Last pulls with balanced regulators

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Leafer

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I am interested to know the difference, if any, between a balanced regulator setup and traditional non-balanced as a tank is hitting "empty". When pulls start getting hard, will there be less breaths to suck out of the tank on a balanced system? I'm guessing so but not sure.

The mechanics of the non-balanced setup will result in harder breathing as the cylinder gets lower on pressure, I'm not asking about this. Rather, I'm asking about when the ambient pressure in the cylinder reaches the pressure at depth.

During training, I think most of us have felt what its like when the valve of the cylinder is closed but in an actual ooa situation, you have the contents of the cylinder that will expand as you ascend to provide you with a few more breaths.
 
With unbalanced diaphragm the IP will increase as tank pressure drops, so will breath easier untill very low cyl pressure.
Balanced will breath the same untill very low pressure.
Bith will then start breathing harder
 
it depends on regulator valves. Upstream or downstream (for unbalanced regs) (Co-flow or Contra-flow).
With decreasing pressure downstream (contra flow) valve will be opened easy, with less forces.
 
For example, with a good balanced second, you won't even notice a difference until the Interstage Pressure hits 60.
An unbalanced second will have a slight, but notable difference if tuned to 140 psi, when IP drops to 125.

Equating that to tank pressures is a little difficult, and is 1st stage dependent. With an unbalanced first and unbalanced second, you'll get clues around 500 psi.

With balanced firsts, you won't know until <200 psi tank. At that point the difference is a bit academic.

Two ways to look at it. If you want to have increasing breathing effort tell you that you're getting low, you're stepping back in time about 30+ years. But it works.

If you know what you've got, and want to suck the last bit out, then a balanced second (or better - balanced both) will allow you slightly more.

Story: My only CESA was deliberate. Was a DM tasked with retrieving the float and down line. It had been a long day, and I'd made one tank last thru two groups of student dives. I rolled up the 100' of navigation line and was at the float screw when I hit 200 psi, in about 25 feet of water.
I figured, "screw it" and started working the sand screw. It wouldn't come. My gauge read 100 psi. I REALLY didn't want to have to swim in to shore and get another tank. I kept working.
The screw wouldn't come free. I sucked my last breath, and did a leisurely CESA to the surface. When I switched tanks back on the beach, the gauge needle didn't even come off the stop.

Glad I had a balanced set, even though it didn't make the difference.

To answer your specific question, when the tank pressure reaches ambient plus interstage pressure, plus a bit for friction losses, things begin to change rapidly. So at 66ft, at 45 + 135 + ~20psi, a good balanced set will begin to pull harder.
If you're diving an 80 cu ft cylinder, you have about 5-6 cu ft left, but only about 2 cu ft usable. Your average consumption as you ascend (plus stress) starting at 66 ft, means that you have about 30-90 sec of usable air left with a pair of balanced regs. With both unbalanced, things would get scarier, quickier. But then you should have known from the breathing effort change, about 300 psi earlier.
 
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Wow fantastic answer! Thank you. My older non balanced regulators can not be serviced so I’m looking to replace them with SP mk17 and A700. In the process of researching I’ve become very interested in CESA and will undoubtedly practice techniques on my upcoming dive trip. Possibly DM assisted.

Just for the purpose of understanding, say my gauge was faulty or I hadn’t check it, I’m at 80ft with my new balanced regulators and no divers are close by… I wonder how many breaths I can get once I notice that the tank pulls hard.

I have heard experienced admittedly old time divers say to just dump air from your bcd, begin ascending and breath normally. Expanding gas in your tank will give you plenty to reach the surface. Thus my curiosity as to whether balanced regulators would result in less air remaining in the cylinder.

In your last paragraph I think you answered this precisely. 30-90 seconds seems a good buffer to quickly assess and make a CESA.

Conversely, when I read of CESA accounts they seem to convey that when that hard pull occurs, the diver should count on that as being the last breath and immediately start for the surface slowly blowing bubbles. (again, where alternate air is not reachable)

I would like to arrange with a DM me sucking a tank dry at 40-80 and completing CESA with him/her at arms length all the way octo ready. I’m guessing that they wont be open to this. If so, I will see what its like sucking one dry on the boat but of course that much different than being at depth.
 
My older non balanced regulators can not be serviced...

Just so you know, I'll bet there might be more than one DIY'er (myself included) who might be interested in some "older non balanced regulators [that] can not be serviced." There is a fair group here that restores vintage equipment that is "beyond repair" for the typical local dive shop.

Maybe post here or in the Classifieds section of Scubaboard?
Cheers!
 
I wonder how many breaths I can get once I notice that the tank pulls hard.

