Diaphragm vs. Piston Regulators

[redacted]

I was a bit confused by some 2nd stage flow rate stats in this thread. Tbone citing the A700 at 71 cfm and Herman citing a generic 2nd at 40 - 60 lpm but an R195 at 1400 lpm made my head hurt. So i hunted down the SP table:.SCUBAPRO - Regulator feature comparison

Surprise, their 2nd stage flow rate databis at 200 bar rather than at IP. I bet the SP marketing staff struck again. So, do we need to rethink this idea of they all provide plenty og gas? A 40 lpm supply may not satisfy an upper bound demand of 7 liters per second.

 

[redacted]

After a little more reasearch, i have concluded that a diver can overbreath any recreational regulator.

The SP chart leads me to conclude that their 2nd stages will provide flow rates of 100 liters per minute to 70 lpm at an IP of 10 bar. Our lung capacity ranges from a maximum volume of 7 liters to a normal tidal vo!ume of .5 liters. Also, breathing rates have a normat range og 12 to 20 breaths per minute but can fall below 10 bpm in a relaxed state or climb to over 30 bpm wigh exercise or LPstress.

So, converting thevreg delivery capacity to liters per second, we are looking at about 1 lps +/-. With a fairly relaxed subject consuming tidal volume, 0.5 l, at a breathing rate of 20 breaths per minute, the inhale phase of each breath should last about 1 second and not present a problem. With good scuba breathing techniques (slow, deep breaths) i'm guessing we are in the same range of inhalation demand of about 0.5 lps with slower, longer breaths. But under stress, say from chasing that turtle, we were to elevate the breating rate to 30 bpm and increase the respiratory volume to 2 liters (no where close to max) that would cause inhalation demand of 2 liters per second, beyond the capability of every one of the SP regs.

The question is not can we overbreath our regs, but at what level of stress or activity will we overbreath our reg. And the lower performin 2nd will become a problem before the high performace model.

 

[joshk]

The SP chart leads me to conclude that their 2nd stages will provide flow rates of 100 liters per minute to 70 lpm at an IP of 10 bar.

The lowest flow rate on the chart here: SCUBAPRO - Regulator feature comparison for 2nd stages is 1400 liters per minute, are you looking at something different?

 

[The Chairman]

For my money, I normally go with Dive Rite regs (diaphragm) for tech diving. My recreational reg is an Atomic (piston). Both get a lot of action and both get neglected from time to time. My stage bottles all have Seac (diaphragm) and a few Hog (diaphragm). I selected these various first stages due to their various configurations rather than breathing specs.

Since we are in the tech section, I can't think of a single tech set up that I've seen that has two divers ever breathing on the same first stage. One second on each first.

 

[tbone1004]

awap, if you're breathing that hard and that fast, you'll pass out rather quickly... that is straight up hyperventilating to be going that fast with short breaths... maybe in hypercapnia, but I don't think it's possible to inhale and exhale that fast and take in 2 liters of air at a go.

 

[redacted]

The lowest flow rate on the chart here: SCUBAPRO - Regulator feature comparison for 2nd stages is 1400 liters per minute, are you looking at something different?

That is 1400 lpm at a 200 bar supply pressure. What do you think tnat equates to at a supply pressure of IP, 9 - 10 bar?

 

[redacted]

awap, if you're breathing that hard and that fast, you'll pass out rather quickly... that is straight up hyperventilating to be going that fast with short breaths... maybe in hypercapnia, but I don't think it's possible to inhale and exhale that fast and take in 2 liters of air at a go.

Do you really think 2 liters is that much??? What size wing do you use and how many breaths does it take you to inflate it?

 

[joshk]

That is 1400 lpm at a 200 bar supply pressure. What do you think tnat equates to at a supply pressure of IP, 9 - 10 bar?

Does the supply pressure alter flow rate linearly? I thought the orifice size was a much bigger factor in flow e.g. a LP hose rupture loses more gas faster than if it was a HP hose because of the orifice size, not the pressure.

All the same, I don't see why they would give the LPM number using a pressure far outside the second stage's normal operating limits. I think that number refers to the supply pressure to the first stage.

 

[redacted]

Gas flow rates:.Calculator: Air Flow Rate through an Orifice | TLV - A Steam Specialist Company (International)

I wonder how many Scubapro customers might be fooled by such marketing deceptions.

 

[tbone1004]

2 liters is absolutely too much for a 1 second inhalation for any length of time, you will hyperventilate. That is a breath 4x as large as the one you are likely taking right now, and being done twice as fast *based on the normal average tidal volume of 0.5l, and roughly one breath every 1.5-2 seconds*