second with adjustment knob, understanding the mechanism.

[Nemrod]

Compared to my unbalanced Mares Proton, it breathed easier. Does the VIVA vane/aspiration make a meaningful difference?

Yes. It does.

Again, if the regulator is properly adjusted, with the VIVA in Max or Dive the regulator will free flow when the purge is depressed and should stop when the VIVA is turned to Min or pre-dive. That is the difference you will see.

I think you may not really understand the VIVA. It is a Venturi control. The denser the air the stronger the Venturi this it becomes more noticeable at depth. The VIVA is not a breathing adjustment. It should be set to Min or pre-dive for an octopus or secondary, for the primary it should be set to Max or Dive position.

The adjustment knob should not need but a turn or two in use from fully out. There is no reason during a dive to turn the knob over it's entire range or even turn it at all from fully out. Properly adjusted, the knob should be fully out in use. For those of us who adjust the cracking effort to lower than specified range, it may be needed to turn the knob in a turn. If swimming/scootering into a current, perhaps another turn. Neither the tension knob or the VIVA are controls that should need constant fiddlling with during a dive.

Note the use of the word "must" in the instructions below regarding the VIVA from the Scubapro regulator owner information booklet. The VIVA in Min position does not eliminate Venturi, it does reduce the effect:

 

[bradlw]

Properly adjusted, the knob should be fully out in use.

i guess I still can't wrap my head around why this knob is even a thing then.

Well I guess in a way, I can. By that measure, the only purpose for the knob is to adjust in a little more seat pressure to compensate for a worn or damaged seat... Is that how you see it? I mean, once a groove gets worn in and it starts to leak, then you could tighten it a bit to get you by until you can service the reg.

Before this comment, I had been thinking that the knob gives an advantage as a "seat-saver" adjustment. So in that application, the proper adjustment would be something in the middle. Loosen it all the way for storage to get the pressure off the seat, but normal operation would be halfway in. Then you'd still have some adjustments to tighten in the event of a seat wear failure mid-trip.

I guess I'm a simplton. All of my dives to date, except for the rental two weeks ago, were on a reg with no field adjustment at all...well, except a pointless little venturi vane that did nothing. Shallow dives, deep dives, high current dives, night dives, technical deco dives...all no problem without a knob at all.

 

[Nemrod]

i guess I still can't wrap my head around why this knob is even a thing then.

Well I guess in a way, I can. By that measure, the only purpose for the knob is to adjust in a little more seat pressure to compensate for a worn or damaged seat... Is that how you see it? I mean, once a groove gets worn in and it starts to leak, then you could tighten it a bit to get you by until you can service the reg.

Before this comment, I had been thinking that the knob gives an advantage as a "seat-saver" adjustment. So in that application, the proper adjustment would be something in the middle. Loosen it all the way for storage to get the pressure off the seat, but normal operation would be halfway in. Then you'd still have some adjustments to tighten in the event of a seat wear failure mid-trip.

I guess I'm a simplton. All of my dives to date, except for the rental two weeks ago, were on a reg with no field adjustment at all...well, except a pointless little venturi vane that did nothing. Shallow dives, deep dives, high current dives, night dives, technical deco dives...all no problem without a knob at all.

I can see you are a bit confused .

It is possible to adjust the knob such that it acts as a seat saver as I have mentioned previously in this thread however per the SMs this is not the purpose of the knob nor is this accounted for in the service manual.

The knob can be used during a dive to increase the preload to reduce free flowing into current or when working head down or as you mentioned to accommodate slight wearing in of the (new) seat or a worn out, indented or cut seat or damaged orifice or even IP outside the specified range on the high side.

When I was servicing (customer) regulators, I always set the adjustable types to the higher end of the specified range, for example, if stated to be 1.0 to 1.6 I would set to something like 1.4 to 1.5 knowing that the seat would break in during use and the cracking effort would reduce but still be in acceptable range per the SM (and importantly not leak afterwards). And, also importantly, not be a customer return reason/irritant after service for the regulator slightly leaking and thus needing the knob turned in to stop the leaking.

