Scubapro MK15 servicing question

[bada3003]

If that o-ring is missing, the 1st stage would never lock up. It is essential.

Hmm... in that case, I may have missed it in the reg. I looked for it but didn't find it. As it wasn't in the service kit either, I didn't verify 100% and left it at that.

I did replace the HP seat which is easy to do by unscrewing the HP retainer in the MK15. Now, the 1st stage IP, when the 2nd stage locks up after purging or breathing, immediately reaches 120 PSI and stays there. No creeping up to 130. One explanation is that the contact point between the plastic HP seat and the orifice of the piston stem left a tiny gap which allowed air to flow from the HP chamber to the LP chamber until the increasing pressure in the latter applied sufficient force to close the gap. Just speculation though.

I've started reading the later chapters of the two reg books (Harlow's first as it's a bit more practically/hands-on oriented) to get a better understanding of what's going on on the performance front. Since the mk15 is not environmentally sealed and I'm not gonna use it in colder waters, I'll have to find a way to increase the IP from 120 to 135 or thereabouts. The MK17 has an IP of 140 so will have to lower it a bit to reduce the likelihood of free flow.

 

[herman]

There is no good reason to change the Ip of the MK-15 or the 17 for that matter. A few psi difference in IP will have very little effect on performance as long as the seconds are tuned to the particualar first. IP value in and of itself has nothing to do with freeflows as long as the seconds are set to the value of the first. The only good reason to change any of the IPs is to match them so you can easily swap second stages between both first stages.. In that case, your best bet is just to lower the 17s to that of the 15. Any time you dissamble a piston reg you stand the chance of damaging the piston, piston orings or creating issues with the HP seat, conversely, the 17 is externally adjustable and you have no risk of damaging anything.

 

[halocline]

I did replace the HP seat which is easy to do by unscrewing the HP retainer in the MK15. Now, the 1st stage IP, when the 2nd stage locks up after purging or breathing, immediately reaches 120 PSI and stays there. No creeping up to 130. One explanation is that the contact point between the plastic HP seat and the orifice of the piston stem left a tiny gap which allowed air to flow from the HP chamber to the LP chamber until the increasing pressure in the latter applied sufficient force to close the gap. Just speculation though.

Two things:

1) If I'm reading this correctly, you had previously taken the regulator apart but did not unscrew the HP seat retainer? In that case, you should not have been able to install the piston if you had removed it. To install the piston, you put a bullet on the end of the piston shaft to protect the HP o-ring, and push the piston in place. The bullet then is removed through the hole that the seat retainer screws into. If you installed the piston without the bullet tool, it's likely you have damaged the HP o-ring in the process and the smart thing is to get a new duro 90 010 size o-ring and do it again. If I've misunderstood your post, sorry.

2) You are correct in that IP creep is caused by a leak between the piston knife edge and the HP seat. That's why replacing the seat is so important. As I mentioned before, once one is out and re-installed, it's likely the piston does not line up exactly as it did before and cuts a new circular groove in the seat. This intersects with the old groove and results in a tiny leak which causes the IP creep.

3) (okay, three things) 120 is on the low side for IP in a MK15; I think 125 is the minimum spec by SP, and they work well at 125 or a bit higher, maybe 130-135. The IP is adjusted by using shims in the ambient chamber that slightly compress the spring. All this is explained in the Harlow book. If your MK17 is at 140 there's no reason the change it, although it's very easy to do so without disassembling the regulator. Same with the conshelf.

 

[bada3003]

Thanks for the advice. I'm not planning on swapping 2nd stages across 1st stages so maybe I can keep the MK17 as is and tune the 2nd stage to match (don't know yet what that entails). I have an AL glacier, the octo version, new, that I'm thinking of using for colder waters. I'm mainly concerned about adiabatic cooling of the 1st stage, which, I heard/read has been a cause of accidents/deaths. I practiced for this what-if scenario by emulating free flow at depth 60+ ft through continuous purging (low 50s to high 40s temp), then engaging in controlled ascent while sipping air from the free flow.

There is no good reason to change the Ip of the MK-15 or the 17 for that matter. A few psi difference in IP will have very little effect on performance as long as the seconds are tuned to the particualar first. IP value in and of itself has nothing to do with freeflows as long as the seconds are set to the value of the first. The only good reason to change any of the IPs is to match them so you can easily swap second stages between both first stages.. In that case, your best bet is just to lower the 17s to that of the 15. Any time you dissamble a piston reg you stand the chance of damaging the piston, piston orings or creating issues with the HP seat, conversely, the 17 is externally adjustable and you have no risk of damaging anything.

