Piston vs Diaphragm OR MK25 vs MK17

[travelrider]

175 scfm is overkill, 225 scfm is overkill and 300 scfm is ridiculous overkill. 300 scfm is "better" only in the same sense that you can do 160 mph in a Shelby Mustang in a school zone. Serves no real purpose, has some serious donwsides and is pretty stupid.

It seems to me that having a first stage that can only deliver just a little more than the max you would ever need would be safer from the perspective of that off chance that you have a problem that's wasting your air while you're trying to get to the surface. A higher flow first stage is just going to empty your tank that much faster. Maybe only a matter of a few seconds difference here depending on the problem, but isn't this a fair argument FOR the lower scfm 1st stage? Or maybe I'm off my rocker here since I'm a newbie and don't really know what I'm talking about yet.

 

[rhwestfall]

you are still valve limited....

 

[halocline]

It seems to me that having a first stage that can only deliver just a little more than the max you would ever need would be safer from the perspective of that off chance that you have a problem that's wasting your air while you're trying to get to the surface. A higher flow first stage is just going to empty your tank that much faster. Maybe only a matter of a few seconds difference here depending on the problem, but isn't this a fair argument FOR the lower scfm 1st stage? Or maybe I'm off my rocker here since I'm a newbie and don't really know what I'm talking about yet.

Not really. First, the tank valve is the slow link in the chain of continuous air flow. If you took two full tanks, put a high flow first stage with all open LP ports on one, left the other with no regulator, then opened both valves, both tanks would empty at about the same rate. I'm sure there are some other factors in there, but generally speaking, the tank valve itself is the bottleneck.

What's important for a first stage in terms of importance is IP stability, drop, and recovery during inhalation. You want a first stage to drop IP as little as possible when the 2nd stage demands air, and in order to do that you need strong flow characteristics across the HP valve. Even if the tank valve is a little slower in terms of continuous flow, having a regulator be able to quickly supply air to the IP chamber is important. In instants of high demand, the HP chamber in the reg, which is fed by the tank valve, could drop substantially as the reg flows air into the IP chamber at a faster rate. But, since the air in the HP chamber is still going to be hundreds (or thousands) of PSI higher in pressure, there's enough of a pressure gradient across the HP valve so that a good 1st stage should still be able to keep IP from dropping precipitously. That would affect the 2nd stage feel. Its not about raw speed for air flow, its about quick response by the reg to keep IP up during demand.

At least, I think that's how it works....

 

[rhwestfall]

that is a lot of words for the same thing I just said above you.....


okay - a step further for you regulator gurus - can a high flow reg actually be a "hazard" in regards to freezing as it passes air to easily - hence possibly icing (freeze-up)?

As I said before, I have had "lowly" MK-10's behave happily (on a 109), while the LDS DM with his brand-spanking-new "top of the line, super flow, wizz-bang" froze up in 38 degree water...

Is "low performance" actually desirable in circumstances like this?

 

[halocline]

that is a lot of words for the same thing I just said above you.....


okay - a step further for you regulator gurus - can a high flow reg actually be a "hazard" in regards to freezing as it passes air to easily - hence possibly icing (freeze-up)?

As I said before, I have had "lowly" MK-10's behave happily (on a 109), while the LDS DM with his brand-spanking-new "top of the line, super flow, wizz-bang" froze up in 38 degree water...

Is "low performance" actually desirable in circumstances like this?

Well, tech divers sometimes de-tune and lower IP on regulators for cold water use, so I suppose that's something like using a lower performing reg. As I said, it's not about sheer flow, it's about IP drop and stability. I wouldn't say that the MK10 is a low performing reg anyway. In your situation, it's very possible that the 109 2nd is responsible for the excellent cold water results. That is a MUCH more freeze resistant 2nd stage than any new plastic case stage.

The one real advantage your MK10 would have over a MK25 in cold water would be it's ability to be packed with silicone or PTFE grease. That essentially seals the ambient chamber and has been proven very effective in cold water.

A MK2 would be an example of a lower performing 1st stage that has good cold water performance. That's due to the difference in design, not specifically because it's lower performing; there was a recent thread where this was discussed.

