Why Piston vs. Diaphragm???

[Nemrod]

This is not an exhaustive or complete study and should not be taken as a general rule beyond what I have observed. I have many regulators of all sorts. I have from time to time done experiments. I have installed a guage into the LP ports and using the same base tank pressure and the specified IP I have generally noted a greater IP drop (% of specified IP) during the demand cycle with diaphram type regulators than with piston types. This should not be taken as a golden rule but only that the difference between the two types is quite small but that maybe piston types offer greater potential for performance--maybe.

Also, many diaphram types have exposed springs etc but the internal parts are not exposed to the envionment as is the ambient side of the piston--which moves along a barrel which is also exposed. Both types can be sealed.

N

 

[Soggy]

Some people have posted that they think this aspect of an unbalanced reg is "dangerous", which is BS and probably right out of the mouth of a salesman trying to sell a "safer" (read: more expensive) balanced reg. If anything, the increase in breathing resistance at very low tank pressure is a safety feature.

The way I see it, is the last thing I want to happen to my reg when I'm low on gas is making it HARDER to breathe, thus increasing my breathing rate and CO2 buildup. But, to each their own.

I have a MK2 also and it works well enough. It's been sitting in a box for 4 years. Anyone want to buy it?

 

[DA Aquamaster]

Depth itself will not have any noticeable effect on on breathing effort with either balanced or unbalanced first stages. Tank pressure will have an effect with diaphragm first stages becoming easier as tank pressure decreases and piston first stages becoming harder as tank pressure decreases. In both cases it would really only become noticeable at about 500 psi and below as thalassamaina indicated.
The unbalanced Le Spirotechnique Mistral double hose, single stage regulators were used by Cousteau below 300 feet.

WHat you are saying regarding the change in IP with tank pressure is true of unbalanced regulators.

More specifically, an UNBALANCED diaphragm first stage will begin to breathe easier as tank pressure changes due to an IP increase that occurs because the seat carrier is upstream of the orifice and the tank pressure helps close the seat. So as pressure falls the assist in closing the seat decreases, meaning less IP is needed on the other side of the seat to keep the forces acting on the seat equal.

The thing is that no one has actually made an unbalanced diaphragm first stage since the early 1970's as balancing the seat carrier is very easy and inexpensive to do. Plus of course a reg that gets easier to breathe right up until it stops delivering air has never been regarded as being all that safe for an inattentive diver.

In an unbalanced piston first stage the opposite is true as the seat is located on the downstream side of the orifice where supply pressure from the tank assists in lifting the seat off the orifice. So in essence they breathe easier (with higher IP) with a full tank and IP fall with inhalation resistance increasing as the tank pressure falls. The increase in inhalation effort usually becomes very noticeable as tank pressure falls into the 300-500 psi range - giving an innattentive diver a hard to miss warning that he or she is about out of gas.

The lower perfromance that tends to be found in an unbalanced first stage is due to a tendency for these regulators to have a a smaller orifice than their balanced counterparts in an effort to keep the piston head size (and over all first stage size) to a fairly small diameter while keeping the IP change in the 20-25 psi range. A larger orifice would require a larger piston head diameter to supply the force needed to keep the forces on both sides of the seat equal without increasing the change in IP that occurs.

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With regard to diaphragm designs and IP drop, the advantage that many balanced psiton designs offered was very high flow rates that minimized IP drop during inhalation. With current diaphragm designs like the Mk 17 with it's high 177 SCFM flow rate, this really is not an isuse and in any case, with adequate flow rate and a balanced second stage, a slight to even moderate IP drop will not be noticeable to the diver even if it is noticeable on the test bench.

 

[DA Aquamaster]

The way I see it, is the last thing I want to happen to my reg when I'm low on gas is making it HARDER to breathe, thus increasing my breathing rate and CO2 buildup. But, to each their own.

I have a MK2 also and it works well enough. It's been sitting in a box for 4 years. Anyone want to buy it?

Sure...shoot me a reasonable price and we can take care of the sale today.

The thing you need to realize is that this noticeable increase in inhalation effort is not going to occur unitl you are into what should be your reserve. At that point if you are still in a situation at depth where you are exerting yourself hard enough to encounter CO2 buildup you are pretty well hosed. You will however probably still be better off than you would have been continuing your dive in blissful ignorance of your low gas supply until your uber easy breathing reg announced you were out of gas by delivering your last breathe with no warning right before you exhaled and tried to inhgale the next one. There is nothing quite like being in serious O2 debt with elevated CO2 levels (which will happen under a heavy exertion regardless of the regulator you are using) and having no air at all in your lungs.

