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whistle in MK25/S650

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Hi all, for a while now ive had a high pitched whistle in my first stage ScubaPro MK25. Much to the 'enjoyment' of my fellow divers, and my own sanity. The whistle only occurs between a pressure of 3000+ psi and about 2500 psi, after which it pretty much disappears completely. Depth also plays a role. Ive had this damn thing serviced twice specifically to get rid of it, and both times they failed. So im gonna give it one more go, but would like to know a bit more about the probable cause for this. It doesnt matter what I breath out of. S650, R250, AIR2, whatever, same problem.

 

Anyone have any idea? Intermediate pressure problem? Internal damage?

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Hi Cor,

I've had it with Mk25's-it is caused by the spring in the first stage resonating. The best way to deal with it is to add plastic shims on both sides of the first stage main spring. I think SP do these as a part, but techs will often add them to one or other side, rather than both.

 

Adam

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the only problem with adding shims is that it raises your intermediate pressure thats ok if your spring is at the low end of the acceptable range but if it is already near the high end then a shim will likely put it over the top,just as often it is caused by harmonic resonance over the end of the piston tube and this is much more difficult to get rid of the scuba pro kits come with several different hp seats and generally a mk 25 will have a + seat i have switched to a t seat in alot of these whistlers and it has solved the problem,you may have to adjust the shimming after switching seats to get the intermediate pressure back into spec but your tech should be able to figure that out no prob.

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the only problem with adding shims is that it raises your intermediate pressure thats ok if your spring is at the low end of the acceptable range but if it is already near the high end then a shim will likely put it over the top,just as often it is caused by harmonic resonance over the end of the piston tube and this is much more difficult to get rid of the scuba pro kits come with several different hp seats and generally a mk 25 will have a + seat i have switched to a t seat in alot of these whistlers and it has solved the problem,you may have to adjust the shimming after switching seats to get the intermediate pressure back into spec but your tech should be able to figure that out no prob.

The MK25 has adjustable interstage pressure anyway via the pre-load screw at the top end....

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