MK 25 or MK 17 for Cave Diving

[Sludge]

why do you prefer the piston over the diaphragm for water above 40?

If I may be allowed to plagiarize a bit, "first stages with air balanced pistons deliver significantly more air to the second stage than any other first stage, while their performance is totally unaffected by the changing tank pressure. A balanced piston allows the use of lighter and more sensitive components, resulting in ultra-fast breathing response, instant delivery of air on demand, and extra high air flow, especially in low tank pressure ranges."

When I started diving, the store owner was so adamant about this that he wouldn't even sell diaphragm first stages. Believe it or not, this is more important in single-tank diving than diving doubles, because there is almost no scenario where two divers will be breathing off the same first stage whereas an air-sharing episode of single-tank divers will involve two divers and one first stage. Doing this at 100' will be beyond the air-delivering capabilities of a diaphragm.

Also, I have a potential reply to your theory of not getting service every year, but I'm not sure I want to go out on that limb.

[deanme]

Wow, great input.

This being the case, it is not clear to me why anyone would purchase a diaphragm first stage, other then cost and cold water diving. I’m going to re-think getting replacements for my two MK25’s with the MK17’s. both my Mk25’s breath extremely well, even with low tank pressures.

Great observation about buddy breathing on a single bottle with a diaphrame. Again, assuming what you have said is correct, this seems to be a technical bit of information everyone should know to better understand the risks of diving at 100 feet with a single bottle.

I dive with an OMS H value on my single tanks. I do a lot of solo diving, and have always felt less stress diving with an H valve and two separate first stages. Also makes it easier to dive a deep wreck on doubles and then do a 60 foot reef dive the next day on a single tank with no equipment re-configuration. Lots of opportunity for fun comments by others on the boat joking about diving with two first stages and a long hose on a 60 foot reef. 

Actually, I’m the one that went out on a limb saying I don’t maintain my deco first stages. I’m not interested in general “your going to die” comments. Just your technical thoughts. I’d also be interested in what components are at risk with no maintenance for an unbalanced piston. Maybe it is time to get a new set of low end OMS first stages? I’ve already signed the release forms for any advise.

But I really don’t think there are significant risks. Breathing on a deco bottle that is easily accessible and can be turned off and on for each breath in an emergency doesn’t seem like much of a risk. Maybe I’m missing something? I wouldn’t have asked if I didn’t want a polite and constructive response.

[Sludge]

You made me do it.

I started an experiment nine years ago concerning servicing. First, on my backgas, I rebuild my regs every February, using new parts kits (as well as my open water stuff and just about everything else I have).

I have one stage regulator that I rebuild every year, but instead of a new kit, I clean and reuse all the o-rings and other parts.

I have a Mark 5 that I use for filling tires and stuff, and I have a Mark 10+ that I use with a blower nozzle for blow-drying regulator parts when I do rebuilds. Neither of these have been serviced since 1999.

I wanted to see how long this stuff would stay within specs. So far, not only the stage reg but also both non-serviced regs are spot on.

Which is not to say everybody shouldn't get their regs serviced, with new parts, every year. By the way, notice how you won't see me in the water with fewer than two regulators that have been serviced properly.

[deanme]

Thanks for the input. Now I realize why you were hesitant in commenting.

Like I said below, I'm going on 4 years for my deco first stages. I use them in the springs and in the ocean. I dive a lot, about 200 dives a year. All the first stages perform like new, even the deco bottle first stages, that I never service.