Beneath the waves diaries
Swim with the fishes
Ready for some deep sea diving, but no oxygen tanks? No problem, if you understand pumps and pressures.
Jonathan Hare's diary about the challenge for the Beneath the Waves programme, from the BBC/OU series Rough Science 5
Today I am given the task of making a pump to supply air to the equipment for the diving system. A standard car foot pump won’t provide enough air so we need to take one apart and make up a much larger version. It is very similar to the way a car foot pump works and is based on a great design by Graham Barguss and used so that we can get the best possible results from the various bits we have. I am given large copper tubing for this.
Given the amount of copper tubing I decide to make up pump tubes about 70 cm long. That’s almost three times the size of those found on a normal car pump, consequentially it should work much better. I will make two of these and connect them so that we can have one pumping while the other is filling up. This way there will be no periods when air is not being pumped so that there is always air for Kate to breathe.
The plungers from the car foot pumps are just a little large for the tubing so I have to sand them down to fit in. This was easier than it sounds as a rubber O-ring seal accommodates for a sloppy fit.
Moving the plungers makes them draw in the air and then they compress / pump it. They do this in the following way. The plastic plunger is slightly smaller than the diameter of the tube (after a little sanding) and about 20 mm depth. Around the outside of the plunger there is a grooved channel where a rubber o-ring sits. This is covered in grease and is itself free to move back and forth (by about 10 mm) when the plunger is moved. There are two holes drilled into the back of the plunger. When the plunger is drawn back the o-ring moves back and lets air flood in through these holes, filling the copper tube with air. When the plunger is then pushed in, the o-ring slides back making a tight seal against these air holes. The air is then sealed from exit and so moving the plunger in therefore pumps / compresses the air.
By the end of the day I have the two pumps set up and blowing up balloons when the plungers are pushed in. Unfortunately when the plungers are pulled out the balloons deflate, not because there is a problem with the o-ring but because the air goes back into the expanding space behind the plunger. We therefore need non-return valves in the design as well as the o-rings. In a car or bicycle wheel this valve would be on the tyre itself and so there is no need for one on the pump.
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Content last updated: 26/01/2005
