Biology 11–12 · Year 12

Evaporative cooling and the limit of sweating: wet-bulb and dry-bulb temperatures

Module 8: Non-infectious Disease and Disorders (Homeostasis: temperature)

Practical, model not builtLow risk

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The idea

Sweat cools the body only by evaporating, and a wet thermometer bulb shows the lowest temperature evaporation can reach, which rises toward air temperature as humidity rises and so sets a limit to thermoregulation.

What you need

Two spirit-filled thermometers or temperature probes reading to 0.1 or 0.5 degrees Celsius, clamped side by side on a stand
Cotton wick or cotton sleeve over one bulb, dipping into a small beaker of water at room temperature
Desk fan; digital hygrometer showing relative humidity; stop clock

How to do it

Record air (dry-bulb) temperature and relative humidity from the hygrometer.
Wet the wick, let both readings settle for 5 minutes in still air and record both temperatures every minute.
Switch the fan on to blow across both bulbs and record every minute until the wet-bulb reading stops falling.
Enter the air temperature and humidity in the model and compare its wet-bulb temperature with your fanned reading.
Use the model to find the wet-bulb temperature for a hot humid day and a hot dry day, and relate both to how well sweating can cool skin at about 35 degrees Celsius.
Compare your results with the published sweating and body-temperature data set on the practicalbiology.org sheet and draw the negative feedback loop for body temperature.

What you should see

The wet bulb reads lower than the dry bulb and falls further when fanned, settling near the model value; at 22 degrees Celsius and 60 percent relative humidity the model gives 16.8 degrees Celsius, a depression of about 5 degrees (computed). At 35 degrees Celsius and 90 percent humidity the wet-bulb temperature is 33.5 degrees Celsius (computed), so evaporation can barely cool skin; Sherwood and Huber (2010) identified a sustained wet-bulb temperature of 35 degrees Celsius as the theoretical limit beyond which the body can no longer shed metabolic heat, but heat-chamber trials with young, healthy adults (Vecellio and colleagues, Journal of Applied Physiology 2022) found the critical wet-bulb temperature in warm, humid air averaged 30.55 degrees Celsius, and lower still in hot, dry air, so the 35 degree figure overstates what bodies can bear. The learner knows it worked when the fanned depression lies within about 1 degree Celsius of the model, the stated accuracy of the equation.

What changes

What you change: air flow (still or fanned)
What you measure: wet-bulb temperature and depression below dry-bulb
What you keep the same: water temperature in the wick
room air temperature and humidity
thermometer placement
settling time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Sweat cools because it is cold; it cools by taking heat to evaporate.
More sweating always means more cooling; in saturated air sweat drips off without evaporating.
Thermoregulation works at any temperature; tolerance limits are reached when evaporation can no longer remove heat.

Safety card

Low riskLearners carry it out

Hazards

Water near the fan's mains supply
Broken glass thermometer

Controls

Keep the beaker away from the fan base and cable
Use spirit-filled thermometers or probes, never mercury; clean up breakages with a dustpan

Note

No hazardous chemicals; RiskAssess (https://www.riskassess.com.au/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-15 BIO11/12-5 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-04 BI-11WS-05 BI-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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