second with adjustment knob, understanding the mechanism.

[bradlw]

Help me to understand the mechanism in these second's that have an adjustment knob.

I've never owned or serviced one so I'm not familiar. I used to like my seconds to be adjusted just a little bit hard to breath. I liked them to be very resistant to free-flowing, so I didn't mind a hint of a delay when taking a breath. Not so much that they were hard to breathe of course, but enough so that they would crack open only when I was sure enough taking a breath. So now I'm trying to understand the best practices for diving and storing a reg with an adjustment.

Specifically something such as the DGX Gears regulators, Deep Six, etc...

What is the mechanism and what is considered the best approach to using and storing such a reg.?

Is it a direct adjustment knob that just simply increases or reduces the closing force of the seat, directly altering the cracking force?
or is it doing something different and complicated, such as changing some opening dynamic response, or ?

So when storing the regulator is there an advantage to back off the adjustment so that the seat has less force pressing it together?



My old US Divers reg had the venturi feature, but I honestly could never tell the functional difference no matter its position. Are these newer regs different?

 

[Mobulai]

I think there’s a lot of “preference” based methods to approaching the tuning/storage question.
The key aspects:

• The adjustment knob influences the spring tension, either it having a micro adjustment option or not
• The micro adjustment allows for fine tuning to accommodate variance and/or degradations in the spring stiffness
• Almost all regs have the knob only influence the spring, and thus CE — there are rare cases where the Venturi function is augmented in the same knob (Aqualung), it’s non practical and goes against the intuition of functional separation

Choices/Preferences:

• Some like their regs to be as lightly tuned as possible
• Some might even want to combine that with the “under tuning” approach where the reg is ever so slightly freeflowing whenthe knob is at max open (lightest spring compression) so that when they store it, it will be in an “anti set” mode; and when they diving it they just tighten the knob lightly
• Some like to tune their regs stiff(er) for freeflow protection (think of drop stages/octos/coldwater..)
• Edit2add: some/lots like to tune it right at the „standard“, the case geometry fault, ~1.2imH20 in most cases, but not lighter than that (you can figure this number out by measuring the distance between the highest point of inhalation diaphragm ie at lever contact point and the high point of exhaust diaphragm, ie point closest to mouthpiece) — attached below

The Venturi:
There are different styles, in barreled regs some have a sleeve that turns around the barrel, some have a vane close to the mouthpiece
I haven’t owned nor worked the inside of either a DGX nor a D6; most of my experience is g250 (and variants) based

The g250 has such a powerful Venturi effect that when tuning on land, a 40% Venturi is stable, and just changing that to 70% (no actual numbers on the teg to indicate it, by feel) and giving it a slight depression on the lever will result in the most violent freeflows; imagine what happens under hydrostatic loading

This can be a bug or a feature; when breathing off the reg, that extra venturi assist is so helpful in reducing the WoB and provides a very light breathing/inhalation feel

If it’s not being breathed, you wanna detune that so it doesn’t freeflow (from +ve to neutral to -ve Venturi characteristics; there’s a lot to be said/geeked out on here, but for the sake of simplicity having the turbulence work with or against you — rarely laminar flow is part of the equation)

@rsingler has made a wonderful 2 part seminar about this on YouTube (and also threads here) titled reg geeks #2 & #3

He even dives into the analysis and plots of response curves of various end balanced 2nd stages. Highly recommend reviewing that for the in-depth knowledge (that’s where I’m sourcing this info from)

 

[Nemrod]

A balanced and adjustable type regulator, of which the G250 is the best example and probably the best of the breed (currently G260), uses a knob to adjust spring tension to increase and decrease seat pressure. There is also a Venturi control vane for dive and pre dive to limit free flow:




Current G260:



The seat spring tension can be adjusted such that all force or nearly all force is removed and thus a seat can last much longer as it takes longer to indent. Some regulators, like the G260 also incorporate a seat saver function by pressing the purge button and rotating it to a lock position (no, it cannot lock in use).

I do not agree with adjusting regulators to be stiff or hard to breath to prevent free flow. A regulator that requires that sort of adjustment is one I would get rid of fast. Here form the G250 manual, read the last few bullets for the VIVA check:



Some, including me, for personal regulators, may adjust the knob such that when fully out the regulator will dribble a little air---bleep---bleep---bleeep-- or hear a barely audible hiss. Then turning the adjustment knob in one turn or less it will stop. This affords the lowest cracking effort and acts as a seat saver.

