Calculator Form
Example Data Table
| Dry Bulb °C | RH % | Pressure kPa | Humidity Ratio kg/kg | Dew Point °C | Enthalpy kJ/kg |
|---|---|---|---|---|---|
| 24 | 50 | 101.325 | 0.0093 | 12.9 | 47.8 |
| 30 | 60 | 101.325 | 0.0160 | 21.4 | 71.2 |
| 18 | 45 | 95.000 | 0.0062 | 6.0 | 33.7 |
Formula Used
Saturation vapor pressure is estimated with Tetens style equations. For temperatures above freezing, the calculator uses this relation:
Pws = 0.61078 × e^((17.2694 × T) / (T + 237.3))
Actual vapor pressure is found by multiplying saturation pressure by relative humidity.
Pw = RH / 100 × Pws
Humidity ratio is calculated from vapor pressure and barometric pressure.
W = 0.62198 × Pw / (P - Pw)
Moist air enthalpy is estimated as:
h = 1.006T + W × (2501 + 1.86T)
Specific volume is estimated as:
v = 0.287042 × (T + 273.15) × (1 + 1.607858W) / P
How to Use This Calculator
- Enter dry bulb temperature in Celsius.
- Enter relative humidity as a percentage.
- Choose altitude based pressure or manual pressure.
- Add airflow if you want a load estimate.
- Enter a target temperature for comparison.
- Press the calculate button.
- Review dew point, wet bulb, humidity ratio, and enthalpy.
- Download the report as CSV or PDF.
Psychrometric Chart Calculator Guide
What This Tool Does
A psychrometric chart calculator helps you read moist air behavior without drawing every chart line by hand. It links dry bulb temperature, relative humidity, pressure, vapor pressure, humidity ratio, dew point, wet bulb, enthalpy, and specific volume. These values matter in air conditioning, ventilation, drying, storage, and comfort design. The calculator uses entered air conditions and standard moist air equations. It then returns values that match the main ideas shown on a psychrometric chart.
Why Air Pressure Matters
Air pressure changes the moisture capacity of air. A mountain site does not behave like a sea level site. That is why this calculator includes altitude and manual pressure options. Use altitude when you need a quick estimate. Use manual pressure when a measured barometer value is available. This gives better results for field testing, lab work, commissioning, and design checks.
Useful HVAC Outputs
Humidity ratio shows the mass of water vapor in each mass of dry air. Enthalpy shows total heat content. Dew point tells you when condensation begins. Wet bulb gives a cooling related estimate. Specific volume helps convert volume flow into mass flow. Together, these outputs help compare coils, rooms, dryers, humidifiers, dehumidifiers, and air handling units.
Chart Based Decisions
A chart is useful because it shows direction. Heating moves air mostly right. Cooling moves air left. Humidification raises moisture. Dehumidification lowers moisture. This calculator gives the numbers behind those moves. You can test a current state and compare it with a target state. The sensible change estimate helps show the energy difference for the selected target temperature.
Practical Notes
Use accurate temperature and humidity readings. Place sensors away from direct sun, wet surfaces, vents, and hot machines. Allow readings to stabilize. Check pressure if the project is sensitive. Results are engineering estimates. For safety critical systems, verify with accepted standards and professional design tools.
FAQs
What is a psychrometric chart calculator?
It calculates moist air properties from temperature, humidity, and pressure. It gives values usually read from a psychrometric chart.
Which inputs are most important?
Dry bulb temperature, relative humidity, and pressure are the main inputs. Airflow is only needed for capacity estimates.
Can I use altitude instead of pressure?
Yes. The calculator can estimate pressure from altitude. Use measured pressure when accuracy matters more.
What is humidity ratio?
Humidity ratio is the mass of water vapor per mass of dry air. It is often shown as kg/kg dry air.
What does dew point mean?
Dew point is the temperature where air becomes saturated. Cooling below it can cause condensation.
Is wet bulb exact here?
The wet bulb value is an estimate. It is useful for quick review, but detailed design may need stricter methods.
Can this help with HVAC sizing?
It can support early checks. Final sizing should include building loads, ventilation rates, equipment data, and local codes.
Why download CSV or PDF?
CSV helps with spreadsheets. PDF helps with sharing, reports, field notes, and project records.