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Ron
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regulator airflow capabilities primer

Mon Jul 27, 2009 2:28 am

This is for the engineering types.

1.) Does more intermediate pressure mean more physical air per breathe? In other words, does a regulator with an IP of 140 give you more air per breath than a regulator with say 110 PSI of IP assuming that the second stage orifices are the same diameter?

2.) I know that regarding flow through an orifice, Q=AVK, where Q equals the flow in cubic feet per second, A equals the area of the orifice, V equals the velocity of air, and K for a volcano style orifice is about .97. What are common first stage orifice diameters? Supposedly the hallmark of a good deep diving regulator is a flow rate of at least 25 CFM with a supply pressure of 2500 PSI+ and I would be interested to see which of our vintage USD regulator qualify as "deep diving regulators". I know most of the double hose ones provide plenty of air, but I would be curious about some of the single hose ones.
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luis
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 10:20 am

The short answer is NO…

A demand valve will only give the amount of air that the diver demands (that is why the catchy name). The moment you stop inhaling, a properly tuned demand valve will stop delivering air. The difference in regulator performance is in how much effort (continues suction) it takes to get that air delivered.

The actual amount of air it delivers is really always up to the diver… It just feels like we are not getting enough air if we have to work hard at getting it.


Higher IP on a down stream simple demand valve is beneficial to balance the spring forces. It also provides in a way some accumulated air reserve in the LP hose to deliver air during the breathing cycle, but this effect is in most cases small.

Your flow equation at best would be for an open orifice would. This is not just an orifice, it is a valve. Keep in mind that due to the high pressure drop in both stages, the flow in these valves is supersonic compressible flow.

The flow area is the perimeter of the orifice times the height the seat lifts up. The pressure drop (and subsequent shock wave discontinuity) occurs on the perimeter of the orifice not on the plane of the orifice.

In this case the larger orifice gives you a larger perimeter; therefore the flow area is proportional to the diameter (not the diameter square like the area of the circle is).
Luis

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INFIDELxx
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 11:58 am

luis wrote:The short answer is NO…

A demand valve will only give the amount of air that the diver demands (that is why the catchy name). The moment you stop inhaling, a properly tuned demand valve will stop delivering air. The difference in regulator performance is in how much effort (continues suction) it takes to get that air delivered.

The actual amount of air it delivers is really always up to the diver… It just feels like we are not getting enough air if we have to work hard at getting it.


Higher IP on a down stream simple demand valve is beneficial to balance the spring forces. It also provides in a way some accumulated air reserve in the LP hose to deliver air during the breathing cycle, but this effect is in most cases small.

Your flow equation at best would be for an open orifice would. This is not just an orifice, it is a valve. Keep in mind that due to the high pressure drop in both stages, the flow in these valves is supersonic compressible flow.

The flow area is the perimeter of the orifice times the height the seat lifts up. The pressure drop (and subsequent shock wave discontinuity) occurs on the perimeter of the orifice not on the plane of the orifice.

In this case the larger orifice gives you a larger perimeter; therefore the flow area is proportional to the diameter (not the diameter square like the area of the circle is).

UUUUMMMMMM.........What? :oops: Man I feel stupid!

I thought the simplicity of a Single Stage Double hose was just that, Simple!..... Man Ive been duped.

Luis,
Why is the HP drop supersonic? Is it because the Drop in pressure is so rapid?
D.I.R.....With Voit !

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luis
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 12:49 pm

The pressure drop on is so large that a shock wave/ pressure discontinuity is formed. The pressure and density difference of the gas before and after each valve is very large.


I am not sure what you are referring to with the single stage… it is a very simple regulator. All the pressure drop occurs in one stage, but the more important aspect is that it is mechanically very simple… very few mechanical parts and very few moving parts.
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 1:00 pm

So Luis, just to make sure I understand this, I tuned an old calypso last night. It had a cracking pressure of .8 and an IP of 130 PSI. My conshelf 14 has a cracking pressure of .8 and an IP of 145. There is no difference to the diver, correct? So for my purposes, the IP is really just a way to make sure that the spring is balanced and does not exert too little pressure (freeflow) or too much pressure (crappy cracking pressure). This also explains why many old regulators have really low IPs (like 110 for an original calypso) but they still have good cracking pressures (lower spring rates).

