Conservative Tech?

[buddhasummer]

Those MK11s can swing pretty badly from full to empty tank, ditto the MK17, I wouldn't call them "balanced" first stages but "kinda balanced".

 

[buddhasummer]

Some for sure but we are talking machine level testing. No diver will tell the difference, remember the second stage is a demand valve so you only get what volume of air you demand regardless of the density of the gas (within reason). The volume you demand is the same at any depth and until you get way past normal rec depths does the density really start to be an issue. Besides the increase in IP at 100 ft is roughly 45 psi of a "normal" reg as opposed to 50ish for an "overbalanced" reg, roughly 5 psi not really enough to make a difference in practical terms. It's enough to make a well tuned unbalanced second stage start to gently freeflow but not much else.

I have a Conshelf tuned as fine as I could at around 1.1" paired with an Apeks DS4, I'll take it out this weekend and see at what depth it starts to free flow. I did an on land experiment last year with a conshelf cracking at 1.2-1.3" IIRC, and it didn't start to free flow until ip was increased by around 10 psi. I've spent many hours seeing just how low I can get a Conshelf cracking and 1.1" seems to be the lowest and remaining stable.

 

[Luis H]

And @herman , without stirring the pot too much, don't you think that over-depth-compensating might be of some value in providing a balanced second with a higher driving pressure, given the air density concerns at 5 atm? Not that I'm sure the Mk11 actually does that.

Some for sure but we are talking machine level testing. No diver will tell the difference, remember the second stage is a demand valve so you only get what volume of air you demand regardless of the density of the gas (within reason). The volume you demand is the same at any depth and until you get way past normal rec depths does the density really start to be an issue. Besides the increase in IP at 100 ft is roughly 45 psi of a "normal" reg as opposed to 50ish for an "overbalanced" reg, roughly 5 psi not really enough to make a difference in practical terms. It's enough to make a well tuned unbalanced second stage start to gently freeflow but not much else.

I agree with @Herman’s post. I don't see much of a point with the over depth-compensation, just marketing, IMHO.

It is always interesting the concern about the increase flow resistance due to gas density.

If you look at Bernoulli’s equation for fluid flow you will indeed see that flow resistance is proportional to the fluid density.

But, if you if you understand Bernoulli’s equation, you will also realize that any venturi educator-jet effect is also proportional to the fluid density.

I am not trying to say that the venturi effect will totally compensate for any increase flow resistance since the amount of venturi effect varies on different regulators, but the actual amount of flow resistance also varies and its effect is often over stated.

The breathing resistance in a regulator is not really due to flow resistance. The resistance divers feel is because the demand valve is design to close when the suction is too low to keep the valve open. The diver has to maintain a minimum suction to keep the valve supplying air. This is to avoid wasting air. The trick is balancing the lowest possible suction required to open the demand valve without it opening and letting excess air go out the exhaust.

The required suction (which is what we feel as resistance) is actually designed into the demand valve. Again, it is just balancing the minimum suction without causing a free-flow.

For the most part, there is plenty (I mean lots) of stored pneumatic energy in the compressed gas, that easily overcomes the minor flow resistance found inside the regulator gas path, that includes any flow bends, contractions, expansions, or the insignificant friction. I can only think of one exception with some very small orifices, but that is a separate subject.


I have never actually notice the increase in the venturi effect (with increase depth) on any commercially available regulator, but I did notice a very strong change in the amount of venturi flow in a balanced single stage regulator that I was designing and testing.

I designed that regulator with an adjustable venturi educator jet and for some dives I adjusted the venturi-jet to be very aggressive on the surface. During the dives the regulator was breathing great at 30 feet deep and a bit deeper. At about 60 feet it was delivering a bit too much air. With every breath there was a bit of extra air going to the exhaust. I had to breath very slowly to avoid wasting air. At about 90 feet the amount of wasted air was way too much. No matter how light I tried to breath, there was extra air going out the exhaust. It was actually a fun, but short, dive. I made a lot of great observations of the venturi response as a function of my breathing .


Regulators are not operating on a steady state flow situation. They are a mechanism that in normal operation is always in a transient state. They are mechanically very simple, but sometimes designing or predicting the gas dynamics with the mechanism is not as straight forward. IMO, that is why venturi effect in commercially available regulators tend to be conservative (not too aggressive) in their design. The dynamic characteristics of the venturi flow can easily become unstable and unpredictable at depth if it was too aggressive.


