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.
Today I had a lot of fun plumbing-in all the copper pipe work and using an oxy-acetylene blow torch. This torch was far more powerful than I needed and nearly melted holes in the copper tubing, so I had to be careful. Tony, the cameraman, gave me loads of tips on how to use the pipe fittings.
I make up one-way valves from mouse balls (computer mouse balls!) These are slightly rubberised and make a really good seal with the copper fittings. Copper adaptors and fittings were used to make a vertical cylindrical container that was about twice the diameter of the copper tubing. Within this construction the ball rested in a funnel like section at the bottom. When air was pumped through the device, the ball would be blown up slightly to allow the air to pass. When there was no pumping (on the return stroke of the pump) the positive pressure above the ball kept it forced-in this funnel, blocking the tube and acting as a one way valve.
Two of these were made up and one fitted to each pump. The tubing following the one way valves were then joined up to form the output of the pumping system. The two pump plungers were then attached to a single wooden arm so that a see-saw action by the operators (Ellen and I) pushed one pump while the other pump refilled with air. Constantly see-sawing the arm constantly pumped air through the system. We had a lot of fun blowing up balloons and plastic gloves to unfeasible sizes before they exploded!
End the day with the sun going down and a little test of all the kit in the water tank.
Today we all go out to film the diving gear working. We spend most of the morning travelling to a lovely location where we are to film. It’s near a private island where the rich and famous go!
The sea water is starting to corrode the outside of all the equipment but the insides all seem to be working OK. Kathy does a test of the air meter and we set up for Kate to start the test. It all works! Ellen and I pump away to get enough air to the tank as possible and Kate seems to be able to stay down and breathe for as long as she wants.
At the end of the day I also get a chance to re-use the hydrophone. I have fitted a new crystal and made up a better amplifier, after re-sealing I manage to get a much better recording of fish at this location. Two successes in one great day!