MK11, MK17, or MK25

[supbihatches]

I am upgrading my first stage, and have decided to get either a mk11, mk17, or mk25. I can't decide though.

I am planning on doing a cavern course within the next few months, and then eventually cave, but I am not very interested in decompression or trimix and I don't dive in water colder than 60 degrees F. I already dive a balanced reg with DIN and steel tanks, but now it's time to upgrade.

Everyone says that the mk25 is the best, especially for tech diving because it's a piston with less parts is and easy to deal with flooding. But am I really going to have to deal with that issue in cavern and cave? If not then the mk11 should be everything that I need right? And if the mk17 is environmentally sealed than essentially flooding shouldn't be an issue there either....? And if flooding is not an issue for the mk17, then wouldn't that be a better choice over the mk25 because the diaphragm is better suited to colder diving?

Also if you have an idea of a better reg for me I would greatly appreciate any advice.

 

[DA Aquamaster]

Here is my .02 as a cave diver and reg tech.

The Mk 25 has oodles of performance - about 300 SCFM in terms of flow rate, but that is a bit more than twice the flow rate you would ever need and it comes at the expense of cold water performance and lacks a sealed ambient chamber and as such is at a disadvantage in silty water as well.

THe Mk 17 has a sealed ambient chamber and the flow rate is 177 SCFM - a lot less than the Mk 25 but way more than the 125 SCFM minimum that you need for a technical diving regulator. THe sealed ambient chamber and excellent heat transfer make it an ideal cold water reg, but more importantly, in silty or sandy water, it will keep gunk out of the reg. That can be a major issue in cave diving if you start getting into tighter and siltier caves.

For example before my most recent trip, I inspected all my back gas regs, stage and deco regs and the only regs that failed were piston regs due to large amounts of crud in the ambient chambers and associated wear on the piston stem and piston head o-rings. That was not as much an oissue for me, until I started diving smaller tunnels and in placed where the viz is less than pristine, but in my opinion it is a major selling popint for the Mk 17 as opposed to the Mk 25.

In terms of hose routing you will find several pictures on line of good doubles hose routing for the Mk 25. Good is the right word as it works for the Mk 25 (and similar Mk 5, 10, 15 and 20), but many divers misinterpret that in some way to mean it is some sort of ideal holy grail of hose routing and that is really not the case. The Mk 17 lets you route all three hoses basically straight down and it is much cleaner - and I say that having switched from Mk 25's to Mk 17's.

In terms of perfromance, I have never noticed any difference at depth between the Mk 17 and Mk 25 even with very finely tuned high performance second stages (D400's X650's, G250's and G250V's). On the test bench, the Mk 17 does not have any nmore IP drop or lag in response time than the Mk 25, and if you can't see it on a test bench, the second stage won't be able to tell the difference either.

I have been using Mk 17's as my back gas regs since they came out and I have had zero issues with them in deep wreck or cave diving.

 

[zider]

I switched from a MK20 setup to a MK17 due to the fact that I started diving during the winter. I really like the MK17 and as DA A. stated, the hose routing is really clean if you start diving doubles. My vote is for the MK17.

 

[supbihatches]

Here is my .02 as a cave diver and reg tech.

The Mk 25 has oodles of performance - about 300 SCFM in terms of flow rate, but that is a bit more than twice the flow rate you would ever need and it comes at the expense of cold water performance and lacks a sealed ambient chamber and as such is at a disadvantage in silty water as well.

THe Mk 17 has a sealed ambient chamber and the flow rate is 177 SCFM - a lot less than the Mk 25 but way more than the 125 SCFM minimum that you need for a technical diving regulator. THe sealed ambient chamber and excellent heat transfer make it an ideal cold water reg, but more importantly, in silty or sandy water, it will keep gunk out of the reg. That can be a major issue in cave diving if you start getting into tighter and siltier caves.

For example before my most recent trip, I inspected all my back gas regs, stage and deco regs and the only regs that failed were piston regs due to large amounts of crud in the ambient chambers and associated wear on the piston stem and piston head o-rings. That was not as much an oissue for me, until I started diving smaller tunnels and in placed where the viz is less than pristine, but in my opinion it is a major selling popint for the Mk 17 as opposed to the Mk 25.

