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Q & A: Vacuum pump suggestions

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Most recent answer: 10/22/2007
Q:
I have a clear acrylic tube. The tube is 5 inches around, and 3 feet long. I have a rubber in both ends, one of which has a small hole in it. I am attempting to create a vacuum in this tube to demonstrate to my students that, in the absence of matter, (gas), that objects in the tube, such as a penny, marble and feather, will fall at the same rate. How can I create this vacuum. I have attempted to attach the tube to my vacuum cleaner, however, I think I burned out the motor, YIKES. Please advise me on the best manner to create the vacuum. The person at the store where I purchased the tube suggested using a "back pump"(?) for an acquarium. What the heck is that??? Please help me. I am very sorry this is so long.
- Rebecca Harris (age 49)
St. Margaret Mary School, Chicago, IL Cook
A:
Hi Rebecca,

It sounds as if you are planning some interesting demonstrations. A vacuum cleaner's suction probably isn't strong enough to pull enough air out of the tube to lower the air resistance enough to make the point. Many teaching supply companies sell inexpensive vacuum pumps which are appropriate for the purpose. There might even be hand-operated ones for low cost.

The name "back pump" is sometimes used to refer to a mechanical piston pump or other kind of pump used for creating a fairly good vacuum but not state of the art. Vacuum pumps for research can evacuate a chamber to very very very low pressures using turbines, or by literally freezing the gas molecules out, or by absorbing them in activated charcoal, or by ionizing the molecules and collecting them electrostatically. These high-tech pumps cannot pump large quantities of air, but they can get most of the remaining molecules out once the bulk of them are taken away by a mechanical pump. The reason the mechanical pump is called a "back pump" is because it pumps on the exhaust (the back end) of the fancy pump so that the fancy pump does not have air flowing the wrong way through it -- these fancy, high-tech pumps often do not work well or at all if their exhaust is at high pressure. For the classroom, you most likely do not need a fancy pump, but can use the "back pump" all by itself.

None of the pumps I've seen for aquarium use would be appropriate, but maybe there are some that are better than others. The ones I am familiar with circulate air through a bubbler in the tank and use a thin rubber diaphragm moved by a little magnet. These do not handle large pressure differernces (as you can easily tell by putting your finger over the input or output of one -- it just doesn't pull or push that hard.) Maybe people have other kinds of pumps for aquariums.

I'd go for the low-cost solution from a teaching supply company. You may need some additional materials, such as a fairly stiff tube to connect your pump to the hole in the stopper, and I'd recommend getting some vacuum grease to prevent the inevitable leaks. You also will probably want a valve or two so you can remove the pump and keep the vacuum in the tube if you need to.

One very low-cost pump is a water suction pump, which has no moving parts except some flowing water. It's hooked up to a faucet, and a third opening (other than the water inlet and outlet) draws a vacuum that can get close to the vapor pressure of water.

You should know that there will be some air friction left even with a good vacuum. The part of the friction that goes as the sqaure of the velocity will be mostly remioved, but another part that is just proportional to velocity doesn't drop off until the vaucuum is much, much better.

Tom J. (w Mike)

(published on 10/22/2007)

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