Sunday, 9 May 2010


I completed the assembly today and managed to get a few tests in.

The first thing I had to do was fit the Schrader valve. These seem to be very difficult to source in the UK unless they are already attached to a tyre. In the end I found a guy on ebay selling these:

They are intended for Mini Metro shock absorbers but they are ideal for me for three reasons:

  1. They have a standard, wide thread (M12 x 1.75) making it easy to source a tap and as the thread diameter is wide, it should give a secure mount.
  2. It is made of brass and so won't rust.
  3. Cost: Only £3.89

The PVC pipe that I am using has a wall width of about 4mm. I was concerned that mounting the above in a tapped hole might result in the PVC becoming distorted, or the valve becoming insecure. To fix this I decided to mount the valve at the union of the 90 degree elbow joint pipe as the elbow joints have a wall thickness of about 6mm.

Because I am paranoid, I also decided to fit an additional PVC shim from an offcut of pipe to give a total wall thickness of 4 + 6 + 4mm = 14mm.

I made the shim by using a hole cutter to cut a 35mm disc from an offcut of pipe.

However, this pipe is of a daimeter designed to fit inside the elbow but needs to sit on the outside of the elbow. This means that the outer diameter is greater than the diameter of the pipe this disc was cut from. To allow this to sit flush on the elbow I 'baked' the disc in the oven at gas mark 6 for 15 minutes to make the PVC malleable. I then pressed it over the elbow until it cooled and hardened. Here is the shim in place on the elbow.

I then solvent welded the disc to the elbow, drilled it out to 10mm then tapped it. I also filed it flat to allow the Schrader valve to fit flush.

Here is the complete assembly:

It was now time to pressure test the system. I fired up the compressor and pressurised to 30psi. There were no leaks. Now it was time to fit the water outlet port.

I decided to use a pneumatic quick fit connector for the outlet as it does not stick out very far and allows quick (dis)connection of the pipe:

This is one of the connectors I had left over from Halloween and has an 8mm pipe diameter.

I used the same 'shim and cook' method to fit this at the very bottom of the tank. My Grandad left me a varied selection of imperial taps, one of which was 1/4" BSP which is the correct thread for this fitting.

I knocked up a basic trigger using a few pneumatic bits and pieces I got from work when they shut down our electronic assembly department. I got the ball valve from ebay.

Here is a close up of the nozzle and ball valve:

Here is a picture of Josh testing it for me at 80 psi.

I was disappointed by the range. Anyone on the receiving end will get drenched as it dumped 3 litres in about 25 seconds. The stream breaks up much earlier than I was expecting. Examining the bore of the nozzle I noticed it was very rough so I drilled it out to 4mm on my lathe.

This had no effect. I then changed to nozzles of 6mm and 3mm. Neither showed any improvement on range. I upped the pressure to 90 psi and got a maximum range of 32 feet. I think that I need to reduce the nozzle diameter or that there is too much resistance in the pipe from the tank to the nozzle. I've filled a nozzle with epoxy and will drill it out when it's dry.

Range could also be increased by upping the pressure. The PVC tube is rated for a maximum of 131 psi, but I feel a bit nervous about taking it so close to its rated limit.

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