Piston vs Diaphram 1st stage

[GotAir]

on this Thread! sense I didnt really ask a direct question I couldnt exspect you to read my mind but the Air flow on a piston reg is greater than on a diaphram thats a good answer. I'm a big guy that use's alot of air so a piston reg maybe better suited for me. Thanks DA Aquamaster for pointing that out!

Mike

 

[DA Aquamaster]

on this Thread! sense I didnt really ask a direct question I couldnt exspect you to read my mind but the Air flow on a piston reg is greater than on a diaphram thats a good answer. I'm a big guy that use's alot of air so a piston reg maybe better suited for me. Thanks DA Aquamaster for pointing that out!

Mike

Boy my typing was bad this morning - sorry.

I have only had one problem with over breathing a reg once and that was with a Scubapro Mk 3 in about 30 feet of water under a bridge in a 5 kt current. The work load of working across the rocky bottom from rock to rock was high and I got to the point of realizing I was about to pass out before I managed to get out of the current behind one of the support piers. The Mk 3 only produced about 30 scfm and just was not up to the task even in shallow water.

I have never had a problem with a Mk 10 even on hard working dives at depths to 130 ft. A Scubapro sales rep told me once that they had something like 15 divers in a pool once breathing off a Mk 20 (with a whole bunch of y's to accomodate all the second stages) and were not able to out breathe it. Only 10 feet of water, but still impressive performance.

 

[rjens]

A Scubapro sales rep told me once that they had something like 15 divers in a pool once breathing off a Mk 20 (with a whole bunch of y's to accomodate all the second stages) and were not able to out breathe it. Only 10 feet of water, but still impressive performance.

Ya mean something like this
Mares MR22 Abyss Record. 25 divers at 40ft.

Not pushing Mares, just passing this along for reference...


RJ

 

[DrySuitDave]

I dove open circuit for 20 or more years using a Oceanic Omega regulator and that things will almost inflate me it breathes so incredibly easy at all depths, and it never gave me drymouth.

I still have it, it still works beautifully and have always been able to get it overhauled.

 

[Lamar Jr]

From a raw performace standpoint, balanced piston designs tend to have higher flow rates. For example an unbalanced piston first stage will provide a maximum of about 50 cu ft per minute, a balanced piston will do about 80 cu ft per minute, a compact Mk 10 style balanced piston design will do around 150 and the enlarged MK 20/25 style balanced piston design will produce nearly 300 cu ft/min.

...... Personally I think 100 cu ft min is plenty for any conceivable situation and 300 cu ft per minute is serious overkill.

Hey DA..I have to admit that I know almost nothing about the internal workings of regulators. However, the flow rates you mention sound shockingly high to me.

Allow me to illustrate my point. Would you agree that many or even most divers have a SAC\RMV rate under normal conditions of less than .60 cu ft per minute? Even an air hog is probably under 1.0 cu ft per min. Now let's double or triple that number for stressful conditions, so we're up to something like 3.0 cfm. Next consider the multiplier effect of depth. Let's look at a dive to 130 feet, which is 5 ata. That would get the diver's air consumption in this example to 15 cfm. That's mighty high, and means he would breath a full 120 bone dry in 8 minutes flat (not suggesting anyone would do that, just pointing out the obvious).

The Mk 3 only produced about 30 scfm and just was not up to the task even in shallow water.

So why would a regulator that flows at double my illustrated rate not be able to provide sufficient air? Why would a high performance regulator need to flow at 100 or 150 cdm in order to be easy breathing?

I'm not challenging the accuracy of your statements or numbers, just trying to understand a little better what is at work here.

 

[DA Aquamaster]

As you point out a Mk 3 is capable of supplying 30 cu ft/min which according to the math will supply you all the air you could ever need down to sane depths and in fact would have enough reserve to supply your buddy under the smae conditions. That is a great theory and was exactly the rationale I used when I bought a Mk 3 over the more expensive Mk 10 in 1983. Which of course left me under a bridge very short on air contemplating that perhaps cheaper is not always better when it comes to life support equipment.

The first problem with an otherwise great theory lies in the concept of scfm as it is essentially a measure of what the reg will flow if you remove a low pressure port and turn on the air. It also assumes the valve has the flow capacity to feed the reg all the air it can flow (not all older valves are up to this). This is not the case in a normal diving situation where the reg has to cycle open and closed.

The second problem with the 30 cu ft/min figure is that it does not take into account the lag that occurs as the pressure drops and the piston or diaphragm moves to open the reg to start the flow. If you are huffing really hard on the reg, an unbalanced piston reg like the Mk 3 will not be able to respond fast enough and you will feel like you are short of air. The internal IP section is also very small and offers very little air for the diver to draw on while the piston opens the seat to it's full flow capacity.

The third problem with the theory is that the IP increases with depth to maintain the same pressure differential over the ambient pressure. So the air flowing through the reg gets "thicker" with depth and with a "flow by" type of seat design squeezes through less efficiently so you almost invariably get a lower flow rate with depth.

So the idea of a minimum scfm of 100 is not directly related to diving requirements but is still important as a measure of first stage having enough flow capacity and to meet the diver's instantaneous inhalation requirements under severe conditions with no noticeable lag.

On the other hand, a very well designed balanced diaphragm second stage will overcome the lag problem differently by including what amounts to an internal reservoir of IP air in the first stage to supply the diver until the seat fully opens and allows a full flow of air from the tank. This tends to make the first stage a bit larger than many other balanced diaphragm first stages but is preferable in terms of performance under harsh conditions. The Scubapro Mk 16 and Mk 18 use this approach and in the case of the Mk 16 it also uses high flow ports for the second stages to improve effiency. If I remember correctly the Mk 16 has a rather medium performace scfm of around 80 but I would still consider it adequate for all but the most demanding situations. If you plan on using a balanced diaphragm second stage for deep diving, I would make sure it has similar design characteristics.

The second stage used can also be a factor. The average downstream poppet valved second stage will also provide an air flow of around 30 cu ft/min and this is more than enough under normal conditions but can come up short under more demanding conditions at depth. The SP "high Performance" second stage on my Mk 3 had a capacity of 30 cu ft/min a figure that by 1983 was already not eactly "high perfromance". (but it was still a very sweet breathing second stage)

For deep diving a higher performance second stage is a better choice. I think the D400 on my Mk 10 had a flow rate of around 70 cu ft per minute and this is closer to the average for a current high performance second stage. And again as with the first stage the extra flow capacity helps to ensure that the instananeous inhalation requirements can be met under demanding conditions and just as importantly at low inhalation efforts. A second stage that delivers all the air you can breath but only with an inhalation effort of 3-4 inches of water is still not much help.