 

[halocline]

The way I see it, is the last thing I want to happen to my reg when I'm low on gas is making it HARDER to breathe, thus increasing my breathing rate and CO2 buildup. But, to each their own.

So you'd rather have no warning whatsoever, just take a breath and then find your tank completely empty? Remember, I'm talking about pretty low tank pressures; the only likely scenario in which someone would be at any sort of depth with 300PSI is if he/she forgot to look at the SPG or it's malfunctioning. And, the increased breathing resistance I'm referring to is not so great that it makes it difficult to get enough air; just a gradual increasing in stiffness. I know, before I bought the MK2 I borrowed one and breathed it right down to 150PSI on a training platform with a buddy. The increased effort was barely noticable until 300PSI.

 

[scubawally]

Ok I have a few simple points to make. I think that we have completely hijaked this thread!!!!

Our initial question was if there was any appreciable difference in the piston and diaphram first stages and why if diaphram first stages are better in cold and polluted water to technical diver prefer the piston type.

tech divers usually choose balanced piston first staged because they can deliver a larger quantity of gas at depth. Or so goes my understanding.

When it comes to hose routing, the mk25 is a very good reg especiall for the right post, however if you are in a drysuit and you have 3 hoses comming off of your left post the mk16/mk17 might be the better option! This reg is smaller and you have the option of placing 2 low pressure hoses (bungeed octo and B.C. inflation) as well as the hp hose on one side of the reg and then put the reg on the tank such that all of the hoses point straight down. While this can be done with the mk25 I dont thing you can get it as streamlined as you can with the mk16/17.

As far as diving in cold water goes I always see the arguement of diaphram vs sealed piston. No I dont know how cold you guys are taking about, or how silty, or how deep, but i know that last sunday me and two of my buddies did a dive to 110 in St. Margaretts bay in Nova Scotia Canada with water surface temperature of 35 degrees F and neither of use had a free flow. I was using doubles with a mk25 on my right post and a mk16 on my left post, on of my buddies was using a mk 10 and the other a mk2+. IMHO the key to not having free flows is in how you deal with your reg in the surface (not breathing in it before you get in the water etc.) rather than if it is a piston, diaphram or environmentally sealed.



now that i have put in my 2 cents worth regarding the replys to this thread ....what can I say to the guy looking at buying the new reg? for recreational diving either of the current or older models of scubapro regulators will work fine in most diving conditions. however if you are looking into tech diving I would recommend you buy the mk25 as when you get deep (150+) the mk2 can get a little hard to breath on.


thanks

 

[oxyhacker]

More specifically, an UNBALANCED diaphragm first stage will begin to breathe easier as tank pressure changes due to an IP increase that occurs because the seat carrier is upstream of the orifice and the tank pressure helps close the seat. So as pressure falls the assist in closing the seat decreases, meaning less IP is needed on the other side of the seat to keep the forces acting on the seat equal.

True is theory but not necessarily in real life. While the cracking pressure will be less, the flow will also be less due to the reduced pressure differential, so the actual amount of air delivered may very well be less. One of the advantages that Cousteau often cited for the old Mistral regs as part of his "manfish" concept was that the breathing effort increased as the tank pressure lessened, allowing the experienced diver to sense his/her depth intuitively without needing a gauge, yet the Mistral was an unbalanced upstream valve.

Plus of course a reg that gets easier to breathe right up until it stops delivering air has never been regarded as being all that safe for an inattentive diver.

You'd think this would be just common sense. But Scubapro on the Mk15 claimed something new and wonderful they called "intermediate pressure inversion", by which the IP, and hence ease of breathing, would increase as the tank pressure dropped, Or, as SP put it "Just when you need it most". This seemed so nutty that I always wondered whether it really was a deliberate feature, or just an attempt to try and make a flaw in the design seem like a feature.

 

[Thalassamania]

You'd think this would be just common sense. But Scubapro on the Mk15 claimed something new and wonderful they called "intermediate pressure inversion", by which the IP, and hence ease of breathing, would increase as the tank pressure dropped, Or, as SP put it "Just when you need it most". This seemed so nutty that I always wondered whether it really was a deliberate feature, or just an attempt to try and make a flaw in the design seem like a feature.

Well ... SP also brought us the DCAR Tank Valve.

 

[TJF]

Great thread, answered the question I too had regarding this subject... Cheers.

 

[shogan]

Sherwood 1st stage and the dry air bleed systerm that by design allows it adjust to ambient pressure but keep all the water gunk out. It's a piston. Wow what a great design. Then it's got the cheapest parts as far as maintence $8 for the whole kit. It comes in unbalanced (brut model) and balanced magnum through-maximums models.

By Contrast the most others require a parts kit around $30.