Air flow rate for 1st and 2nd stages

[DA Aquamaster]

The information above is pretty accurate. A truely low end unbalanced second stage will have a flow rate around 30 SCFM. A high end reg will be in the 67-70 SCFM range.

The first stage flow rates noted above are fairly close. In general anything over about 120-150 SCFM will never be utilized due to the second stage limitations and as noted above the valve poses a finite limit anyway. In the case of really old valves, the dip tube is pinched shut and there are 2 small holes perhaps 1/16" in diameter in drilled in the side of the dip tube. At low tank pressures that will present the limit on flow rate regardless of what reg is attached. (On those valves I recommend replacing the dip tube with one with a fully open bottom.)

As noted above, on an unbalanced reg like the MK 2, there is a significant drop in IP as tank pressure decreases. On older balanced piston regs, there is also a 6-8 psi drop in IP as it is not perfectly balanced. Current balanced piston regs are often designed with an enlarged piston head to compenstate fo the area of the seating surface, so the IP is very stable. The same is true with most current balanced diaphragm regs where the IP is also very stable.

In all case, the amount of drop on inhalation is important as is the response time of the reg.

And as Mattboy notes, second stages differ greatly in the quality of the airflow. Numbers and work of breathing charts are great marketing ploys, but I have yet to find a reg that actually feels better in the water than a well tuned D300, D350 or D400. The flow rate is very high (around 67 SCFM) but it is also very smooth and very natural in feel.

 

[islanduni]

Your right Mattboy, S600 Mk25. Although I have seen Kirby Morgans that could suck the last bit of air out of a surface supplied volume tank when all the rest of the regulators stopped working.

 

[Nemrod]

Almost all regulators will flow all the air you could possibly need, you are chasing a Jaberwhocky.

N

 

[AquaNSun]

Thanks for another very well informed postings from everyone. I was trying to get a general feeling of different flow rates in current regulators and learned to know some limiting factors. This is just an assumption, IMHO, I think the "Real" limitation in flow rate is human lung breathing rates. Take for example a male has a lung capacity of 6 liter = 0.211888 cubic ft. On normal breathing, you can say about 16 breaths/minute. So, that's about 3.4 cubic ft/min of breathing rate. Double that for hyperventilating and you have 6.8 cubic ft/min. Double that again for 2 divers hyperventilating and you have 13.6 cubic ft/min. This number seems to be way lower than anything we have seen, even the tank can do much much better. I guess it's true that no matter how fast a human can breath, it's never gonna beat even the tank rate. Female has smaller lung capacity and therefore generally has more air on the same dive. On top of that, age, height, weight, and physical fitness all have an effect on lung capacity as well. In the end, breathing is kinda subjective I guess, one is good for me but not necessary for another person.

 

[Greg Barlow]

You also have to take into account that of the average 6 liters (average sized male) lung capacity only around 4.5l is the true vital capacity. That meaning no matter how hard you exhale, around 1.5l is going to remain in the lungs. Of that 4.5l, a realistic amount of gas capacity moved in and out of the lungs with normal breathing is closer to 2.5l. That is the reason why ANSTI testing is done with 2.5l exchange.

An aerobically fit diver will likely be breathing at a rate between 12-18 breathes per minute while swimming at an average pace. Someone swimming at a somewhat hard pace is likely to record 20-25bpm, and "all out" will be in the 25-30bpm area.

The problem is that when you breathe faster than around 30bpm, there is generally not enough time for adequate gas exchange to take place. Your body's cells go into oxygen starvation, and the CO2 levels get dangerously high.

As DA Aquamaster has written, a reg that delivers more than what two divers can possibly use is just overkill.

As I mentioned, I regularly use first stages that flow a "weak" 140-175cfm per minute. At depths around 150ffw on air, and battling pretty significant currents, I have yet to "beat" my regs. Factor helium blends into the picture below 150ffw, and the regs breathe even better.

Marketing....

Greg Barlow

 

[Blue Quaker]

Some of the manufacturers reg tech clinics have got into this on several occasions but the numbers that meant the most to me was when when the engineers started talking about tank valves...max 28-32 cuft /min flow rate on a yoke valve @3000psig supply, and max 65 cuft/min flow rate from a 300 BAR DIN valve @3500psig. Remember that the flow rate will drop as the tank pressure drops so you will never be able to flow a reg at its max flow rate in the first place....
Cheers,
Norman H.