Scubapro Mk 20 odd whistling problem

[Isaac-1]

Last week while I was on vacation on Roatan I ran into an odd problem with my Scubapro Mk 20 first stage (blunt piston), thankfully I had a recently rebuilt spare Mk 10, and swapped it out. The problem started on my 2nd or 3rd dive of the week, and would only happen with a partly full tank starting at around 1,500 psi going on until about 1,100 psi. The problem can be described as a loud high pitched whistle, and bubbles coming out of the 1st stage vent holes, the whistling was constant, not associated with breathing. There was no air leak or unusual sound with a full tank, I am not sure exact pressure where the air leak would start, however the whistling would stop around 1,100 psi, and leak would continue down to about 900 psi. I would catagorize the leak as moderate rate, fast enough to leak down pressure from a turned off tank valve in about 45 seconds. I have never serviced this particular Mk 20 myself (I bought it off ebay), I did take it into the local Scubapro shop this summer for the yoke nut recall, while there they messed up the switch on the attached D400, I took it back again, and they put the D400 aspirator in backwards. (this should teach me to let other people work on my gear). I did not have the parts with me to service the Mk 20 (I now have a new piston and service kit), or a full service bench, just my basic field kit, multi tool, allen wrenches, picks, (rebuild kits for just about everything else I own) etc. So sitting in the club house I removed the cover noted the old lube was somewhat gummy , cleaned and relubed the piston O-ring, etc., I reassembled, and it seemed to fix the problem, although I did not do extensive testing. Has anyone else ever seen anything like this, 2 other scubapro techs (one a scubapro shop owner), that saw me working on it had never seen this problem before.

thanks Ike

 

[DA Aquamaster]

Whistling in a flow through piston 1rst stage is usually related to an inadequately lubricated piston stem o-ring. I suppose in a really bad case it could result in gas leaking past the o-ring, but a leak is normally caused by the o-ring getting pinched with little cresent shaped cuts resulting. If the plastic bushing wears excessively (as in it has not been replaced in a few years) it could have enough clearance to allow o-ring extrusion and pinching.

Another source of Mk 20/25 whistling is installing the upper bushing upside down (there are 4 little notches in it and they should face away from the o-ring.)

In my experience, leaks past the piston head o-ring tend to be constant - which makes sense as the IP is constant, so I don't see that as being the problem with a leak that only occurs in a specific pressure range.

Either way it needs a full annual service with new piston stem o-rings and bushing as well as a careful inspection of the piston stem and the o-ring bearing surfaces in the swivel cap.

 

[Isaac-1]

I tend to think it was in some way associated with the gummy lube, otherwise everything looked good. O-rings seemed in good condition, no visbile crushing or nicks. The thing that really struck me as odd about this one was the limited pressure range, with both an upper and lower end not only for the whistling, but also for the air leak, I just can't visualize the cause like I might be able to with a leak above or below X pressure. (Note I did not remove the seat end of the Mk 20, only the turret and cap end, I did not have the right sized allen wrench). Except for the gummy lube I was impressed to see that everything else looked like nearly new (the second stage that it came with was much the same)

 

[redacted]

I tend to think it was in some way associated with the gummy lube, otherwise everything looked good. O-rings seemed in good condition, no visbile crushing or nicks. The thing that really struck me as odd about this one was the limited pressure range, with both an upper and lower end not only for the whistling, but also for the air leak, I just can't visualize the cause like I might be able to with a leak above or below X pressure. (Note I did not remove the seat end of the Mk 20, only the turret and cap end, I did not have the right sized allen wrench). Except for the gummy lube I was impressed to see that everything else looked like nearly new (the second stage that it came with was much the same)

I guess you were not able to inspect the piston HP o-ring. The symptoms make it (or the top bushing) sound like the culprit(s). They are exposed to and must respond to the full range of tank pressure variations. Maybe you'll get lucky and be able to see a fault with one of those. If not, you may just have to be satisfied with changing them out because "what else could it possibly be?" With the leak being that large, hopefully you will be able to see some defect that would have caused the problem.

 

[halocline]

Since the two guys that have taught me what little I know about regulator repair have already posted, I don't have much to add, except I suppose it's possible that the limited supply pressure range of the whistling and leaking indicates that the HP o-ring is extruding and pinching some. Maybe when the tank is full it's extruding enough so that it doesn't leak but is really pinched against the piston, then within the range it's extruding enough but not pinched so much that it is leaking, and below a certain supply pressure it's not extruding enough to cause the problem. That's a WAG. Either way, you need to replace the o-ring and the bushings. I bet if you hooked up an IP gauge you'd find the IP much higher at high supply pressure.

You might try a true 90 duro polyurethane o-ring when you rebuild. I've only used them on my MK5 and MK10s, and they do correct the drop in IP over the supply range I have experienced with those regs; something that I assume is caused by HP o-ring pinch.

I wonder if the last guy that rebuilt this used the MK20 tool or just installed the seat carrier first and hoped he did not dislodge the bushings when installing the piston. The comment that DA Aquamaster made about the bushing orientation might also yield some insight; I could very easily see an inexperienced or careless tech installing the bushing upside down.

Or, maybe it's just worn out. Personally every time I buy a reg on ebay (too frequently, I admit) I immediately rebuild it. I have re used almost all the o-rings at times, but never the HP piston one. I would not take one on a dive trip without having had it apart, but I'm paranoid.

 

[redacted]

Since it still has the old style piston, it may well also have the old style bushings that were split to allow for material contraction in cold water. Those splits may facilitate wear or damage leading to o-ring performance issues.

 

[Isaac-1]

Or, maybe it's just worn out. Personally every time I buy a reg on ebay (too frequently, I admit) I immediately rebuild it. I have re used almost all the o-rings at times, but never the HP piston one. I would not take one on a dive trip without having had it apart, but I'm paranoid.

I had planned to rebuild before the vacation, just have been WAY TOO BUSY, thankfully I did take a lot of spares on this trip. My fiancee also blew an HP hose at the 1st stage fitting while on the trip.

 

[jac11819]

DA Aquamaster is right on the money. Take his advice and for sure, double check the upper bushing. Most inexperience technicians make the mistake of replacing either the upper bushing, lower one, or both upside down. On Saturday, I just rebuilt one for customer that was doing exactly what you described. I asked him if he had tinkered with the reg but, he swore up and down he didn't.