I already attached the Scubapro generic OM, you should read it, taken directly from the manual below attached. This explains the intended purpose of the knob.

 

[Nemrod]

You know, for the cave persons amongst us (I can include myself) and those who just cannot fathom doodads, the diving world really could use a modern equivalent to the wonderful AL 1085, unbalanced, no user adjustments, fixed Venturi, tough as nails:



What does 1085 mean? That was the part number in the USD/AL catalog. Just as the Scubapro 108, 109 etc. were also the catalog numbers for those models.

Well, there is one, but it does have a tension adjustment but nothing else and it is not balanced and you might see some similarity to the 1085 from which most think it evolved:

 

[Mobulai]

Simply put, the Venturi is the label of a phenomenon that happens in the aerodynamics/pneumatics/hydraulics world

Well actually 2 closely knit phenomena:
- Venturi effect: when a change in the (pipe) geometry occurs (mainly the effective diameter), for the same flowrate, a change in pressure and speed will occur like so

Now this has a very famous application in the aerospace field (but out of scope here), and even in plumbing, venturi meters, that capitalizes on such a controlled constiction to measure airspeed (or flowrate in plumbing examples)

- Reynolds Nr. (a unitless measure):
this is where things get a bit.. turbulant (lol); this is number that represents the extent of turbulance in a stream of fluid (gases included); it haa to do with pressures, pipe diameters, flow rates, and viscousity (among other lesser variables)

Lets consider uniform pipes:

• at a low Re.Nr., we get laminar flow is smooth, we almost never encounter it in scuba
• at a "moderate" Re.Nr., we get some small whorls that might disrupt the flow a bit; but in an “unwalled” setting like an aeroplane wind it can throttle you (or provide the lift needed)
• at high Re.Nr., very turbulant flow that can dramatically resist the flow

Now add a drastic change in geometry, like a hose turning into 2nd stage barrel then the 2nd stage body:

• the volume dramatically increased --> the pressure drops drastically
• this will create a "rush" of gas, the "local" flow rate increases
• then the air diverges from the 2nds stage case (big) to the mouthpiece (small) —> pressure up again; this acts like a throttle
• there is a constant fluctuation in both the Re.Nr. and also the local conditions (multiple venturi constrictions)
• there is a lot of thought out / coincidential local venturi occurances that capitalize on the turbulance happening at the right spot (behind the flow) that effectively reduce flow resistance (like a tail wind, or even closer : as if you hit a boost pad in MarioKart)

So far I am considering only a non adjustable knob nor venturi knob, the idea of venturi assist has been there since olden double hose regs
this contributes to lowering the WoB of the 2nd stage already, nothing to do with the orifice or the spring
(edit: note in the 1085 cutaway that Nemrod posted above The various parts this is indicated, if I’m not mistaken there might be even a tech adjustment Venturi port right behind the nut in the middle — at least SP 108 counterpart has it)

Now add a vane that you can control, that allows you to tune the venturi effect to either enhance it further, or take it down a notch based on the need; that becomes an effective tool to manage freeflows or further assist when gas density is tremendous.
(this is independant of the reg being balanced or WoB adjustable; I have downstream 2nd stages that have venturi knobs; there is also the whole thing about Mares VAD)

there are even some cases (high flowrates, high density) that the venturi effect gets too much that the reg would shake and crossover into unacceptable levels and would be back to working against you -- and someone very clever (@rsingler ) discoverd that all you need to do in such cases is take the venturi knbo just a bit down (from max VIVA to like 60-70%; the reg in question was the c350 I believe; this is dscussed in reg geeks 3 -- again highly reccomend you watch this; it's easier to get and visualize.. well, visually)

I hope this along with @Nemrod 's input bridges the gap

 

[bradlw]

I received my new DGX D6 reg package, and jumped into the pool to check it out!

