Air pressure in pumps
Air has weight, so it pushes on everything. At sea level, air presses with about 100 000 Pa (100 kPa). It presses on the water in a well too.
Inside a pump pipe, when the piston goes up, the space under the piston gets bigger and the air gets spread out. The pressure falls below the outside air pressure. The stronger outside pressure pushes water up the pipe until the pressures are equal. So we say the water is pushed up by air pressure, not pulled by the piston.
Parts of a piston pump
- Cylinder: a tube in which the piston moves.
- Piston: a disc that fits tightly and slides up and down, joined to the handle by a rod. It has a one-way flap, valve B.
- Valve A (inlet valve): a one-way flap at the bottom of the cylinder. It opens upwards only.
- Pipe: dips into the water.
- Spout: where the water comes out.
A one-way valve lets water pass in one direction only.
How it works, stroke by stroke
- Up-stroke: piston moves up. Pressure below it falls. Valve B shuts. Valve A opens. Air pressure pushes water in from the well.
- Down-stroke: piston moves down. Valve A shuts so water cannot go back. Valve B opens. The water moves to the top of the piston.
- Up-stroke again: valve B shuts. The piston lifts the water above it and it pours from the spout. Meanwhile new water enters below.
The first strokes may only fill the pipe. This is why old hand pumps are "primed" with a little water first: a dry piston leaks air and cannot lower the pressure enough.
How high can the water go?
Air pressure can hold up a water column of height h where P = ρ × g × h. With P = 100 000 Pa, ρ = 1000 kg/m³, g = 10 N/kg: h = 100 000 / (1000 × 10) = 10 m. In real pumps it is a little less, about 8 to 9 m, because of air leaks. For deeper wells, a pump is put down in the well and pushes water up, or an electric submersible pump is used.
Build and test a simple pump
You need a plastic syringe (no needle), a small transparent tube that fits, a glass of water and a coloured liquid or ink.
- Dip the end of the tube into the coloured water and join the other end to the syringe.
- Pull the plunger back slowly. What do you see in the tube? Water rises because the pressure in the syringe is lower.
- Push the plunger and watch the water squirt out.
- Test: how many pulls for a full cup? What happens if the tube is lifted high above the glass? What if you make a small hole in the tube? (Air leaks in and it stops working.)
This is also how a bicycle pump and a medicine syringe work. Write down your results in a table: number of strokes, volume pumped.
Try it
Use the 3D pump above. Move the handle slowly up and watch valve A turn green and the water rise. Then move it down and watch valve B. Before you move it, say aloud: "Which valve is open now?"
Key formulas and definitions
- Water column held by air: P = ρ × g × h
- h = P / (ρ g) ≈ 100 000 / (1000 × 10) = 10 m
- Atmospheric pressure ≈ 100 kPa = 100 000 Pa
- Pressure inside < outside → water is pushed in
- Up-stroke: valve A open, B shut. Down-stroke: A shut, B open.
Worked examples
1. Which valve is open during the up-stroke of a piston pump?
Valve A (the inlet valve at the bottom). Valve B on the piston is shut, so the piston lifts the water above it.
2. Calculate the greatest height to which air pressure of 100 000 Pa can raise water (ρ = 1000 kg/m³, g = 10 N/kg).
h = P / (ρ g) = 100 000 / (1000 × 10) = 10 m.
3. A pump gives 0.2 litre of water in each full stroke cycle. How many cycles are needed to fill a 10 litre bucket?
Number = 10 / 0.2 = 50 cycles.
Common mistakes
- Saying the piston "sucks" the water. It lowers the pressure and the outside air pressure pushes the water.
- Mixing up the valves: A is at the bottom of the cylinder; B is on the piston.
- Thinking air pressure can lift water to any height. The limit is about 10 m.
- Forgetting that a leaking piston or pipe stops the pump from working.