Servicing my own gear

[halocline]

The MK17 should hold a more stable IP than that.... The pistons are the ones that usually see the IP creep up as the supply pressure goes down. The 17 should go up at a rate faster than the ambient pressure due to the thumb tank principal in the article linked to above.

1) You have it backwards with piston regs; they drop in IP as supply pressure drops.
2) You are confusing IP change through the supply pressure range with IP change through depth changes. They are two separate issues.

 

[bl6394]

... "overbalanced" usually refers to ...

I hear you halocline. As I recall, Rene was providing a simplified categorization:

• unbalanced: IP drops as Supply pressure drops
• balanced: IP remains consistent - regardless of Supply pressure
• over-balanced: IP increases as Supply pressure drops

If it really bothers you - (given the context previously provided) - you should probably take it up with Rene... I'm indifferent as to what you call the behavior.

Whatever you want to call it ... expect that behavior when you vary the supply pressure to your Mk 11 / 17 / 25's.

 

[wizbangdoodle]

It seems to me that you would never really want you IP to change (much). Therefore, the balanced unit is a far superior to others. Can anyone give me an example of why you would want an unbalanced or overbalanced unit?

 

[tbone1004]

unbalanced used to be nice because before SPG's were a thing, and even when they first came out they cost more than the whole regulator set, you could "feel" the reg starting to breathe harder if you had an unbalanced first and unbalanced second. Balanced second stages minimize the effect of an unbalanced or poorly balanced first

over-balanced I see no reason why you would ever want a regulator to do that. The marketing department will probably tell you that you want it to breathe easier when the tank pressure starts going down but if the second stage is tuned right it shouldn't matter since it is at the threshold already.

over-compensating is told to us by marketing that the IP increases over ambient to aid in gas flow as the gas gets more dense with depth. With a properly balanced and well tuned second stage, you frankly won't be able to tell a difference. With a non-balanced second stage you might be able to tell but aside from Poseidons which want lower IP for easier breathing, I don't know of anyone doing deep diving on unbalanced second stages anymore...

properly balanced are ideal but are VERY difficult to get with a small IP swing. Frankly I don't buy the MK25 swinging as little as 1psi because I just don't think it's possible. Would love to be proved wrong though because I'm trying to find a swivel with as little IP swing as possible for my Poseidons... 5psi I can start to believe but I have seen fresh out of the box MK25's with a 15psi+ IP swing from 200psi up to 3600psi cave fill. The repair manual doesn't give any indication of what it is supposed to stay in, but gives a 20psi range from low supply to high supply that it can stay within. Maybe it's the extreme where it falls off pretty dramatically below 500psi and creeps pretty quick above 3000 since we didn't log the swing, but it was pretty dramatic. Atomics do it as well. Either way, with properly balanced second stages it doesn't really matter if there is a 10-15psi IP swing because the second stages will take care of it.

 

[halocline]

It seems to me that you would never really want you IP to change (much). Therefore, the balanced unit is a far superior to others. Can anyone give me an example of why you would want an unbalanced or overbalanced unit?

Please forget about this "overbalanced" thing that bl6394 says the teacher told him about in the weekend seminar he took. You should get the Vance Harlow book on regulator repair for a good primer on how regulators actually work and what "balanced" really means.

It's true that exact IP stability throughout the supply range is in general a good thing, but it's not crucial to first stage performance, and it rarely exists anyways. Piston regulators will generally drop in IP over the supply range and diaphragms will generally rise over the supply range. The harlow explains why this is the case in a nice clear manner. The bumper sticker version is that piston regs have some downstream bias while diaphragms have some upstream bias. This means that the air "pushing" on the piston or poppet from the tank has some influence in whether it takes less pressure (piston) or more pressure (diaphragm) to close the valve as the tank empties. IP is the force required to close the valve.

What you want to avoid in a first stage is a wide swing (20 PSI+) from full to empty, and even that is not necessarily a bad thing; the SP MK2 drops a good 20 PSI and with a balanced 2nd stage it is still a very solid performer. There are other issues more important in 1st stage performance, like IP dip and recovery under demand, and especially quick, consistent 'lock up' at IP over a large number of dives.

 

[halocline]

Frankly I don't buy the MK25 swinging as little as 1psi because I just don't think it's possible. Would love to be proved wrong though because I'm trying to find a swivel with as little IP swing as possible for my Poseidons... 5psi I can start to believe but I have seen fresh out of the box MK25's with a 15psi+ IP swing from 200psi up to 3600psi cave fill. The repair manual doesn't give any indication of what it is supposed to stay in, but gives a 20psi range from low supply to high supply that it can stay within. Maybe it's the extreme where it falls off pretty dramatically below 500psi and creeps pretty quick above 3000 since we didn't log the swing, but it was pretty dramatic. Atomics do it as well. Either way, with properly balanced second stages it doesn't really matter if there is a 10-15psi IP swing because the second stages will take care of it.