---------- Post added March 2nd, 2014 at 12:51 AM ----------

On 1) you misunderstood. The MK15 was disassembled and put together using the mk15 bushing tool and piston bullet as you described.

On 2), after understanding the possible cause of 10 psi creep, I'm belatedly realizing that putting in additional shims (there was only one in the reg, a thin plastic ring) to help raise IP may have been a good idea. For the reason you mentioned, disassembling now to add more shims runs into the danger of the piston stem orifice not aligning tightly during reassembly which might necessitate getting a new HP seat to fix a resultant IP creep (not sure I'm using the terms correctly). I also remember reading about a related issue in Wolfinger's 1st stage chapters.

On 3), 120 seems a tad low so I looked for mk15 HP seats on the web. And shims. In so doing, I found a scubaboard thread that you started a couple of years back on a related problem. It would appear Trident has some that may work. However, their web site says to contact retailers (LDS) which I haven't done yet.

Two things:

1) If I'm reading this correctly, you had previously taken the regulator apart but did not unscrew the HP seat retainer? In that case, you should not have been able to install the piston if you had removed it. To install the piston, you put a bullet on the end of the piston shaft to protect the HP o-ring, and push the piston in place. The bullet then is removed through the hole that the seat retainer screws into. If you installed the piston without the bullet tool, it's likely you have damaged the HP o-ring in the process and the smart thing is to get a new duro 90 010 size o-ring and do it again. If I've misunderstood your post, sorry.

2) You are correct in that IP creep is caused by a leak between the piston knife edge and the HP seat. That's why replacing the seat is so important. As I mentioned before, once one is out and re-installed, it's likely the piston does not line up exactly as it did before and cuts a new circular groove in the seat. This intersects with the old groove and results in a tiny leak which causes the IP creep.

3) (okay, three things) 120 is on the low side for IP in a MK15; I think 125 is the minimum spec by SP, and they work well at 125 or a bit higher, maybe 130-135. The IP is adjusted by using shims in the ambient chamber that slightly compress the spring. All this is explained in the Harlow book. If your MK17 is at 140 there's no reason the change it, although it's very easy to do so without disassembling the regulator. Same with the conshelf.

 

[ams511]

On 3), 120 seems a tad low so I looked for mk15 HP seats on the web. And shims. In so doing, I found a scubaboard thread that you started a couple of years back on a related problem. It would appear Trident has some that may work. However, their web site says to contact retailers (LDS) which I haven't done yet.

You should check the IP of a regulator before the rebuild to know if additional shims are necessary. There was a post from either Herman or Luis on modifying shims from eBay to use in regulators, maybe someone remembers where it was. Trident makes HP seats for MK-5/10 (same seat). MK-2s and for some other brands. I have not seen MK-15 seats in their catalog.

 

[bada3003]

You should check the IP of a regulator before the rebuild to know if additional shims are necessary. There was a post from either Herman or Luis on modifying shims from eBay to use in regulators, maybe someone remembers where it was. Trident makes HP seats for MK-5/10 (same seat). MK-2s and for some other brands. I have not seen MK-15 seats in their catalog.

Indeed. I haven't yet finished Harlow's book but that's what he says for piston 1st stages which, in hindsight, is obvious. Not a big deal since I'm experimenting with the MK15 (the patient found an intern instead of the chief resident, never mind a specialist) and I expected a fair amount of missteps for jumping in w/o reading all the relevant parts of the two books.

On a positive note: When discussing shims, Harlow actually mentions MK15 in Chap. 6 as having an externally adjustable IP tuning knob similar to diaphragm designs -- basically, the HP seat retainer that can be rotated to vary the distance from the piston stem orifice which affects IP. Since I disassembled the thing, it's not that I didn't notice this possibility but I assumed the retainer was meant to be screwed tight. Trusting Harlow (the reg was pressurized ) I loosened the retainer a bit (actually almost 2 full rotations) and the IP increased from 120 to 130. Stable, no creeps.

Harlow says that only some reg techs knew of this MK15 feature (maybe most took shimming as granted and SP didn't highlight it enough), but those that figured it out, on occasion used it too liberally (I'm paraphrasing) whereby the piston stem would get wedged in the HP seat, thereby closing off the gas supply to the 2nd stage. Obviously, not a good thing since a key fail safe feature of regs is that they remain open upon failure. Harlow says that SP dropped this feature, moved from MK15 to MK20, in part, for this reason.

