First stage whistling noise?

[andshand]

Hi, i hope you dont mind me adding to this as there seams to be a fair bit of knowledge here.
I bought my SP mk20 from ebay, sold as noted with a whistle. I took it for a service and mentioned this, on picking my reg up the LDS said the top O ring was folded over and this MIGHT have caused it. However when i used it i noticed there was no whistle until about15 dives in then it came back. any ideas?

 

[Couv]

Hello andshand,

We do not mind at all your asking, if you do not mind the best guess answers we come up with.

The tech may have been a bit stingy with the Christo-lube or may have failed to replace and lube the piston stem o-ring. Another possibility is that there is a little corrosion on the piston stem itself that needs to be cleaned off. After the last service, even if the tech properly lubed the stem and o-ring, corrosion on the dynamic sealing surface could be causing the friction and noise.

Now, having said all that I have to give a disclaimer. I have never worked on a MK 20 but I know there are slightly different procedures when installing the piston and the accompanying bushing system. Incidentally, ScubaPro changed designs on the bushing system and composite piston a few times in the last several years. A couple of changes involved eliminating a cut along the axial length of a stem bushing, using a softer material and polished stem to help alleviate whistling. In fact, one bulletin (Engineering Bulletin #272) says, "Lubricant is now added, in very small amounts, only to the piston stem o-ring, and not directly to the stem itself. At very low temperatures typical in ice diving, traces of lubricant on the piston stem act as nucleation sites for ice."

So, back to the shop I would go and tell them the noise has returned after a few dives. Gently suggest the stem be looked at closely for corrosion and that the service kits installed are up to date.

I see in your profile you are in NW England and wonder if you are doing cold water diving.

Hope this helps,

couv

 

[DA Aquamaster]

Hi, i hope you dont mind me adding to this as there seams to be a fair bit of knowledge here.
I bought my SP mk20 from ebay, sold as noted with a whistle. I took it for a service and mentioned this, on picking my reg up the LDS said the top O ring was folded over and this MIGHT have caused it. However when i used it i noticed there was no whistle until about15 dives in then it came back. any ideas?

Normally a folded over o-ring results in a large leak, especially when it is the top one. In any case the tech should have been able to feel the piston seat harder than usual in the swivel cap when he assembled it. The fact that he did not probably speaks volumes about his experience level and/or attention to detail.

As couv inidcated, the issue is still one of inadequate lubrication. Christolube was and still is promoted as the greatest thing since canned beer and while it is a lot slicker than silicone, you still need enough to do the job. In my opinion, Scubapro created an issue with the service bulletin couv referenced as they stated that less lubrication was better and specified "very small amounts". That may be the case on the stem (although I think the more recent TIS boots on the piston stem make their original premis about ice formation obsolete) but it is still not the case with the piston stem o-ring and bushings. I ahve never seen a Mk 20 or Mk 25 fail due to too much lubrication in that area, but I have seen scores of them whistle due to not having enough.

So...the whistling problem would almost certainly be resolved with the application of additional Christolube to the piston stem o-ring and bushings, but at this point I would also replace the piston stem o-ring as it may now be a bit rougher than it should be where it drags against the piston stem. The 0-ring costs something like $0.42.

 

[skynscuba]

I still haven't gotten around to getting the reg checked out again -- no time to dive for a while yet. Mare I did put it on tanks at various pressures. The sound seems most noticable on one at 1900psi, a bit less at 3100, and pretty much not at all on a 350 psi tank. It's not a piercing sound, and is most noticable when barely purging a 2nd stage, or breathing very gently.

 

[skynscuba]

Update: sent the reg back. I don't know if they did anything to the 1st stage or not, but the whistle is pretty much the same. The repair sheet just showed 2nd stage cracking pressures.

We got in the pool to work on BB's drysuit setup, and I could hear the whistle on her reg whenever she would breathe. I'm pretty sure I would have noticed that when diving with her previously, so I think it is a new "feature" after the annual service (1st since new).