Thought this might be of interest to anyone considering making their own sphere pressure tester. As you can see from the picture its made from an old CX rear strut and an axle stand. By coincidence the hole in the axle stand matched the outside diameter of the strut. The collar at the end of the strut stops it popping right through the hole and out the other side. The pipe connection appears above the level of the axle stand top. A 200 bar pressure gauge is fitted to the pipe inlet via a short section of pipe.
Here's how it works:
A bottle jack is located under the strut and pushes the piston of the strut upwards thus displacing the fluid out of the strut and into the sphere. The sphere of course is screwed into the strut.
When the sphere is screwed to the strut there's an inevitable air bubble trapped between them. Added to that there's some slack in the whole assembly. Therefore the first 3 strokes of the bottle jack handle don't result in any reading on the pressure gauge. Only after all the slack is taken up and the air bubble pressure is up to the mark does the pressure gauge start to read something due to the scale being so coarse over 200 bar. After the slack is taken up the gauge presure
starts to rise. At this point the little hard disc at the bottom of the diaphragm is firmly pressed against the inside of the sphere orifice. Continue pumping the handle of the bottle jackand the pressure gauge will rise. At a certain point further pumping of the jack handle will not result in any more rise in pressure. This is because the diaphragm has lifted off the orifice and fluid is now entering the sphere. At this point the fluid pressure as recorded by the pressure gauge equals the gas pressure in the sphere.
The quantity of fluid pumped per stroke of the jack handle is very small, probably a few cc's. Don't forget the sphere capacity is around 400cc depending on model. Clearly you will run out of puff on the jack long before the sphere is full of fluid. As the fluid is pumped into the sphere, theoretically the gas pressure will rise
because it's mass is staying the same but the volume is being reduced. This will follow Boyles Law. Theoretically if the volume is reduced to zero the pressure will be infinite. In practice this wouldn't be achievable. The sphere tester would start to break apart at the seams. In reality you probably wouldn't be able to reduce the
volume much beyond about a third of the original volume before the pressure required by the tester starts to become unfeasible. Therefore the diaphragm would never bottom out.
The sphere pressure quoted is measured 'at rest' ie with the sphere neck open to atmosphere.
I'll try to track down a table of sphere presures and add it to this post for reference.
Go on - have a go at making one. There's nothing complicated about it. If you need some help just shout.
Cheers
noz