The most complex fluid dynamics regulator I have design is a balanced single stage regulator. Mechanically a single stage regulator is incredible simple, but analyzing, predicting, and designing the transient gas dynamics was a bit of a challenge. Having a steady IP supplying a demand valve is what makes the modern two stage regulator so much more predictable than the vintage single stage regulator.


Servicing and diving a vintage single stage regulator is very easy, very predictable, and a lot of fun. But during the original design I am guessing there was a lot of trial and error to get it right.


Some general information for those not familiar with all these regulators:

I should point out that the single stage regulator I am talking about are all two hose regulators, one inlet hose and an exhaust hose. The regulator demand valve is mounted on the tank valve and it is supplied directly with full tank pressure. There is not step down first stage valve.

Most double hose regulators are actually a two stage design just like the single hose regulators. The only mechanical difference is that the two stages are built into one body and they are side-by-side, there is normally no hose between the first and the second stage.



Note: I have re-adjusted my balanced single stage regulator and it performs great at any depth now. When I have more time I am planning on continuing the design of my balanced single stage regulator. Just for the fun of it. My designs are somewhat vintage style, but I am using modern design tools (including computer modeling).


Sorry for the long post. I hope it is helpful.

 

[Ojai Diver]

Thanks for the replies. What strikes me is that both the manufacturer (through their specs) and the tech seem averse to free flows at any time, in any situation, thus reducing even high end regulators to doggy performance.

Conclusion: I think I am going to have to learn to make my own adjustments.

 

[Luis H]

Divers will complain a lot louder even about a minor free flow than they will about a slight increase in suction required. The free-flow is immediately obvious and considered a malfunction. Many divers will abort a dive due to even a minor free flow or leak.

High suction (resistance) to most divers, is just an inconvenience. It would have to be very bad for someone to abort a dive.

 

[herman]

There is a good bit of truth to your statement. Most divers have no clue how their reg works and having one set to the performance edge is a sales and possibly a legal disaster. A reg set that close to the edge of performance will often violently free flow due the venturi built into it if not dove with an understanding of how the reg works and it will be prone of slight free flows due to components ageing, which will require minor adjustments from time to time. Both of these conditions would have the average diver going back the shop complaining about the reg "not working right", I don't blame the company and tech for getting ahead of the problem by being conservative when adjusting a reg.
If you are interested in learning about regs and doing your own work, start off by learning how they work. Get copies of "Regulator Savvy" and "Scuba Regulator Maintenance and Repair" , between these 2 books and the regulators service manuals you will quickly learn what you need to know to service and adjust your regs to your liking.

SCUBA REGULATOR MAINTENANCE AND REPAIR by Vance Harlow

Regulator Savvy Book | Scuba Tools

 

[rsingler]

Thanks for the replies. What strikes me is that both the manufacturer (through their specs) and the tech seem averse to free flows at any time, in any situation, thus reducing even high end regulators to doggy performance.

Conclusion: I think I am going to have to learn to make my own adjustments.

@Luis H 's comments are really important here. Once you get your cracking effort where you want it, the dynamics of second stage flow become really dependent upon venturi effects, which are also user adjustable in many second stages.
A few shops can run a dynamic flow chart for you (see earlier posts in that Atomic thread above), and adjust the venturi vane or deflector so it gives you improved flow during the breath cycle, without "crossover" into freeflow. Now, Luis may comment on the problem of trying to equate high flows at 1 atm with normal flows at 5atm, but that's outside of what shops can provide. In other words, you may need to do just what Luis did with his reg: reduce your venturi assist for your deep dives.
It's fun playing with it all.

 

[rsingler]

Lol, sorry @herman . We were replying at the same time.

What you said.

 

[Ojai Diver]

Thanks. Is there a specific shop manual for my regs, including specs? Can I get parts? Thanks again.

 

[rsingler]

Not for the Mark 11 - but it's basically a Mk 17 with no seal. I have a mfr's cheat sheet and the Mk 17 manual.
Yes on the A700. PM me.

But realize that you are taking on a not insignificant risk if you tinker without some training. These devices are dead simple, but incredibly precise.