Dry Bulb Wet Bulb Calculator

Calculate relative humidity, dew point, and other psychrometric properties from dry bulb and wet bulb temperature measurements.

How to Use This Calculator

Enter the dry bulb temperature (actual air temperature)
Enter the wet bulb temperature (from a sling psychrometer or similar device)
Adjust atmospheric pressure if needed (default is standard sea level)
Select your preferred temperature unit (Celsius or Fahrenheit)
Click Calculate to see psychrometric properties including relative humidity and dew point

Formulas Used

RH = 100 × (Pws(Twb) - P × (Tdb - Twb) × 0.00066) / Pws(Tdb)

Where:

RH = Relative Humidity (%)
Pws(T) = Saturation vapor pressure at temperature T
Tdb = Dry bulb temperature
Twb = Wet bulb temperature
P = Atmospheric pressure

Tdp = (243.12 × ln(RH/100) + (17.62 × Tdb)/(243.12 + Tdb)) / (17.62 - ln(RH/100) - (17.62 × Tdb)/(243.12 + Tdb))

Where:

Tdp = Dew point temperature
RH = Relative Humidity (%)
Tdb = Dry bulb temperature

Example Calculation

Real-World Scenario:

An HVAC technician measures a dry bulb temperature of 25°C and a wet bulb temperature of 20°C in an office building. Let's calculate the relative humidity and dew point.

Given:

Dry Bulb Temperature = 25°C
Wet Bulb Temperature = 20°C
Atmospheric Pressure = 101.325 kPa (standard)

Calculation:

1. Calculate saturation vapor pressure at dry bulb temperature (25°C): Pws(25°C) = 3.169 kPa

2. Calculate saturation vapor pressure at wet bulb temperature (20°C): Pws(20°C) = 2.339 kPa

3. Calculate relative humidity: RH = 100 × (2.339 - 101.325 × (25-20) × 0.00066) / 3.169 = 65.2%

4. Calculate dew point: Tdp = (243.12 × ln(0.652) + (17.62 × 25)/(243.12 + 25)) / (17.62 - ln(0.652) - (17.62 × 25)/(243.12 + 25)) = 17.8°C

Result: The relative humidity is 65.2% and the dew point is 17.8°C, indicating comfortable indoor conditions.

Why This Calculation Matters

Practical Applications

HVAC system design and troubleshooting
Indoor air quality assessment
Weather forecasting and meteorology
Agricultural climate control
Industrial process control

Key Benefits

Optimizing energy efficiency in buildings
Preventing mold and moisture damage
Ensuring occupant comfort and health
Improving agricultural productivity
Maintaining proper storage conditions

Common Mistakes & Tips

Ensure the wet bulb thermometer has a properly wetted wick and adequate airflow. The wet bulb temperature should never be higher than the dry bulb temperature. For accurate readings, use a properly calibrated sling psychrometer or electronic psychrometer.

At higher elevations, the reduced atmospheric pressure affects psychrometric calculations. Always adjust the pressure setting when working at elevations significantly different from sea level, as this can lead to errors in humidity calculations of 5% or more.

Be consistent with your units throughout the calculation. Mixing Celsius and Fahrenheit values will lead to incorrect results. Also ensure pressure units match (kPa, inHg, etc.) when using different psychrometric charts or calculators.

Frequently Asked Questions

Dry bulb temperature is the actual air temperature measured by a regular thermometer. Wet bulb temperature is the temperature a parcel of air would have if it were cooled to saturation (100% humidity) by the evaporation of water into it. The difference between these two temperatures indicates the moisture content of the air.

Wet bulb temperature is typically measured using a sling psychrometer, which consists of two thermometers - one regular (dry bulb) and one with a wetted wick (wet bulb). The device is spun to create airflow around the thermometers, causing evaporation from the wick and cooling the wet bulb thermometer. The difference between the two readings is used to calculate relative humidity.

Wet bulb temperature is crucial in HVAC because it helps determine the heat content (enthalpy) of air, which affects cooling load calculations. It's also essential for properly sizing and controlling cooling systems, particularly evaporative coolers and dehumidifiers. HVAC technicians use wet bulb readings to diagnose system performance and ensure optimal comfort conditions.

References & Disclaimer

Scientific Disclaimer

This calculator provides estimates based on standard psychrometric formulas. For critical applications, professional measurement equipment and consultation with HVAC or meteorology experts is recommended. Results may vary slightly from actual conditions due to measurement errors and local variations.

References

ASHRAE Handbook - Fundamentals - American Society of Heating, Refrigerating and Air-Conditioning Engineers
WMO Guide to Meteorological Instruments and Methods of Observation - World Meteorological Organization
Indoor Humidity and Health - Environmental Protection Agency

Accuracy Notice

This calculator uses the Tetens equation for saturation vapor pressure and standard psychrometric formulas. For temperatures below freezing or extreme conditions, specialized formulas may be required. The accuracy of results depends on the precision of input measurements and the appropriateness of the formulas for the specific conditions.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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science physics dry bulb wet formula

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