Agriculture and Food 7–12 · Years 7–8

Measuring humidity with wet and dry bulb thermometers

Food and agricultural practices

Practical, model not builtLow risk

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The idea

Evaporation cools a wet thermometer bulb, and the difference between wet and dry bulb readings measures relative humidity, one of the growing conditions that affect plants and animals.

What you need

wet and dry bulb hygrometer or whirling hygrometer with spirit-filled (not mercury) thermometers
distilled water for the wick
maximum and minimum thermometer
rain gauge
digital temperature and humidity sensor on a microcontroller (optional, for comparison)
recording sheet or spreadsheet

How to do it

Wet the muslin wick with distilled water. Either hang the hygrometer in still shade, or hang it in a Stevenson screen if the school has one, or whirl a whirling hygrometer for 1 minute in open air. Record which of the three it was, because each has its own psychrometer coefficient.
Read the wet bulb first and then the dry bulb, to the nearest 0.5 °C: once a whirling hygrometer stops, its wet bulb is no longer ventilated and warms quickly. Whirl again and read again until the wet-bulb reading stops falling.
Calculate relative humidity with the model, choosing the setting for the instrument used: still shade (the FAO non-ventilated coefficient), screen (the Bureau of Meteorology relative humidity equation) or whirled (the FAO ventilated-psychrometer coefficient). Enter the station level pressure, measured if the school has a barometer and otherwise read from the Bureau's table of standard pressures for the school's altitude; the answer barely moves with pressure, so an unmeasured pressure is not what limits this measurement.
Take readings inside a greenhouse or polytunnel and outside it within five minutes of each other; note whether the greenhouse plants were watered that day.
Read the rain gauge at 9 am each school day, as the Bureau does, and read and reset the maximum and minimum thermometer.
After four weeks, compare the class totals and mean maximum and minimum temperatures with the long-term monthly means for the nearest Bureau station, and relate the temperatures to the comfort range the Animals in Schools guidelines give for laying hens (20 to 28 °C).

What you should see

The setting must match how the wet bulb was ventilated. At a station level pressure of 998.3 hPa (the Bureau's standard pressure for a station 0 to 250 m above sea level), a dry bulb of 25.0 °C with a wet bulb of 20.0 °C gives 54.9% relative humidity for a hygrometer hung in still shade, 61.0% for one in a Stevenson screen and 63.4% for a whirled hygrometer, because each instrument ventilates the wet bulb differently and so carries a different psychrometer coefficient. At 30.0 °C with 20.0 °C the three settings give 26.9%, 36.0% and 39.5%. Choosing the screen setting for an instrument merely hung in still shade puts the answer 6.1 percentage points high every time, because it is a bias in the coefficient and not a reading error: repeat readings do not reduce it, while the 0.5 °C reading spread below is at least partly random. Pressure, by contrast, hardly matters: in the screen setting at 25.0 and 20.0 °C the model moves 0.013 percentage points of relative humidity per hPa, so the table pressure 998.3 hPa and a measured 1013.2 hPa give 61.0% and 60.8%, and the whole 880 to 1030 hPa range of the slider spans only 62.5% to 60.6%. Within one of the Bureau's 250 m altitude bands a table pressure can be at most about 28 hPa from the measured one, which is 0.4 of a percentage point. The Bureau's note that a humidity calculated from an accurately measured pressure is expected to vary by 2 to 3% from one calculated with a table pressure therefore cannot be read as percentage points of relative humidity at a single station: this equation would need a pressure error of about 156 hPa to move the answer 2 percentage points. Reading each thermometer to the nearest 0.5 °C is what limits precision: at 25.0 and 20.0 °C in a screen, readings half a degree out in opposite directions give anywhere from 54.6% to 67.9%. The wet bulb never reads above the dry bulb, and equal readings mean 100% humidity. Warming air without adding water vapour lowers relative humidity, while watering and transpiration inside a greenhouse add vapour and raise it, so the paired readings show which effect was larger that day. A single month's rain gauge total can differ widely from the long-term monthly mean. For comparison, Wagga Wagga AMO (station 072150, 212 m above sea level) has a January mean maximum of 31.9 °C and a July mean minimum of 2.8 °C (1942 to 2026) and a mean annual rainfall of 571.8 mm (1941 to 2026). The learner knows it worked when the wet bulb reads below the dry bulb, repeat readings agree to 0.5 °C, the setting used matches the instrument, and the digital sensor's reading falls within the range that half-degree reading errors allow.

What changes

What you change: location (inside or outside the greenhouse)
What you measure: dry and wet bulb temperatures (°C) and relative humidity (%)
What you keep the same: same hygrometer
readings within five minutes
instrument shaded
wick wet with distilled water

Common misconceptions

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

Relative humidity tells you how many grams of water are in the air.
Both thermometers are in the same air, so they should give the same reading.
One month of school rain gauge readings should match the long-term average for that month.
Any wet and dry bulb pair can be read off the same humidity table, whatever the airflow over the wet bulb.

Safety card

Low riskLearners carry it out

Hazards

broken glass thermometers
a whirling hygrometer striking others
heat inside a closed greenhouse

Controls

spirit-filled or digital thermometers only
whirl only in a clear space
hats and water; short visits to a hot greenhouse

Note

No chemicals are used. Complete the school's risk assessment for the outdoor readings and the greenhouse visits before the first lesson.

Curriculum references

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

Agricultural Technology Years 7–10 Syllabus (2019)AG4-11 AG4-12
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9AC9TDE8K04AC9S8I03

Sources

The pages the author read to write this activity.

All Concept Studio activities