What it shows
The air you breathe out can be measured by the water it pushes out of an upturned bottle. It gives your tidal volume and your vital capacity, and shows how they compare between people.
The science
The volume of a normal breath in and out is the tidal volume, around 500 mL in adults. After a deep breath, forcing all the air out gives the vital capacity, about 3 to 5 litres in adults and less in children. Some air always remains in the lungs, the residual volume, so you cannot measure total capacity this way. Volume depends on body size, age and fitness. The method works because the exhaled air takes the place of the water in the bottle.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 2 litre clear plastic bottle with a marked scale you make by pouring in known volumes
- A large basin or tub
- About 1 metre of clean plastic tubing
- A permanent marker and a measuring jug
- A fresh disposable straw cut to length, one for each person
- Soapy water and a clean cloth
- A notebook
Safety
For this project: Use a fresh straw for every person and wash the tube end with soapy water between users. Do not test anyone with asthma, a cold or breathing trouble. Stop at once if anyone feels dizzy; an adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Calibrate the bottle: stand it upright and pour in 250 mL at a time from a measuring jug, marking the water level after each pour until the bottle is full.
Fill the basin half full of water and the bottle completely, then invert the bottle in the basin with its mouth under water, supporting it upright.
Slide one end of the tube into the bottle mouth under water, and keep the other end outside the basin.
Fit a fresh straw on the outside end of the tube for each person.
Breathe in normally, then breathe out normally through the straw and note the mark where the water level drops to; this is the tidal volume.
Refill the bottle, then take the deepest breath possible and blow out as much air as you can in one go; record the volume as the vital capacity.
Repeat three times for each person and calculate the average.
Compare results with height, age or sports habit in a table and write what you notice.
What you should see
Tidal volume is small, about 300 to 500 mL for most students, and vital capacity is several times larger. Taller or athletic people tend to have larger volumes, and your table shows the pattern.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is vital capacity?
The most air you can breathe out after a deep breath in.
2Why can this method not measure total lung capacity?
Some air always stays in the lungs after the strongest breath out.
3Why is a fresh straw used for each person?
To keep it hygienic and to avoid passing germs.
4Why does the water level fall in the bottle?
The exhaled air rises and pushes water out of the bottle.
Take it further
Compare vital capacity before and after four weeks of daily breathing and stretching exercise among willing volunteers.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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