Measuring lung pressure

Class practical

Apparatus and Materials

For each student group

• Mounted U-tube, 3 m tall (see Technical note 1)
• Mouthpiece tubes, disposable, 2 (see Technical note 2)
• Food colouring
• Metre rule
• Trap (to prevent the water being 'sucked' into the mouth)

Health & Safety and Technical Notes

Read our standard health & safety guidance

The large U-tube manometer is made of clean transparent plastic tubing, secured to a board. It should preferably be attached to the wall. You may find it easier to avoid air bubbles in the manometer if the tube is filled before fixing it to the board. Make a preliminary estimate of the length of water column required. Immerse one end of the tubing in a beaker of coloured water and suck at the other end until there is the required length and finally fix the tube to the board.

For hygiene, disposable mouthpieces are desirable, of the sort used for medical purposes.

Place a trap between the gas tap and manometer to prevent water from getting into the gas supply.

Procedure

1. Half fill the U-tube with water together with some food colouring as a dye (it makes the water clearly visible and is non-staining).
2. Measure your lung pressure by blowing into the manometer and measuring the difference in water levels between the two arms.
3. Insert a small trap. Repeat by sucking.

Teaching Notes

• Students may discover for themselves when using short (0.5 m) manometer tubes that if they blow into them then the water can be blown out. That is the time to introduce them to a 3 m tube so that they can measure their lung pressure as the difference in levels between the water in the two arms.
• A statistical distribution of the class results can be produced and explanations for the variation sought. Perhaps you can tell who smokes, or who has asthma.
• In their efforts to get the water up and out of the tube some may discover resonance while bouncing the water over. This can be messy but it is a delight too.
• Some may try to see how far below atmospheric pressure they can go by sucking the water. Ensure that there is no danger of the water being swallowed by using a trap.

The experiment was safety-checked in July 2007