MK25 IP increase as tank pressure drop

[jacksatlanta]

James R. has it correct as I learned in the Scuba Pro Pro Seminar just last year. Please also make sure if you are adjusting a HOG second stage, or just about any, that when you go in and tighten the Allen Key orifice you are pressing in on the purge button or lever, failure to do so will score the soft seat and then you will be experiencing continuous and growing seepage problems until you service the second stage and replace the soft seat. I have seen way to many field technicians go at the orifice and forget to pull the knife edge away from the seat (by depressing the 2nd stage lever while adjusting) only to make their small problem worse.

 

[Fishpie]

You know, in over 25 years of servicing scuba kit, I have never damaged, or seen a seat damaged, by not pressing the purge button......I must admit I usually do press the button or depress the lever, but by no means do I always do it.

Maybe it's one of those mask on the forehead deals that just get perpetuated by the community.

 

[DA Aquamaster]

You know, in over 25 years of servicing scuba kit, I have never damaged, or seen a seat damaged, by not pressing the purge button......I must admit I usually do press the button or depress the lever, but by no means do I always do it.

Maybe it's one of those mask on the forehead deals that just get perpetuated by the community.

I've heard this caution for 25 years as well - and again I've never actually seen it happen. Like you I normally follow this practice as a good practice, but not universally.

---------- Post Merged at 11:20 AM ---------- Previous Post was at 11:16 AM ----------

Actually, on the MK25 the IP will increase with decreasing tank pressure.

"Intermediate Pressure Limits: IP limits must be between 130-142 PSIG at both 500and 3000 PSI supply. Verify and expect an IP inversion/increase of approximately 5 PSI from 3000 PSI to 500 PSI supply. IP inversion: As tank supply decrease, IP will increase. For best results, set IP @ 142 PSI, with a supply pressure of 500."

They did not mention that in March of 2011.

This would be unfortunate as it would suggest Scubapro has in fact "overbalanced" the design.

The only valid argument for doing this is to improve flow rates at low supply pressures, but the Mk 25 already delivers bucket loads of gas at low tank pressures.

As noted then, if the IP increases at low tank pressures, the second stage must be detuned to prevent any free flow at low tank pressures - and that in essence detunes the reg across the entree range of tank performance.

In that regard it's not an improvement or a design feature, but rather an engineering mistake. What is this fascination with screwing with IP when IP should be a very stable thing to maximize second stage performance? It makes me suspect the marketing folks are back in charge of the engineering department.

It's also a powerful argument for getting a Mk 17.

----

However, I would suggest actually checking the IP on a sample of Mk 25s and see what really happens.

And, they are apparently suggesting 142 psi at 500 psi to maximize the IP and allow greater air delivery. However, many techs with cold wearer customers reduce the IP to the 120 psi range to reduce gas flow and reduce the freeze flow potential, and often detune the second stage to prevent any inadvertent free flow that might cool the reg and lead to a freeze flow.

 

[James R]

I will do a little experiment this weekend if my LDS has the bench open. I want to hook up a Mk25 on the bench at 500psig, set the IP to 142 if it isnt already, then set an A700 to crack at 1.1" on the magnehelic. I will then change the bench outlet to 3000psig and re-check where the a700 cracks. I will also note the IP change.

 

[jacksatlanta]

If you are doing adjustments in 1/16th turn increments or smaller your not likely to see a problem, but if you screw the knife edge orifice all the way in (as I have seen some do recently because they were never properly taught how to make that adjustment) then you can cut a grove sufficient to allow seepage past the knife edge from that time.

 

[eelnoraa]

James, if you end up making this measurement, please let us know the result. Thank you

I will do a little experiment this weekend if my LDS has the bench open. I want to hook up a Mk25 on the bench at 500psig, set the IP to 142 if it isnt already, then set an A700 to crack at 1.1" on the magnehelic. I will then change the bench outlet to 3000psig and re-check where the a700 cracks. I will also note the IP change.

 

[Zung]

Actually, on the MK25 the IP will increase with decreasing tank pressure.

"Intermediate Pressure Limits: IP limits must be between 130-142 PSIG at both 500and 3000 PSI supply. Verify and expect an IP inversion/increase of approximately 5 PSI from 3000 PSI to 500 PSI supply. IP inversion: As tank supply decrease, IP will increase. For best results, set IP @ 142 PSI, with a supply pressure of 500."

Surprising, as the numbers I posted were taken with the lastest and greatest composite piston with the flare at the stem, fitted in a MK20 body, basically the same as the MK25 sans the IP adjustment.

The only thing I'm not 100% sure of is the way I dropped the tank pressure: I didn't drain the tank, I just turned off the valve, purged to the desired "tank" pressure as read from a SPG, and read the IP. This method was reported by halocline and I can see nothing wrong with it.

Assuming the procedure is correct, you can see the IP is constant at 9.4bar/136PSI all the way down to 50bar/725PSI of tank pressure, then it starts to drop off.

Or, is there yet another piston?

I look forward to the outcome of your experiment.

 

[halocline]

I can't imagine any real-world scenario in which the IP on a balanced piston reg of the general SP design would increase significantly as supply pressure drops. There would have to be a really big flare in the piston edge, and I would venture a guess that that would cause some problems in installing the piston through the tight-fitting HP o-ring. There's some friction at the HP o-ring, too, that is bound to have some effect on IP.

 

[James R]

Unfortunately I wasn't able to get to that test over the weekend. Hopefully I will be able to get up there later in the week or next weekend.