Mk25

[Packhorse]

Hi all,

Been awhile since I posted here, mainly due to moving to CCR.

Anyway, got a friend who has just started diving and I recommend he get a G250v and MK25.

While in discussions with a well respected service tech, it was said that the piston bushes need to be cut before installed when a MK25 is serviced and a 2nd stage reg should not be used in the 5th ( bottom) port of a MK25 due to such hi flow rates which can cause pulsating IP.

I have serviced a few MK20's and 25's and never heard of cutting the bush nor is it mentioned in the service manual for the MK20 ( have not seen the service manual for the 25) I have also used the 5th port for 2nd stage regs with out issue and also not that the "DIR" set up for the back up 2nd stage uses the left posts MK25's 5th port.


Anyone care to comment?

 

[herman]

The BS meter just hit the top end... Why exactly would the 5th port have any more flow than the rest? All the gas makes the same turn at the HP seat and comes down the same piston shaft, the only difference is the turn at the end.. While it is likely true that wide open, unrestricted flow as measured on a machine is likely slightly higher in the 5th port due to one less turn the effect, however when the flow is being throttled by the entire second stage assembly that extra capacity is not going to make any difference. Besides, the factor limiting flow is the second stage and hose so none of the ports are ever going to get to max flow and even if they did, this overlooks the fact that no human can possibly breath at the rate any modern reg can flow. Add to all of that, flow is going to be controlled by demand on the second stage, the first stage does not operate at max flow or closed but rather is throttled during each breath cycle. If IP of a MK-25 is that unstable (and it's not) SP would pull it off the market and fix the design...
As for cutting the bushing…if SP wanted/needed that bushing cut it would come from the factory that way…cutting a high precision part by hand in the field makes no sense at all. Sounds more like a shortcut or a lack of understanding of the proper procedure to me.
The old saying “If you can’t dazzle them with your brilliance, baffle them with your BS” comes to mind. .

 

[halocline]

I agree, this is a particularly entertaining one.

The end port does supposedly have higher flow capacity than the others, due to it's proximity to the top of the piston where the air comes directly up the piston shaft. However, this is higher flow in theory, with all ports wide open and an unlimited flow HP supply. In reality, any of the ports supplies far more air flow than any 2nd stage can flow under full purge. So I'm not sure where the 'pulsating IP' bit came from.

I once did an experiment to measure IP drop under full purge in several SP piston stages, and it is interesting to note that if your IP gauge is on a LP inflator hose like mine, you get some strange readings, like much higher initial drop with MK10s than with MK5s, and least drop with MK2s. Go figure that one......I even called Peter Wolfinger and asked him about it. He had no idea what would cause it. My best guess is that there are venturi effects in the turret that cause initial pressure drops in the ports not being used by the primary 2nd stage.

IP drop and recovery in the MK25 or MK20 retrofitted with the composite piston is truly remarkable; it hardly budges under anything less than full purge with 2 2nd stages, and I suspect that's mostly due to flow limitations at the tank valve.

Never heard anything about 'cutting' the bushings on the MK25. I've looked at those bushings (actually MK15 bushings) with a loupe and occasionally seen tiny burrs that I've cleaned up with micromesh, the idea being that the HP o-ring is pushed against the bushings and we don't want anything damaging that o-ring. But I'm probably on the obsessive side; I never once heard of an o-ring being damaged by those bushings. DA Aquamaster might have some comments about this.

 

[Zung]

+2

Regarding the bushes, I looked up the schematic (Config. D, 4/2004), and the only thing called "bushing" is that black plastic part right under the crown of the piston. Still, I see no reason to cut it.

Maybe the tech is just trying to buy more "respects".

 

[James R]

+3

I wonder how many thousand Mk25, MK20, Halcyon (and more) are configured in doubles with a reg coming out of a 5th port. If this were true, it would be a very common complaint, and would have been fixed long ago.

 

[CPH]

Actually, it comes from ScubaPro themselves.
When you buy a high performance 2nd stage, in the book that comes with it is a paragraph that says "not to be fitted to the end port of the Mk25 1st stage"
I noticed it when I bought my last A700 and commented on it to the shop and they basically said it was just a legal thing.
Myself and the wifey both have Mk25's (7 sets) and use them in full tech setups with GV250s, A700s and S600s with no problems, even under ice.

 

[redacted]

Actually, it comes from ScubaPro themselves.
When you buy a high performance 2nd stage, in the book that comes with it is a paragraph that says "not to be fitted to the end port of the Mk25 1st stage"
I noticed it when I bought my last A700 and commented on it to the shop and they basically said it was just a legal thing.
Myself and the wifey both have Mk25's (7 sets) and use them in full tech setups with GV250s, A700s and S600s with no problems, even under ice.

the warning that I found in this online manual (http://www.scubapro.com/media/59907/scubapro_regulators-english.pdf) refers to R-series 2nd stages.

 

[beaverdivers]

The Scubapro MK25/G250V is an excellent reg.

I do remember hearing the 5 port on the end had 17% more flow due to the fact it did not have to make a 90 degree turn. A long time ago, we used the 5 port for the added performance.

Maybe we should give it another go just for fun.

 

[halocline]

The Scubapro MK25/G250V is an excellent reg.

I do remember hearing the 5 port on the end had 17% more flow due to the fact it did not have to make a 90 degree turn. .

This is perfect example of how half-truths get mixed in with sales pitches. If SP says "the end port has 17% more flow" the implication is that the added port will give you some measurable performance improvement. This is simply not the case, for the reasons that are outlined earlier in the thread by both herman and me. It just means that with an unlimited supply pressure and flow rate, when all the ports are open 17% more gas flows out of the end. This, of course, has zero to do with actual diving performance. Yet somehow a SP dealer comes along and says "17% more flow...added performance" and on it goes.

 

[Packhorse]

Thanks all, pretty much what I expected.