Diaphragm vs. Piston Regulators

[tbone1004]

it doesn't matter, remember the flow rate is determined by the IP and the density of the gas. Once you get below recreational rates, there is helium in the mix to lower the gas density to allow for better breathing. Apeks tried to combat this as did other mfg's with larger primary regulator ports or orifices, however it was proven to be a marketing gimmick and held no scientific validity.

Most of the regs on the market are variants of a handful of designs. So, let's look at some specs from scubapro because they post it.
Their highest performing reg is the A700 in terms of flow rate and it can flow 71cfm. This is purging mind you. Their lowest rated first stage, the MK2+ flows 92cfm. The combination of 1 second stage, with 1 first stage can drain an AL80 in about a minute. Not of concern at any depth.

Now, let's assume that we are diving DEEP like 200m deep, we can assume a 1cfm breathing rate, which translates to 21cfm adjusted. Two divers, at that depth, require 46cfm out of the first stage. That is half of what the MK2+ flows. Obviously you don't want to be near the limit of any regulator, but even with an old design basic piston, you are well within the limits.

 

[2airishuman]

I know divers have their personal preferences and brand/model loyalty but strictly from a functional stand point what are the advantages and disadvantages between balanced diaphragm 1st/2nd stages and balanced piston 1st/2nd stages?

I have both piston and diaphragm regs in my stack of dive stuff, and have dived and serviced them.

It is my experience that there is little difference. There are differences in service difficulty and parts cost from one regulator to the next, but that has more to do with the individual make and model of regulator than with the piston vs. diaphragm design.

 

[Aqua-Andy]

I am definitely not trying to dispute you. This is a serious and sincere question: Is that true for depths beyond recreational limits? Or maybe I should ask, at what depths do the differences between "good" current 1st stages and "less good" ones come into play?

Actually it really does not matter, the limiting factor is the tank valve. The valves the majority of us are using (Thermo/XS Scuba) are rated for 50 scfm @3000 PSI. Even SP's cheapest first stage the Mk2 at around 70 scfm can outflow the valve.

 

[herman]

I am definitely not trying to dispute you. This is a serious and sincere question: Is that true for depths beyond recreational limits? Or maybe I should ask, at what depths do the differences between "good" current 1st stages and "less good" ones come into play?

After a quick search of the internet I found a Scubapro document that listed flow rates of it's regs. The MK-2, the lowest by far, has a max listed flow of 2600 l/m (the next model up was 5500 and the best was 8500). SPs worst second stage R195 has a max flow of 1400 L/M, (the best was 1850) . Another search turned up a number of 40-60 L/M for an adult under moderate exercise (5-8 at rest). SO with this data it's pretty easy to see 2 divers under moderate exercise will breath roughly 100L/M combined. The limiting factor here is not the first stage but the second which is still capable of 14 times the needs of 2 divers and the worst first stage is capable of almost double that. While depths in excess of rec limits does limit flow some, until you start getting into really deep tech dives, it makes little difference since even the poorest reg is capable of a lot more flow than is needed. One would hope that anyone diving to those depths was using high end equipment.

I went back an looked, the rated flow of the MK-2 is 92 SCFM, the conversion factor is 0.0353.

 

[redacted]

Perhaps the problem is that we are trying to look at this in liters per minute when our lungs could try to take in up to 7 liter in one second. Within the range of normal testing parameters, WOB does increase with increasing RMV.

 

[stuartv]

it doesn't matter, remember the flow rate is determined by the IP and the density of the gas. Once you get below recreational rates, there is helium in the mix to lower the gas density to allow for better breathing. Apeks tried to combat this as did other mfg's with larger primary regulator ports or orifices, however it was proven to be a marketing gimmick and held no scientific validity.

Most of the regs on the market are variants of a handful of designs. So, let's look at some specs from scubapro because they post it.
Their highest performing reg is the A700 in terms of flow rate and it can flow 71cfm. This is purging mind you. Their lowest rated first stage, the MK2+ flows 92cfm. The combination of 1 second stage, with 1 first stage can drain an AL80 in about a minute. Not of concern at any depth.

Now, let's assume that we are diving DEEP like 200m deep, we can assume a 1cfm breathing rate, which translates to 21cfm adjusted. Two divers, at that depth, require 46cfm out of the first stage. That is half of what the MK2+ flows. Obviously you don't want to be near the limit of any regulator, but even with an old design basic piston, you are well within the limits.

Just to put a bow on this, if we're assuming a SRMV of 1.0 cu-ft/min, then that means what's actually happening is we are inhaling at some higher rate for part of the minute and exhaling for part of the minute, right? If our breathing was exactly symmetrical (which it's probably not, but for the sake of discussion), then that would mean you're inhaling half the time, so the flow rate of the reg would need to be 2.0, instead of 1.0, right?

Or, in the example, 2 divers at 200m, both breathing at an SRMV of 1.0, means the 1st stage would need to be able to flow 84 cu-ft/min to keep up, right?

Still not a big deal, as we're talking about 200m depth. 660 feet? Right. I'll try to remember to use something better than a Mk2 when I go that deep next time. ;-)

 

[tbone1004]

you use RMV which is a term to describe a number of respirations per unit time against the volume per respiration.
So, total lung capacity is call it 6l for a normal male, which for easy math let's call it .2cf. You can average about 90 inspirations per minute if you are hyperventilating, this equates to 18cfm. Unrealistic because you need shallower breaths to do that. More realistic is a 2-3l inspiration, so call it 0.1cf, and you can average 40 breaths per minute, for a rough SAC of 4. Each inspiration by that mark is 1.5 seconds long, and even at the largest theoretical breath size, would be 8cfm flow rate out of the first stage. Two divers doing this in unison, so 16cfm, by the Scubapro MK2, it can deal with that to 6 atmospheres, or 160 ish feet. This is still greater than the flowrate of the valve, is predicated on an impossible breathing rate both in volume and speed, while magically done in sync between two divers. Keep in mind, even at the surface, an AL80 at that flow rate would be drained in 5 minutes...

All completely theoretical, and I don't think I have actually seen a real world SAC rate exceed 2cfm under hypercapnia conditions. Not to say it can't happen, but no one has to worry about outbreathing a regulator...

 

[redacted]

Over breathing a reg is not a matter of trying to get more gas than the reg can deliver. It is trying to get more gas than your reg can comfortably deliver and that gets us into WOB. I suspect that all regs are subject to result in high, uncomfortable breathing effort under suffuciently stressful conditions. And specific regulator performance will be a variable.

Overbreathing my Mk2/R190 trying to work against a 2 knot river current is what caused me to buy a Mk20/G500. It made a readily noticable difference.

 

[tbone1004]

and that is a much more important differentiation, the reg can flow that amount of air, but if the cracking pressure and work of breathing are escalating, then that is another discussion.

 

[Oldbear]

So does the design (diaphragm or piston) have any bearing on WOB?

BTW - very informative posts, it is causing me to do a lot of thinking.