So how do you know if a regulator will flow enough air at depth then?

Between you, Greg B, and Douche I feel like my brain is the size of a grape :lol:
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 5:23 pm

Between you, Greg B, and Douche I feel like my brain is the size of a grape :lol:

Welcome to the club Ron. LOL This old dog has learned a lot of new tricks here!

Jim

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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 6:15 pm

Slonda,

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!

Greg
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 6:41 pm

Wow. Yet another reason I hang my hat at VDH. People on Scubaboard are stilling arguing about jacket BCs and we are having this kind of discussion. Thanks Luis and Greg! So I guess the short version is that most regulators will afford you plenty of air at depth. Is the limiting factor the first stage orifice diameter? I know pretty much every regulator that I have worked on with a downstream second stage has an second stage orifice diameter of .028 square inches, so I gather that most second stages will flow plenty of air.
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 7:09 pm

slonda828 wrote:Wow. Yet another reason I hang my hat at VDH. People on Scubaboard are stilling arguing about jacket BCs and we are having this kind of discussion. Thanks Luis and Greg! So I guess the short version is that most regulators will afford you plenty of air at depth. Is the limiting factor the first stage orifice diameter? I know pretty much every regulator that I have worked on with a downstream second stage has an second stage orifice diameter of .028 square inches, so I gather that most second stages will flow plenty of air.
Ron,

Yes, the first stage orifice diameter is typically the limiting factor. That is why a balanced diaphragm can flow so much gas compared to a diaphragm design. Considering current designs, it would be pretty tough to get a diaphragm reg to flow more than 200cfm. Again, why do you need such a flow rate anyway?

To give some additional info to consider...Very few second stages can flow more than 75cfm in a fully purged situation. So...My reason for feeling that my Mares MR12 first stages meet my needs for my cave diving doubles. Plus, real high flow rates can lead to icing conditions in cold water. When you start flowing those high gas volumes, adiabatic cooling can really take its toll.

I'll bring one of my early MR12s for you to try out at Portage. Just keep it a secret that we are using "one hose" on a few dives! Rob and Bryan will never let me live it down...

Greg
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 7:14 pm

Whew, thanks Luis and Greg, I for one love these post.
The more I learn about these regs the more I realize that there is so much I don't know. Keep post like these coming.
Herman

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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 8:05 pm

Well, Greg and Luis, FWIW, I have dove with you guys and determined that with the SAC you have you cannot be human. Ron, you will observe this at Portage. Last year I was happy with about 45 minutes out of my 72. Greg and Luis went about another 30 minutes. What can I say? I'm not worthy! LOL

Jim

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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 8:10 pm

Just to get some stuff in perspective I will try to help Greg by converting some units:

62.5 lpm (liter per minute) = 2.2 cfm (cubic feet per minute) this is about the max flow rate needed by a human according to Greg’s first post.

A Scubapro Mk-20 first stage can deliver:
300 cfm = 8495 lpm

A diaphragm first stage can deliver about:
140 cfm = 3964 lpm

A decent second stage can deliver:
75 cfm = 2123 lpm
That is over 30 times the needed amount by a human (if we needed 75 cfm, a steel 72 would last less than one minute)… It is true that we are talking about continues steady state gas flow, when in reality our breathing rate is not continues flow, it is in cycles.

The issue is not the flow rate… the issue is the effort that it takes to get that flow rate.


The common statement “this regulator delivers a lot of air” is as I said earlier not a precise statement. Most regulators will deliver all the air any diver can need, the difference is how much effort it takes to get that air.

If the effort to get the needed air actually causes the diver to work hard and in turn need even more air, then is what would be commonly known as over-breathing a regulator.