 

[halocline]

that is a lot of words for the same thing I just said above you.....

except for the fact that I was addressing why a reg with a higher flow than the tank valve is actually useful....

Flow rates themselves are certainly not the best indicator of regulator performance, but they do corollate (in part, I suppose) with IP drop and recovery. I bet you can almost certainly tell a difference between a 100SCFM 1st stage and a 40SCFM one, even if the tank valve is lower than both of those numbers.

 

[DA Aquamaster]

I have always been interested in trying it and have never gotten around to it, but I suspect a tank with a reg on it and the LP ports removed will empty a tank faster than just a tank with an open valve, simply because the valve is the limiting factor.

It's a matter of where the pressure drop occurs. With a given flow passage, the higher the gas pressure in the passage, the more air you can push through it in a given amount of time. In that regard with an open valve, the pressure drop is occurring somewhere along the passage in the valve, and thus reducing the pressure in the passage itself. In contrast, when the valve is attached to a regulator, the drop in pressure is occurring in the larger passages in the first stage, keeping the pressure higher in the tank valve itself. Or maybe not - thus the desire to try it.

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Theoretically, the Mk 25 has more external surface area and should have better heat transfer than the smaller Mk 10, but in practice a TIS protected Mk 10 is slightly more reliable in cold water than a TIS protected Mk 25. Although that is also saying that a TIS equipped Mk 10 is freeze prone where a SPEC protected Mk 10 is almost bullet proof in cold water.

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Halocline is correct that what matters is response time and IP drop. If you start pushing a first stage to its maximum flow rate, you'll begin to get excessive IP drop and two things will happen:

1. the inhalation effort will increase as the reg will lose some of the downstream assist it has at higher IPs. That's true even with balanced second stages due to the downstream bias that has to be built in for safety.

2. the flow rate through the second stage will fall as the amount of gas that can pass through the LP hose and secnd stage valve is proprotional to the pressure in the hose and regulator. So in effect, if the IP drops you lose the turbocharging effect.

Thus some excess flow rate is important, all other things being equal.

 

[halocline]

I wonder if the valve and/or reg would freeze, and if so, which one would freeze first? It might be worth trying it submerged in very warm water.

 

[ajduplessis]

@travelrider The very same reason people say BMW is better than Merc. It's a matter of opinion. The 25 can be dived in ice (2 Celsius) with a G250 or A700, many divers here have done so without issues. SP launched the A700 on the 25 while ice diving.

The argument around the piston spring getting stuck in duty water is BS. Ask you dealer to give you a piston spring as see if you can compress it with your hands. Then tell what needs to be in the water to make it get stuck!! The 25’s also route very well on doubles.

 

[DA Aquamaster]

I see Mk 5s, 10s and 20/25s come in for service from PSDs and commercial diver with the ambient chambers nearly filled with sand and silt. Some of them make me want to put on a hazmat suit before servicing them.

I have never seen one quit, but the o-rings and metal parts do experience a lot more wear.

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The Mk 25 will work in ice diving conditions - but it will require near perfect cold water technique - no use of the reg before the reg is fully submerged (not even testing the reg or inflating the BC or dry suit), short bursts on the dry suit and wing inflators, not breathing and inflating simultaneously, pulling the rubber hose covers back off the metal fittings, to improve heat transfer and not overbreathing the reg.

Even then it probably won't survive the unexpected. For example, what if the second stage slips out of your mouth? the resulting freeflow of gas wil most liekly cause a Mk 25 to freeze flow. If not, the water in some second stages (noteably the S600) will in a few minutes find its way onto the poppet and cause a slight freeflow, that then pushes the Mk 25 over the edge into a freeze flow.

Also, saying a Mk 25 will work on an ice dive means one thing when you are dinking around under the ice in near the surface in 2-30' of water. It means something entirely different when under the ice in 90-100' of water, or in 35 degree water at 150-200' where the amount of gas flowing through the first stage is 5 to 6 times greater than it is near the surface.

Finally, fresh water versus salt water makes a difference given the lower freezing point of salt water as it allows more margin at a givne temp than fresh water.

When everything is considered, especially for technical diving under a soft or hard overhead where you cannot exit the water within a couple minutes, the Mk 25 is just a poor choice in extremely cold water.