The MK25 is pretty consistent up to around 3000 (< 5 PSI) and then it starts to rise at higher pressures due to increased friction and extrusion in the HP o-ring. Probably the best way to address this is with a 90 duro polyurethane o-ring, well lubricated, in the HP piston spot. If you are looking at one, I would suggest going with a MK20 and using the MK25 composite piston. That way you don't have the added complexity of the external IP adjustment, but you still get the MK25 flow rate and performance. It doesn't really matter......

The earlier SP balanced piston 1sts have a greater IP swing; my MK10s go at least 10 PSI from 3000-500. It doesn't bother me in the least, but for HP tanks, you'd be better off with either a MK15 or 20, which have the bushings for addressing extrusion at higher tank pressures. I think the higher duro PU o-ring is very helpful.

 

[bl6394]

...Please forget about this "overbalanced" thing that bl6394 says the teacher told him about in the weekend seminar he took. You should get the Vance Harlow book on regulator repair for a good primer on how regulators actually work and what "balanced" really means.
...

The Rene in previous comments was, Rene Dupre, Technical Manager at Scubapro. He's responsible for teaching the equipment repair courses for Scubapro and seems extremely knowledgeable about the Scubapro product line and techniques used for servicing them. The weekend class Halocline referred to was a hands on two day seminar describing the processes for servicing Scubapro regulators and is consistent with the manufacturer's present expectations for servicing their products. Halocline seems to take exception with some aspects of their course content. No one will loose sleep about this. I've participated in equipment seminars from a number of manufacturers - and the Scubapro presentation was one of the best.

Some anonymous guy on the internet who describes himself as a salinity gradient may know more about the Scubapro product line than the manufacturer... who knows. In any event, Halocline is certainly a passionate amateur enthusiast.

Halocline does have a great suggestion in Vance Harlow's Scuba Regulator Maintenance and Repair (as are the other books from Airspeed Press). Pete Wolfinger's Regulator Savvy is also a great book. Manufacturer classes can also be a great tool - if you can find a local shop to sponsor your participation.

 

[bl6394]

It seems to me that you would never really want you IP to change (much). Therefore, the balanced unit is a far superior to others. Can anyone give me an example of why you would want an unbalanced or overbalanced unit?

There are always pros and cons with any choice.

The Mk2 is an unbalanced piston regulator. I think even Halocline will agree with that. Compare the schematics of the Mk2 and Mk25 - and note all the additional parts and extra engineering.... A pro of the Mk2 is that its simple to maintain -- and bullet proof. I use it as a deco regulator - its good with up to 100% O2 (with the right seat / service kit.) And does everything I need it to do - reliably - at 20 ft. Now - would I choose to use the Mk2 at 300 ffw - probably not. I'd prefer a regulator that had better performance at depth. And I would accept that it had a higher cost to maintain (service kit) and perhaps was a bit more finicky than the Mk2.

In cold fresh, silty water - that would probably be the Mk17. When I'm sidemount in a tight cave passage - I like that the environmentally sealed Mk17 ((over)balanced/diaphram) is not subjected to any of the silt / grit that can get thrown up. When I'm teaching a deep class in cold fresh water with a student that has a high breathing rate - I put them on a Mk17 - to make it less likely they will freeze up their regulator. That same student in warm water - meh - they can use a Mk 25. It has a theoretically higher performance than the Mk 17 - and is a great seller at the retail counter.

Know the characteristics of your regulator - and you can choose the right tool for the right job.

 

[wizbangdoodle]

And this discussion has raised another question. How do you identify 1st stage models. I was looking at my wife's and the only thing that identifies it as a Mk25 is the knob for tightening it on the tank. Do you just have to become familiar with each model and what they look like?

 

[tbone1004]

most of them look pretty different from each other that you can ID them at a quick glance. Some are less obviously, but most of the major revisions have had a pretty substantial cosmetic change. Apeks are super easy, though they all use the same parts kits so it doesn't actually matter. Scubapro are pretty straight forward now, though there have been a few that are slightly different, but can be upgraded to the "new" ones IIRC. This was mainly on the second stages IIRC, but I'm not a SP guy, so someone more familiar can correct that. Atomic is a little weird to ID because they look mostly the same, but I think they take the same kits and function the same so it's not really that critical. Most mfg's have a pretty obvious change in the cosmetics when they make a substantial revision to a regulator so you do just have to "know", but it's not as subtle like ID'ing a 68 vs 69 GTO