Most of the performance of any decent regulator is controlled by the second stage. The first stage needs to deliver just a constant IP to help the down stream demand valve of a non-balanced second stage (a vintage second stage). The more constant the IP during the breathing cycle, the better the second stage will perform with the assistance of the pressure force.

The higher the IP does have some other minor advantages, but they are minor. One example is that the higher the IP then the momentary pressure drop during inhalation becomes a smaller percentage drop of the total IP
Luis

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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 9:21 pm

Luis,

I have noticed that. My conshelf drops a lot less pressure than my aquarius on a full inhale. Since the cracking pressures (full tank) are the same, I cannot tell any difference though. So it seems like other than cracking pressure, all the other factors to a diver are negligible. In car racing they refer to a "butt dyno", which is when you can feel that your car is faster or slower. My diving dyno tells me that all my regs breathe about the same except for the unbalanced ones, which breath harder or easier depending on first stage valve configuration (downstream or upstream). FWIW, I experienced Luis' lack of a SAC rate (it's like he does not breathe) in the Bahamas. He must have a very efficient respiratory system.

Maybe we should make this a sticky in the technical section, I bet other people could use something this...technical.
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 9:43 pm

Luis makes some excellent points. My focus was to explain how flow rate compares to metabolic needs. Regulator performance involves so many variables that few can be isolated from others. For example, the pressure sensing diaphragm's position relative to the lungs has a dramatic impact upon perceived performance. Changing you orientation in the water column clearly demonstrates this effect.

Double hose regs have easier exhalation (compared to single hose models) while in a normal swimming position primarily due to the exhaust valve being located in shallower water than the diver's lungs. However, that same water pressure issue causes them to have higher inhalation effort. Fortunately, for those of us who love diving with them, it is physically easier for the lungs to inhale rather than to exhale. This is the main reason why early single hose regs couldn't get USN approval due to the tiny exhalation valves. The MR12 ushered in large diameter valves, and it earned the coveted award. It didn't take long for the other manufacturers to do the same.

Mechanical resistance to parts moving plays a big role as well. As Luis and I have discussed, one of the main reasons why the Voit Trieste is not a great breather is due to the bell shape of the diaphragm. The curvature resists deflection, thereby creating more inhalation effort. These "little things" can add up to where the overall work of breathing is compromised.

Another example is the substitution of using a silicone diaphragm in place of the more rigid originals. Doing nothing but changing a diaphragm can demonstrated a marked decrease in cracking effort. Luis's redesigned wagon wheels, along with the silicone one-way valves is but another example.

The cracking effort of a reg needs to be as low as the case geometry will allow for various swimming positions. Any venturi action needs to be controlled so that the lung's are not required to do all the work of keeping the diaphragm depressed. However, too much venturi assist will cause you to be "force fed" air. If you don't believe that is possible, then ask Rob if you can dive with his restored USD DX. He claims it can blow your tonsils down your throat. Again, all of these issues are big factors in reg performance.

I appreciate the compliments about Luis and me regarding our air consumption. I bet a young guy like Slonda will demonstrate how youth can show us a thing or two. My 50 year old lungs are still pretty efficient, but Allan can show us all a healthy lifestyle makes those 72s last a lot longer. I can only hope that I can match him if, and when, I get to his age.

Greg
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INFIDELxx
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Re: regulator airflow capabilities questions

Mon Jul 27, 2009 11:35 pm

luis wrote: I am not sure what you are referring to with the single stage… it is a very simple regulator. All the pressure drop occurs in one stage, but the more important aspect is that it is mechanically very simple… very few mechanical parts and very few moving parts.
Yes, your right they are simple. I was was making fun about the "supersonic" speeds. What I meant to say (And poorly I might add) is that anything that is supersonic could not be so simple.

I think this winter I am going to pull up all these type of posts and try and really grasp as much as possible. Then I may attempt re-building some of my own reg's.

Thanks!
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