Wet Bulb to Relative Humidity Calculator

HVAC commissioning: quick humidity checks during system startup.

Process air monitoring: verifying that dryer or humidifier performance is in expected range.

Educational demonstrations of psychrometric principles.

In HVAC commissioning and field troubleshooting, the sling psychrometer (or the more modern digital psychrometer with thermistor or capacitive RH sensor) is the standard instrument for measuring wet bulb and dry bulb temperature, with the relative humidity calculated from the readings.

The '5° rule' approximation is sufficient for first-pass checks during commissioning (confirming that the return air RH is in the expected 45–55% range, or that the outdoor air wet bulb is in the expected range for the design conditions), but the full ASHRAE psychrometric analysis is used for the as-built documentation.

Process air monitoring in compressed air systems, environmental chambers, pharmaceutical manufacturing, and food processing uses wet bulb temperature to verify that the humidification or dehumidification system is performing to spec — a pharmaceutical cleanroom maintained at 45% RH / 70°F has a wet bulb of about 57°F (T_db − T_wb = 13°F, 100 − 5 × 13 = 35% RH by the rule, but the actual RH is 45% because the rule is approximate).

Industrial process cooling and drying applications use the wet bulb temperature as the limiting parameter for evaporative cooling: a cooling tower can cool the water to within 5–10°F of the ambient wet bulb, and the wet bulb depression determines the cooling tower's approach temperature and the achievable cold water temperature.

Power plant cooling tower performance specification uses the design wet bulb as the primary input (typical design wet bulb 75–80°F for US inland sites, 78–82°F for humid sites like Houston or the Gulf Coast), and the actual cooling tower performance is rated against the design wet bulb.

Agricultural and horticultural applications (greenhouse climate control, post-harvest produce storage, mushroom cultivation) use wet bulb to control humidity, with target wet bulb temperatures specific to each crop (e.g., 85°F wet bulb for cucumber, 78°F for tomato, 70°F for cut flowers).

Mine ventilation air cooling in deep hot mines (South African gold mines, Australian coal mines) uses evaporative cooling to lower the intake air wet bulb to maintain safe working conditions, with the wet bulb depression determining the cooling capacity.