I have heard experienced admittedly old time divers say to just dump air from your bcd, begin ascending and breath normally. Expanding gas in your tank will give you plenty to reach the surface. Thus my curiosity as to whether balanced regulators would result in less air remaining in the cylinder.

In your last paragraph I think you answered this precisely. 30-90 seconds seems a good buffer to quickly assess and make a CESA.

Conversely, when I read of CESA accounts they seem to convey that when that hard pull occurs, the diver should count on that as being the last breath and immediately start for the surface slowly blowing bubbles. (again, where alternate air is not reachable)

Again, the answer is complicated. It all depends upon how sensitive you are (vs. distracted) to a slight increase in inspiratory effort. With an unbalanced first and second, you'll see a perceptible change as early as 15 min before you're OOA, depth-dependent of course.
With a balanced first and second, and a little distraction, you may indeed be 1 1/2 breaths away from empty. Yes, you may get a breath or two more from expanding gas in the tank and tubing as you ascend. But don't make that part of your planning, practically speaking. Leave your reg in your mouth, make a CESA and take what you can get.

Your planning efforts are laudable at this stage in your diving. But I would recommend an ounce of prevention, as the better course.
1) Don't run OOA (duh :))
2) Dive with a buddy (duh :))
3) As you gain experience, consider Self-Reliant Diver training, not necessarily to start diving solo, but to learn more about the techniques you're pondering here, to KEEP yourself out of trouble, rather than GET yourself out of trouble.

If you're working solely on logic re: this question, you might conclude, "Why NOT go back to 1970's style diving, so I'll always get an alert from my unbalanced regulators?" And if that's the only consideration, you'd be right.

But now we have air-integrated computers to help you watch, and a habit of regularly checking our gauge (remember - we didn't always even have SPG's - hence the MkVII Honker 35 years ago). So things like ease of breathing become relatively more important. And there's nothing like the ease of breathing from a well-maintained, well-tuned pair of balanced regulators.

My 2 cents.
Good on you for asking the questions.
Happy diving!

EDIT: Referred to unbalanced piston first stages. Adding a diaphragm to the discussion made it too complicated.
 
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Equating that to tank pressures is a little difficult, and is 1st stage dependent. With an unbalanced first and unbalanced second, you'll get clues around 500 psi.

I concur. It is why I believe that new and infrequent divers are better served by unbalanced first and seconds. They don't need the super-high performance or zero warning that they are low on air. Screwing up should not be a life threatening situation when using a much less expensive and very reliable regulator is the alternative. I imagine dive shops and manufacturers hate hearing that any regulator that meets EN250 standards is more than 90% of the divers in the world will ever need.

This performance characteristic is the main reason that divers in the pre-SPG days rarely had an OOA (Out Of Air) emergency. The low on air signal was hard to miss and gave you plenty of time to surface since safety stops didn't exist then.
 
his performance characteristic is the main reason that divers in the pre-SPG days rarely had an OOA (Out Of Air) emergency

OOA could hardly be called an emergency when it was the primary indication that your dive was ended, especially for new divers. Also the training of the day prepared myself and others for the reality of that situation.

The low on air signal was hard to miss and gave you plenty of time to surface since safety stops didn't exist then.

The normal 60'/min ascent rate was really handy then, as well.


Bob
 
... I believe that new and infrequent divers are better served by unbalanced first and seconds. They don't need...zero warning that they are low on air. Screwing up should not be a life threatening situation when using a much less expensive and very reliable regulator is the alternative.
...divers in the pre-SPG days rarely had an OOA (Out Of Air) emergency. The low on air signal was hard to miss and gave you plenty of time to surface...

Sad to say, you're absolutely right.
Philosophically, I have a hard time when someone recommends that I stay on a bike with training wheels because I'll hurt myself from task overload if I don't. It offends my ego.
But, sad to say, I think you're right.
We have great students who charge into diving and learn everything they can. They indeed get the most out of our sport.
But then we have a larger group who either want all the fun without paying the learning dues, or who just want to dabble in the water and check off that "experience" box and move on.

In the interest of growing the industry, our largest training organization has made that easier, by breaking training down into more manageable bites. While continued training is encouraged, I think it's fair to say that with many schools, initial OW divers come out with less comfort in the water than back when the sport was tiny, and training took six weeks.

So if you want to rent a Mk2/295 to some unknown diver so he'll figure out he's running out of air before he kills himself, I'd say that's a good business decision. But if I arrive at your shop without regs because of baggage limits or a short notice decision to dive, and that's what you offer me...I'd have to reply, "That's the best you can do for me?"
 
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