- Select Celsius or Fahrenheit.
- Enter the total air pressure and choose its unit.
- Select one moisture input mode.
- Provide relative humidity, dew point, or humidity ratio.
- Press the calculate button.
- Review enthalpy, humidity ratio, dew point, and other psychrometric results.
- Use the CSV or PDF buttons to save your output.
Example Data Table
| Case | Dry-Bulb (°C) | RH (%) | Pressure (kPa) | Humidity Ratio (g/kg) | Enthalpy (kJ/kg dry air) | Dew Point (°C) |
|---|---|---|---|---|---|---|
| Office Cooling | 24.0 | 50.0 | 101.325 | 9.297 | 47.811 | 12.972 |
| Warehouse Air | 30.0 | 60.0 | 100.000 | 16.256 | 71.744 | 21.423 |
| Laboratory Room | 20.0 | 40.0 | 101.325 | 5.795 | 34.829 | 6.018 |
| Humid Process Area | 35.0 | 70.0 | 99.500 | 25.636 | 100.995 | 28.737 |
Frequently Asked Questions
1. What does moist air enthalpy represent?
Moist air enthalpy represents the total heat content of air and its water vapor. It combines sensible heat from temperature and latent heat from moisture into one useful engineering property.
2. Why is pressure included in the calculation?
Pressure affects vapor pressure relationships and humidity ratio. Using actual pressure improves accuracy, especially for high-altitude locations, controlled chambers, and process systems that differ from standard atmospheric conditions.
3. Can I use Fahrenheit inputs?
Yes. Select Fahrenheit in the temperature unit menu. The calculator converts the value internally, performs the psychrometric calculation, and then displays temperature outputs back in your selected unit.
4. What is the difference between relative humidity and humidity ratio?
Relative humidity compares actual vapor pressure with saturation pressure at the same temperature. Humidity ratio measures the mass of water vapor per kilogram of dry air, making it useful for direct mass and energy balances.
5. Why can dew point not exceed dry-bulb temperature?
Dew point is the temperature where condensation begins for the current vapor content. It cannot be higher than dry-bulb temperature in normal unsaturated air because that would imply supersaturation.
6. Is this calculator suitable for HVAC work?
Yes. It is suitable for many HVAC, comfort, and educational psychrometric checks. For regulated design work, always compare against your project standards, validated software, and official engineering references.
7. What does the graph show?
The graph shows how enthalpy changes as relative humidity increases while dry-bulb temperature and pressure remain fixed. It helps you visualize the effect of added moisture on total heat content.
8. What is included in the CSV and PDF exports?
Both exports contain the submitted state and calculated results. The CSV is useful for spreadsheets, while the PDF creates a clean report for sharing, records, or quick project documentation.