In terms of hose routing you will find several pictures on line of good doubles hose routing for the Mk 25. Good is the right word as it works for the Mk 25 (and similar Mk 5, 10, 15 and 20), but many divers misinterpret that in some way to mean it is some sort of ideal holy grail of hose routing and that is really not the case. The Mk 17 lets you route all three hoses basically straight down and it is much cleaner - and I say that having switched from Mk 25's to Mk 17's.

In terms of perfromance, I have never noticed any difference at depth between the Mk 17 and Mk 25 even with very finely tuned high performance second stages (D400's X650's, G250's and G250V's). On the test bench, the Mk 17 does not have any nmore IP drop or lag in response time than the Mk 25, and if you can't see it on a test bench, the second stage won't be able to tell the difference either.

I have been using Mk 17's as my back gas regs since they came out and I have had zero issues with them in deep wreck or cave diving.

So if I was to dive in a silty cave would the mk11 definitely get contaminated since it has no environmental seal?

 

[adm3745]

In extended exposure to silt or sand, is a Mk11, or a mk25 going to hold up better in between servicing.

 

[DA Aquamaster]

The Mk 11 and MK 25 are both fairly open, so they are easily contaminated, but also makes it easier to rinse - as long as the internal parts are not coated wth christo lube - and not doing so makes them a bit more prone to freezeups and in some cases to corrosion, so it is a mixed bag. If I had to pick betwene a mK 25 and a Mk 11, I'd pick the Mk 11 as at least any silt sand or contamination would not be in contact with any sealing surfaces for the o-rings. In the MK25, the piston head o-rings and the HP piston stem o-ring are all exposed to silt or sand, and they are all dynamic o-rings where wear induced by sand or silt could be an issue.

A fully sealed reg like the Mk 17 is preferrable as it prevents any contmaination at all in the ambient chamber.

As an aside, a cave instructor I dove with last week commented that he took his reg in for service and was told "Oh...you've been diving P III a lot" due to the condition of the reg. When a tech can tell you where you have bene diving due to the condition of the reg (effects of silt hydrogen sulfide, etc,) it creates a strong arguement for a fully sealed design.

 

[NorthWoodsDiver]

I own almost a dozen mk17 first stages and have used them for everything from public safety diving to caves to under the ice without any issues. I used to have mk25's...

 

[adm3745]

This has inspired some concern over my mk25 wear. If I dive 100 dives a year on it (in Fl springs/sinks) and clean it thouroughly after each dive, is piston head errosion going to be a serious issue? I mean during a dive the piston is nearly static, and when it decompresses there should be no chance of (pushing) and crud into the piston orings? If so, should I be inspecting/cleaning the reg and replacing those orings far more often than once a year?
This is a factor I never seriously considered when choosing a mk25 over the 17/11.

 

[DA Aquamaster]

The Mk 25 has two piston head o-rings. One, in essence, acts as a scraper ring and prevents most larger contaminants from reaching the second o-ring. Really fine clay silt is perhaps another matter, as I suspect it could be fine enough to be embedded in the wall of the swivel cap and get past the first o-ring, especially if that first o-ring has started to get frayed and degraded a bit pushing sand around.

Also, when the reg is pressurized, the piston is down on the seat and the head is at it's extreme travel into the ambient chamber, so anything forming when the reg is pressurized and inactive will be just beyond the bearing surface for the o-rings. Consequently the larger bit that can contact the o-rings' bearing surface could only do so when the piston is off the seat and gas is flowing.

Where it can get interesting is on a stage reg where the reg may end up being unpressurized and exposed to significant sand and silt. In that regard the Mk 25 is better suited than the Mk 5 and Mk 10 where there is only one o-ring and both those regs have seen lots of use as stage regs.

So it is not a major problem but it is something you need to be aware of in terms of daily rinsing and in terms of annual servicing. It is not a case of the Mk 25 being bad, but rather of the fully sealed Mk 17 being better by avoiding the potential problem entirely.

Based on my experience, my stage regs (all piston regs) will get serviced more frequently than in the past due to what I have found when servicing them this year.

 

[walke121]

I have 4 MK 25's now and I really like them. I like piston regs better than diaphram regs but that is just me. I have them serviced every 1.5-2 years and they all look like new, inside not out.