It's set up pretty much as you've described @Nemrod.
& I don't like it! When I'm on a dive the last thing I want to be doing it putzing around with free-flowing regulators and wasting gas...even if it is only a negligible amount of lost gas

with the adjustment screw all the way out and the venturi in "dive" mode the thing free-flows like a tornado if you look at it sideways.... pretty much even when the mouthpiece is pointed down

with the venturi in "not-dive" at least it mostly stops when the mouthpiece is pointed down

with the adjustment screw all the way tight it mostly doesn't free flow, but it wants to bubble a bit

I get it that with the screw fully loose, the WOB is zero...good. Especially if you're diving 250ft down in challenging current where the gas is dense and the diver's effort is high...

but for recreational depths, this just doesn't seem correct to me!

I think that before my dive, I'm going to pull out my tools and gauges, verify where they have set the IP, and 'detune' these seconds just a hair. Maybe just to the point that with the adjustment all the way tight, they won't blast bubbles when i do a giant-stride

 

[lexvil]

Return the new regs, here is the perfect set up for you and it’s very cheap

G

For Sale Post in thread 'ScubaPro R295/MK2 (yoke), and Oceanic Alpha 8'

Sunday at 9:31 AM

Price drop. $125 including shipping to lower 48

• GregT943

 

[Nemrod]

I received my new DGX D6 reg package, and jumped into the pool to check it out!

It's set up pretty much as you've described @Nemrod.
& I don't like it! When I'm on a dive the last thing I want to be doing it putzing around with free-flowing regulators and wasting gas...even if it is only a negligible amount of lost gas

with the adjustment screw all the way out and the venturi in "dive" mode the thing free-flows like a tornado if you look at it sideways.... pretty much even when the mouthpiece is pointed down

with the venturi in "not-dive" at least it mostly stops when the mouthpiece is pointed down

with the adjustment screw all the way tight it mostly doesn't free flow, but it wants to bubble a bit

I get it that with the screw fully loose, the WOB is zero...good. Especially if you're diving 250ft down in challenging current where the gas is dense and the diver's effort is high...

but for recreational depths, this just doesn't seem correct to me!

I think that before my dive, I'm going to pull out my tools and gauges, verify where they have set the IP, and 'detune' these seconds just a hair. Maybe just to the point that with the adjustment all the way tight, they won't blast bubbles when i do a giant-stride

Whoa, whoa! I never said nor endorsed tuning a regulator to behave as described in post #16. Maybe Dive Rite regs are just not any good or difficult to tune and set up. I am a Scubapro guy, what a Dive Rite might do who knows. It sounds to me like I would get rid of something like that and get me a good G250. Those sound to be quite squirrelly.

 

[bradlw]

Whoa, whoa! I never said nor endorsed tuning a regulator to behave as described in post #16. Maybe Dive Rite regs are just not any good or difficult to tune and set up. I am a Scubapro guy, what a Dive Rite might do who knows. It sounds to me like I would get rid of something like that and get me a good G250. Those sound to be quite squirrelly.

I was probably misleading with my words
English is hard.
I'm often reminded of Garrison Keillor's A Prairie Home Companion.... to paraphrase, "There's never an English major around when I need one."

This new regulator is behaving pretty much exactly as you described in post #11.
I would have thought that it was mal-adjusted...but because of this thread (and reading a couple other old conversations here too) I'm starting to understand that this is at least kinda-sorta per OEM specification
And I can see that in a theoretical sense it's beautiful.
It's just in the perspective of MY common sense, which is no doubt muddied up with all sorts of bad habits and incorrect assumptions, that it seems wrong

It's just as described in post #11...so on the bench it's probably spot-on to the spec. As I see it, the only problem is that dynamically in the water it's too close to the verge of freeflow... dynamic pressure on the diaphragm, inverting the reg with an open mouthpiece "up" and jiggling it gently, etc...

I understand that it's not meant to be a seat saver, but I rekon that as they sit now nice and cozy and dry in my spare room, the knob is screwed all the way out.... and there's probably just enough seat pressure so that a spider or mite can't crawl past the seat, but the seat has nearly no pressure digging into it to form a groove