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Equipment build planAir & Water Pressure

Venturi Vacuum Pump

A short piece of tubing pushed up into the airflow of a plastic hose T turns a stream of compressed air into a surprisingly strong vacuum pump. It demonstrates Bernoulli's principle and the push of atmospheric pressure, and Bruce built his for about two dollars.

Written from Bruce's original workshop handout. Bruce's final review of this plan is in progress.

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Venturi Vacuum Pump
DifficultyBeginner
Build time30 minutes
CostAbout $2-5 (plus an air compressor)
SafetyAdult build / supervision
Good forTeachers, Makers

What it shows

Blow air through a plastic T fitting and it sprays water, crushes bottles and cans, and holds Magdeburg hemispheres together, with no moving parts.

Key ideas: Bernoulli's principle, venturi effect, atmospheric pressure, vacuum

Safety (Adult build / supervision). An adult should run the air compressor, and everyone nearby should wear safety glasses. Never point the air stream at anyone's skin, eyes or ears. Hoses can pop off under full pressure, so use small hose clamps. Never connect the pump to a glass bottle or jar: the vacuum may be strong enough to shatter it. Use only plastic bottles or metal cans.
Construction

Build it

  1. Mark the airflow direction

    Write an arrow on the T showing which way the air will travel. It is easy to lose track when you take it apart, and the feed tube only works facing one way.

    Step 1

  2. Cut the feed tube at an angle

    Cut one end of the tubing at about 45 degrees.

    Step 2

  3. Sharpen the point

    Sand the two sides of the angled cut so the tip becomes narrower and more pointed. This shape gave Bruce the best results so far.

    Step 3

  4. Wrap with Teflon tape

    Wind a few turns of Teflon tape around the tube just below the angled end so it seals tightly inside the T.

    Step 4

  5. Insert the feed tube

    Push the feed tube up through the stem of the T most of the way into the main passage, with the long (high) side of the angled cut facing the incoming air, so the opening faces downstream. Bruce's tip almost touches the far wall of the passage. If you put it in backward, it blows air out the feed tube instead of drawing in.

    Step 5

  6. Check the arrangement

    The tube now forms a constriction inside the T: the air must squeeze past it, speeding up right where the opening sits.

    Step 6

  7. Connect the air supply

    Push a short piece of hose over the upstream end of the T and onto the compressor's air gun, and clamp both ends.

    Step 7

  8. Test it on water

    Put the free end of the feed tube in a bottle of colored water and give a burst of air: the water should rise up the tube and spray out in a fine mist. You can even get a spray by blowing through the T by mouth.

    Step 8

  9. Crush a plastic bottle

    Drill a hole in a bottle cap that grips the feed tube tightly, push the tube through, cap an empty plastic bottle and run the air. The bottle collapses. Put a finger over the exhaust end and it pops back out; lift your finger and it collapses again.

    Step 9

  10. Crush a can

    For aluminum cans, fit a soda can cap, push a rubber stopper into its opening and the feed tube into the stopper. Cans usually crush partway and then spring a leak, which is still impressive. Bruce has also collapsed 1-2 gallon metal cans.

    Step 10
The physics

How it works

Air rushing through the T has to squeeze past the feed tube that pokes into its path. Where the passage narrows, the air speeds up, and faster-moving air has lower pressure (Bernoulli's principle).

The opening of the feed tube faces downstream, sheltered behind its own tip, so the air flowing past drags air out of the tube and the pressure inside the feed tube drops well below the pressure of the room.

There is really no such thing as suction. When the pressure in the tube drops, the ordinary pressure of the atmosphere, about 14.7 pounds on every square inch at sea level, pushes water up the tube or squeezes a sealed bottle flat. The material that comes up the tube is caught by the fast air and blown out as a spray.

With a vacuum gauge, Bruce measured about 19-20 inches of mercury, roughly two-thirds of the way to a complete vacuum. That is enough that one person cannot pull a pair of Magdeburg hemispheres apart.

Using it

  • Spray demo: let the class predict what will happen when air blows across the water tube, then explain why the water rises.
  • Crush a sturdy plastic juice bottle, then cover and uncover the exhaust to make it inflate and deflate on command.
  • Connect the feed tube to a set of Magdeburg hemispheres in place of a vacuum pump and challenge students to pull them apart.
  • Discuss real devices that use the same idea: paint sprayers, garden fertilizer sprayers, pressure-washer soap injectors, carburetors and sandblasters.
  • Use a cheap vacuum gauge to compare different tip shapes or insertion depths.
Go further

Variations

  • Run it on water instead of air by connecting it to a faucet; the same T works as a water-powered vacuum pump.
  • Put the feed tube in fine, dry sand to make a small sandblaster (eye protection and an outdoor space are essential).
  • Experiment with different tip angles and shapes; Bruce plans to keep improving his.
If it doesn't work

Troubleshooting

If this happensAir blows out of the feed tube.Try thisThe tube is in backward. Turn it so the long side of the angled cut faces the incoming air.
If this happensWeak or no vacuum.Try thisPush the tube farther into the airflow, re-wrap it with Teflon tape so it seals in the T, and check every hose and cap for leaks.
If this happensHose pops off.Try thisAdd small hose clamps at both ends.
If this happensCan only crushes partway.Try thisThat is normal; thin cans often leak at the seam once they start to fold. Try a sturdier can or a better seal.
Free PDFDownload this planPrintable build plan with photos ExperimentUpside-Down Water GlassFamily version, worksheets and lesson plan

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© Bruce Yeany · Homemade Science with Bruce Yeany · youtube.com/@YeanyScience
Free for personal and classroom use. Please don't resell or republish. Build and use at your own risk; follow the safety notes and supervise children.