There are and have been non-balanced adjustable regulators such as the early Scubapro 109 and the current Kirby Morgan Superflow:

 

[Mobulai]

Some, including me, for personal regulators, may adjust the knob such that when fully out the regulator will dribble a little air---bleep---bleep---bleeep-- or hear a barely audible hiss. Then turning the adjustment knob in one turn or less it will stop. This affords the lowest cracking effort and acts as a seat saver.

I was thinking of you actually when I wrote this

Some might even want to combine that with the “under tuning” approach where the reg is ever so slightly freeflowing whenthe knob is at max open (lightest spring compression) so that when they store it, it will be in an “anti set” mode; and when they diving it they just tighten the knob lightly

BTW when I said some like it stiff, I only meant in a relative sense, like ~1.4inH2O instead 0.8-1.2; ala octo
I don’t like stiff regs; if it wasn’t for the constant switching of 2nds on sidemount I would have tuned my regs lower than 1in — but I’m trading off for stability

 

[bradlw]

Thanks!
So based on those cross-section illustrations I see that the adjustment knob is a very straightforward screw pushing into the seat's spring. Exactly what I thought it would be...

The only time I've ever dove such a regulator was a rental a couple weeks ago on day 1 of the trip, it was doing what @Nemrod described here as a dribble. It was just a constant blurp...blurp.... blurp....
The adjustment knob made no difference. I tried...but had no idea what the knob was doing... well actually I did ...but i was guessing at that point.
The instructor leading the trip was also a service tech and had his tools. After the dives he adjusted the seat to make it stop. It worked great for day two...but since the knob didn't make it better or worse, I'm thinking there was a bigger issue with that reg.

I do like the idea of a stiffer reg, but I'bvea slo always wanted this "seat-saver" idea. The ability to have it both ways seems like it could be a good thing!

 

[Mobulai]

but since the knob didn't make it better or worse

I can think of 2 scenarios here:
1. (a/b) The seat has such a deep groove / the orifice is so scratched that no seal is happening and it needs immediate service
2. the knob was actually not an adjustment knob, but a „sleeve around the barrel“ type Venturi — that means it was just changing the Venturi adjustment with no impact on spring

Do you remember what reg was it? And di that knob do multiple turns or just most/1 turn?

I do like the idea of a stiffer reg, but I'bvea slo always wanted this "seat-saver" idea. The ability to have it both ways seems like it could be a good thing!

A g250v or g260 do both features beautifully
Even some older g250 s did it that had the „special HP cover, but „rarer“ to confirm)

 

[bradlw]

Do you remember what reg was it? And di that knob do multiple turns or just most/1 turn?

it was a HOG, but I don't know which model. It looks like a Classic 2.0
there was a seperate lever for a venturi (I assumed) that also did nothing
I don't recall how many turns the knob would make, but it was stiff and didn't feel right to me

 

[Mobulai]

it was a HOG, but I don't know which model. It looks like a Classic 2.0
there was a seperate lever for a venturi (I assumed) that also did nothing
I don't recall how many turns the knob would make, but it was stiff and didn't feel right to me

Ok so definitely a seat/orifice that didn’t seal well kinda situation — another reason I like to dive and service my own gear
I was asking about turns just to confirm if it wasn’t a venturi thing (they only do 90-180degrees)

 

[Nemrod]

there was a seperate lever for a venturi (I assumed) that also did nothing

Th Venturi check for that reg would be similar to that outline in the Scubapro G250 instructions. The regulator should free flow when flow is initiated by pressing the purge with the Venturi in Dive and it should stop when turned to pre-Dive. The difference in breathing feel in pre-Dive, at the surface under low demand can be subtle.

The regulator you were using is in serious need of service. The seat at the least and possibly the orifice need replaced and the regulator should then be adjusted per the OM. Edit to add, a leaking balance chamber will not stop leaking by screwing the tension knob in whereas a bad seat/orifice may, temporarily, at the expense of a very poor performing reg.

Again, you guys can set your regs up to breath stiff, go ahead, IMO that is potentially dangerous, can lead to CO2 retention, exhaustion and only increases the WOB and increases air consumption. If swimming/scootering into a stiff current the knob can be turned in to prevent free flow and/or the Venturi could be moved to pre-Dive. But y'all do as you wish. When I was a working shop guy, I set regs per the OM/MM. Some cold water reg sets will specify a higher cracking force range and even a lower IP to reduce the chance of icing.