 

[redacted]

Indeed. I haven't yet finished Harlow's book but that's what he says for piston 1st stages which, in hindsight, is obvious. Not a big deal since I'm experimenting with the MK15 (the patient found an intern instead of the chief resident, never mind a specialist) and I expected a fair amount of missteps for jumping in w/o reading all the relevant parts of the two books.

On a positive note: When discussing shims, Harlow actually mentions MK15 in Chap. 6 as having an externally adjustable IP tuning knob similar to diaphragm designs -- basically, the HP seat retainer that can be rotated to vary the distance from the piston stem orifice which affects IP. Since I disassembled the thing, it's not that I didn't notice this possibility but I assumed the retainer was meant to be screwed tight. Trusting Harlow (the reg was pressurized ) I loosened the retainer a bit (actually almost 2 full rotations) and the IP increased from 120 to 130. Stable, no creeps.

Harlow says that only some reg techs knew of this MK15 feature (maybe most took shimming as granted and SP didn't highlight it enough), but those that figured it out, on occasion used it too liberally (I'm paraphrasing) whereby the piston stem would get wedged in the HP seat, thereby closing off the gas supply to the 2nd stage. Obviously, not a good thing since a key fail safe feature of regs is that they remain open upon failure. Harlow says that SP dropped this feature, moved from MK15 to MK20, in part, for this reason.

SCUBAPRO ENGINEERING BULLETIN #234 rev.B

 

[bada3003]

Thanks, awap. Turns out my MK15 is the one with the updated kit, so the HP seat retainer, unlike in the original model with the hex slot, was not meant to be loosened. I've screwed it tight and IP is also back to 120. Will need some shimming before diving with it. A second major newbie blunder, with potential danger attached.

It's become clear that reading the two books to completion would have answered most of my questions, quite effectively at that. And gathering all the available technical specs/docs on any specific reg before embarking on serving it. Cuts down a lot on backtracking and overhead, although some trial and error seems unavoidable. It's for specialized issues that this DIY group is most helpful, tapping into deeper expertise and experience.

One other thing I learned in this ~3 week crash course: regs are pretty well-designed and safe. I read scubaboard mostly for the accident/incident reports, and it seems very few are reg failure related. It seems evident why. The worst that regs can do is spring a leak and that's not a big deal in rec diving. But practicing for free flows (1st and 2nd stage), an extreme form of leak, is important. And doing common sense checks as outlined in Harlow's book on page 99. Of course, in overhead environments and deco diving, leaking regs can have more severe consequences so commensurate caution needs to be exercised.

Thanks to everyone for helping a newbie with basic questions. I won't be posting questions for a while, probably until it comes to service the MK17 and Apeks DST.

 

[ams511]

The worst that regs can do is spring a leak and that's not a big deal in rec diving. But practicing for free flows (1st and 2nd stage), an extreme form of leak, is important.

The worst that can happen is you loose a turret or puncture a diaphragm and your tank empties in a couple of minutes. Thankfully, these mishaps are rare and for the most part avoidable.

---------- Post added March 5th, 2014 at 02:40 PM ----------

Thanks, awap. Turns out my MK15 is the one with the updated kit, so the HP seat retainer, unlike in the original model with the hex slot, was not meant to be loosened. I've screwed it tight and IP is also back to 120. Will need some shimming before diving with it. A second major newbie blunder, with potential danger attached.

On good thing about Scubapro is that they incorporate improvements in the existing products, however it also makes DIY servicing a bit confusing because you do not know if you have the current configuration or not or if your schematic is current.

---------- Post added March 5th, 2014 at 02:44 PM ----------

Thanks to everyone for helping a newbie with basic questions. I won't be posting questions for a while, probably until it comes to service the MK17 and Apeks DST.

One nice thing about Apeks regulators is that the kits are easier to find and buy. Ness sells Apeks kits as well as some other vendors.

 

[wgmontgomery]

I have a Scubapro Mk15/G250 combo in need of service before a dive that's coming-up. Money is a bit tight & my local SP dealer wants $$$ (a lot) for service. Does someone want to earn some extra cash by servicing the gear?
I plan on buying the tools, books etc. to learn to do it myself, but it'll have to wait until I have more cash.
PM me if interested & thanks!