I wanted a vacuum pump for some experiments and looked all over the internet but couldn't find a pump that was easy (for me any) to make. Most involved fabricating from scratch or using bicycle pumps (but none of the kind I had) so I figured out how to make my own. Of course this isn't close to an industrial pump but it removes quite a lot of air and the valve seals (not perfectly) enough that there is time to seal whatever the air was taken out of.
You could use this for pumping water (I haven't tried it this way but it is possible), shrinking marshmallows, sealing food, or in a vacuum chamber that doesn't require high precision.
And the good news is that you can still use it for its original function, blowing up balloons!
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Step 1: What You Need for This
One dollar store balloon pump.
The cap from a drink bottle.
A pen or other similar tube cut to the size you want. (That's what the small tube is. The pen is just there for show.)
A hot glue gun.
LOTS and LOTS of glue!
(Ignore the black round thing in the picture. I didn't have to use it.)
Step 2: Creating the Inlet
A balloon pump usually has two rubber flap disks as one-way valves on the inside, one on top and one on bottom. One valve opens on an in-stroke and closes on the out-stroke, the other valve doing the opposite so there is a continuous flow of air. We only need to put an inlet over the bottom valve.
The cap must be large enough to cover the inlet holes without blocking them and a hole just a little bit smaller than the tube should be made in the cap so that the tube is a tight fit. If it's not super tight don't worry because that's what glue is for, but please try and make it tight as possible. As you can see the caps are a different colour because I made the hole too big in the blue cap and had to redo it.
The tube should have a small notch cut out of the bottom because it's going to be resting against the bottom of the pump and air needs to go through it. The notch should not be cut more than half the height of the cap or this might cause air leaks.
Step 3: Insert Tube
As you can see the tube is in the cap and the notch does not extend past the top of the cap but extends a little past the bottom.
Place the cap on the pump and push down so that the tube rests against the pump and there is no gap around the base of the cap. If the tube doesn't move it might be too high and push it down until it stops against the pump. If it's too low then push the cap down until it rests against the pump. You're setting the height of the tube.
Now GLUE!!! LOTS AND LOTS OF GLUE!!! Both on the inside and outside of the cap around the tube. You want to make this airtight.
Step 4: More Glue! We Need More Glue!!!
Glue the cap on and seal any other openings that you think might leak. USE LOTS AND LOTS OF GLUE!!! If you think you have done everything right and the pump still leaks try to find the source of the leak, put glue over the hole, place your finger over the inlet tube and slowly draw the pump handle out drawing a slight vacuum. You want to draw air out and the vacuum will pull the hot glue into the opening sealing it quite well. Don't draw too much vacuum or you'll suck the glue into the valve and that will ruin the pump.
Step 5: Enjoy!
Have fun with your toy, new water gun, marshmallow cruncher, or whatever you want to use it for!