You need to keep in mind the capabilities of an average human. My science background is actually in the area of human anatomy and physiology, so the engineering pieces are best explained by Dave and Luis. Don't feel bad, as Luis and I have had conversations that had my head spinnin'.
First, you need to understand that in order for necessary gas exchange to occur there needs to be a short delay between inhalation and exhalation. For example, if you hyperventilate real quick, you will get very dizzy due to poor gas exchange in the lung's alveoli. There needs to be adequate time for gases to diffuse in and out of the alveoli. If breathing occurs too rapidly, then the cardio-pulminary system cannot keep up with the demand of the body's other tissues. A person will pretty much reach the limit of their conditioning within reason. For example, while you may be able to swim a 25 meter race without breathing, you could never do so for a longer race, such as 500m.
The gas exchange is commonly recorded by using the capacity of gas that can flow in and out of your lungs in one minute. The vital capacity, or amount of volume in the lungs, for an average male is about 4.5l and for a female around 3.5l. Now, that is with deep, full inhalations and exhalations. A person running, or exercising at a pace where it would be difficult to carry on a conversation would be around 2.5l. So, the USN uses the 2.5l measure for regulator testing. They also use the breathing rate in calculations. To determine the RMV (Respiratory Minute Volume), you simply multiply the rate times the 2.5l figure. For example, a very fast breathing rate or 25bpm (about the maximum rate for effective gas exchange) you would be moving 62.5 liters of gas in and out of the pulmonary system in one minute. If you were really going hard, say at a full speed sprint, you would likely have a RMV of 75 liters. Most highly trained athletes can maintain that type of work load for a minute or so. An example would be someone sprinting a 400m dash.
So...With that in mind, consider that most divers swimming along at a leisurely pace would be using a RMV of around 15-25lpm. That is because your volume is fairly low, probably at about 1 liter per breath and around 15-20 breaths per minute. Swimming at a pretty fast clip, like around 1-1.25 knots, you would likely be somewhere in the ballpark of 30-50lpm. Get up to 50-60, and you are likely wishing that you had taken up golf.
Now...Throw in the depth, and correspondingly denser gas flowing through a regulator. Any reg that can display an overall work of breathing score of 1.4j/l at a depth of 198', and with a RMV of 62.5lpm is going to get an "A" rating. That simply means that if you take a well trained athlete and have them swim at a fast pace such as for a 200m race, the reg can provide all the gas that their body needs to function normally.
When I hear people say that their reg couldn't provide enough gas, I have very serious doubts as to the validity of their claims. In all honesty, it is likely that their cardio-vascular system couldn't keep up. It merely seemed like the reg was incapable of meeting their demands. Making a statement like that is similar to claiming that the atmosphere can't provide enough oxygen for you when you get winded from running long distance.
Sorry to be so long winded, but that bit of information is necessary to understand the airflow capabilities of a regulator. Most modern regs can meet/exceed the USN Class "A" standards due to the tremendous flow capabilities of the first and second stage. In fact, many regs can flow gas at a higher rate than what the cylinder valve can deliver. For example, a Scubapro MK 20 first stage has such a large bore piston that it can deliver up to 300cfm with a constant air delivery of 3,00psi. Yes, I know that isn't within the capabilities of a scuba cylinder, but it makes my point.
Heck, even a smallish diaphragm first stage like the modern Mares MR12 has a flow rate of 140cfm. The Mares V42 has a rate of around 175lpm for comparison. As you can see, those flow rates far exceed the RMV rate of 75lpm. Even an unbalanced piston like the Mares R2 or Scubapro MK 2 can flow between 90-100lpm. The intermediate pressure does fall more on a reg with lower capacity. The Mares V42 might have an IP drop of 10-12psi during full purge, while the MR12 might go down around 15psi. The Scubapro MK 20 would likely only drop 6-8psi or so.
Now, the vintage regs....When working for Rodale's I had some regs tested on an ANSTI simulator. You have to remember that when those regs were developed, few divers used an octopus. Buddy breathing was the norm, so lower capabilities were not an issue in the engineering phase. Even so, many of the first stages such as the Voit MR12 delivered 110-120 lpm, and the second stage could deliver around 45lpm. Again, that is on a system with continuous supply pressure. Regs are tested in the same manner with USN standards being a 1,500psi supply pressure without a cylinder valve.
To sum it all up....One of my Voit MR12 regs delivered a WOB score on the ANSTI of 1.68j/l at a depth of 132'. That was at the 62.5lpm rate. At 165' the WOB went to 2.15j/l. Still, not too bad! Heck, it would earn the CE stamp of approval! The USD Calypso VI delivered a very impressive score of 1.48j/l at the 165' depth. There are several new regs that can't match that.
The regs that had very low scores were the tilt-valve models. The Voit Little Gem produced a score of 4.10 at 99' with 20bpm. Not one I would dive with below 50' or so!
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