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Description

Making a hygrometer to measure humidity.

See https://www.scouts.org.uk/activities/it-s-a-breeze/

Resources

You will need
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Two small thermometers (available online or from pet shops)
Cotton wool balls or thin cleaning cloth (for example, J-cloth)
Clean, empty plastic bottles (500ml)
Sticky tape
String
Scissors
Access to water
Access to the internet

Instructions

What's a hygrometer
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A hygrometer measures humidity. Humidity’s the amount of water in the atmosphere – high humidity is what gives you that sticky feeling on a hot day.
Humidity’s often measured as ‘relative humidity’ – the amount of water vapour in the air compared to the maximum moisture the air could hold. Fog is 100% relative humidity; humans are most comfortable with anything between 30% and 60% relative humidity.
At this base, people will make a type of hygrometer called a psychrometer. Psychrometers are a ‘wet and dry bulb’ hygrometers: you calculate the humidity using temperature readings from two basic thermometers, one with a dry bulb and one with a wet bulb.
Psychrometers work because evaporation from the wet bulb causes the temperature reading to drop so it shows a lower temperature than the dry bulb. You calculate the relative humidity by comparing the readings to the ambient temperature using a special chart or webpage.
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Take one of the thermometers and tie a cotton wool ball (or a small piece of cloth) around the bulb.
Pour some water on the cotton wool or cloth to turn this thermometer into the wet bulb thermometer.
Tie some string around the neck of the plastic bottle. It should be really tight so it won’t come loose.
Tape the wet bulb thermometer to one side of the bottle.
Tape the other (dry bulb) thermometer to the opposite side of the bottle. Make sure both thermometers are really secure – it might be best to use a couple of extra pieces of tape.
To take a measurement with the finished hygrometer, hold onto the string and whirl the whole thing around for around about a minute.
Take readings from both thermometers. Put them onto a webpage like this one to work out the relative humidity.
Depending on how much equipment you have, people may need to dismantle the hygrometer after they’ve taken a reading, so the equipment can be reused by the next group.

Tags

• hygrometer
• Meteorologist

Badge Links

• Meteorologist - Measuring