noz
November 13, 2004 04:19 pm
Hi there, Guest,
I'm not quite sure what you're getting at. The sphere consists of a hollow metal sphere, a 'rubber' diaphragm and a semi-spherical plate which keeps the diaphragm edge pressed and sealed against the side wall of the sphere (ignoring for a moment the damping mechanism in the neck).
In the early days of spheres I saw one cut in half (by Westroen as far as I can remember) revealing a device at the plug hole with sharp spikes on. If the pressure in the sphere goes too low before recharging and the diaphragm touches the spikes then the diaphragm will be punctured. I presumed this was Citroen's way of making the sphere fail catastrophically once it got too old and forcing you to buy a new one.
I usually test the sphere pressure before removing the plug out of interest. In fact it's the residual pressure which I use to determine whether the sphere needs recharging or not. When recharging spheres I have sometimes seen leakage at the plug using leak detector spray. Tightening the plug usually cures the problem however, once I've had to depressurise the sphere and replace the o-ring.
The common mode of failure for a sphere is diaphragm leakage. This can either be that the diaphragm has parted company with the side wall of the sphere or the diaphragm has ruptured for some reason. There are some theories around that suggest the large number of deflections which the diaphragm performs in it's life gradually weakens the 'rubber' and it eventually fails. I personally don't believe this for reasons that would take too long to explain here. However, diaphragms do leak there is no doubt about that.
If I understand you correctly, you are suggesting there is some kind of test you can perform on the sphere against which you make the judgment whether to recharge or not. I'd be very interested in such a test. The sphere is constructed of two halves and is welded together at time of manufacture. I'd love to hear how the diaphragm is 'tested' when the only access is through the plug hole with the plug removed. I'd especially love to know how the results of this test determine the 'condition' of the diaphragm. It either leaks or it doesn't.
Personally speaking I have never managed to recharge a sphere which has a leaking diaphragm. The gas just bypasses the diaphragm and comes out the damper hole in the neck. I don't understand your claim that some people recharge spheres where the diaphragm won't hold the pressure. ( The fill pressure varies between 40 and 70 bar depending on which particular application the sphere is for.)
The 'daft' looking ends to which you refer are almost certainly the Schraeder type which are primarily used in the refrigeration industry. The reason they are so big is that there's a non-return valve built in so that they can be recharged just like recharging a tyre with air. The original plugs are just that, plugs. They have a small o-ring under the shoulder to seal the gas in. I personally re-use the original plugs. However, the drawback of this method is that the sphere must be removed from the car to recharge. The single biggest advantage of the Schraeder valve method is that the sphere can be re-charged in-situ. When you consider that the biggest and riskiest part of the job is getting the sphere off the car (especially rear ones from CX's, BX's etc where the whole strut must be removed) then you can see why the Schraeder method has it's advantages.
In terms of ripping people off, I don't charge for the service. All I'd ask for is a contribution towards the cost of the Nitrogen which I have to pay for when I get the bottle refilled.
Why not register as a user and join the forum? The more comments the merrier.
cheers
noz