Calculator inputs
Example data table
| Case | Dry bulb | Relative humidity | Pressure | Airflow | Expected use |
|---|---|---|---|---|---|
| Return air | 24 °C | 50 % | 101.325 kPa | 1200 CFM | Find room air chart state |
| Supply air | 13 °C | 90 % | 101.325 kPa | 1200 CFM | Compare coil outlet state |
| High altitude site | 26 °C | 45 % | Altitude method | 800 L/s | Adjust chart values for pressure |
Formula used
The calculator uses common HVAC psychrometric relations. Saturation vapor pressure is estimated with the Buck equation.
Actual vapor pressure: Pv = RH × Pws / 100
Humidity ratio: W = 0.621945 × Pv / (P − Pv)
Moist air enthalpy: h = 1.006T + W(2501 + 1.86T)
Specific volume: v = 0.287042(T + 273.15)(1 + 1.607858W) / P
Total coil load: Qt = mass flow dry air × (h entering − h leaving)
Sensible load: Qs = mass flow dry air × (1.006 + 1.86Wavg) × temperature drop
Latent load: Ql = Qt − Qs
How to use this calculator
- Enter the entering dry bulb temperature.
- Select Celsius or Fahrenheit.
- Enter the entering relative humidity.
- Choose pressure by direct value or altitude estimate.
- Add airflow when coil load is required.
- Add leaving air values for process and load analysis.
- Press the calculate button.
- Download the result as CSV or PDF when needed.
Understanding Psychrometric Chart Reading
A psychrometric chart shows how air and water vapor behave together. Air conditioning work uses the chart to read comfort and coil performance. The dry bulb line gives normal air temperature. The relative humidity curve shows how full the air is with moisture. The humidity ratio scale shows water mass per mass of dry air. Enthalpy lines show total heat content. Specific volume lines help estimate air density and fan flow.
Why It Matters In Air Conditioning
Cooling equipment changes both heat and moisture. A coil may only cool air, or it may cool and remove water. The difference depends on coil surface temperature, airflow, and room load. Reading the chart helps a technician see this process quickly. It also helps compare return air, mixed air, supply air, and outdoor air. That view is useful during balancing, commissioning, and troubleshooting.
Using Calculated Chart Values
This calculator turns field readings into chart values. Enter dry bulb temperature, relative humidity, pressure, and airflow. The tool estimates vapor pressure, humidity ratio, dew point, wet bulb, enthalpy, and specific volume. Add a leaving air state when you need coil load. The result shows sensible load, latent load, total load, and condensate rate. These values help confirm whether equipment is removing enough heat and moisture.
Interpreting The Results
Dew point is the temperature where moisture begins to condense. A lower dew point means drier air. Humidity ratio is often more useful than relative humidity. It shows actual moisture content. Enthalpy includes sensible and latent heat. A high enthalpy drop across the coil means strong total cooling. Sensible heat ratio compares dry cooling with total cooling. A low ratio means more moisture removal. Always compare results with actual system data. Filter condition, coil cleanliness, airflow, and refrigerant charge can change readings.
Good Field Practice
Use accurate instruments. Allow sensors to stabilize before recording values. Measure pressure when site elevation is high. Take return and supply readings away from direct coil splash or sun. Repeat readings after the system runs steadily. Psychrometric calculations are estimates, but they are very useful. They make chart reading faster, clearer, and easier to document.
Keep notes with dates, locations, instruments, and operating mode for later comparison too.
FAQs
What is a psychrometric chart?
It is a chart that shows air temperature, moisture, enthalpy, humidity ratio, wet bulb, dew point, and volume relationships.
Why does pressure matter?
Pressure changes the humidity ratio, air density, and specific volume. High altitude locations need pressure correction for better HVAC readings.
What is humidity ratio?
Humidity ratio is the mass of water vapor per mass of dry air. It is useful because it shows actual moisture content.
Is wet bulb exact here?
The wet bulb value is an approximation. It is good for quick field estimates, but lab-grade work needs more detailed equations.
What does enthalpy mean?
Enthalpy is total heat content in moist air. It includes sensible heat and latent heat from water vapor.
How is coil load calculated?
The calculator multiplies dry air mass flow by the enthalpy difference between entering and leaving air states.
What is sensible heat ratio?
Sensible heat ratio compares sensible cooling with total cooling. Lower values show a larger moisture removal share.
Can I use Fahrenheit readings?
Yes. Select Fahrenheit as the temperature unit. The calculator converts values internally and displays both Celsius and Fahrenheit results.