Wet-Bulb and Dry-Bulb Temperature: Evaporation Changes the Observation

Define the Phase or Process Boundary

Vaisala's wet-bulb explanation describes cooling as water evaporates from a wetted thermometer. The difference from dry-bulb temperature is a temperature depression; determining humidity requires the applicable state relationship, including pressure.

Review a Hypothetical Comparison

Consider fictional readings of 38 °C dry bulb and 26 °C wet bulb. The depression is 12 K. Calling that 12% relative humidity confuses a temperature interval with a vapor-pressure ratio. Treating 26 °C as the dew point would assign a condensation threshold from an evaporation-cooled sensor. If the two readings came from different air locations or times, their difference would not even describe one paired state.

Keep the Evidence with the Decision

Preserve simultaneous, co-located state evidence and whether the sensing surface was actually wetted. Record air pressure and the applicable conversion method. If the decision depends on condensation, obtain the dew-point quantity appropriate to the air state instead of relabelling a wet-bulb reading.

The readings are assigned and no humidity value is derived from them. This is not a psychrometer operating procedure or a claim that measured and thermodynamic wet-bulb temperatures are exactly identical.

Which measured temperature represents the ambient air and which represents an evaporating wetted sensor?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Vaisala's wet-bulb explanation
• Reference from hmpharmamachines.com