Question Are most people confused about unbalanced first stages?

[sunny_diver]

I've been told many times by very experienced divers that unbalanced first stages are not good at depth.

From what I understand, all first stages are "balanced" with respect to ambient pressure. At a given tank pressure, all first stages will deliver constant IP no matter the depth.

If a first stage is "balanced", it will also deliver constant IP no matter the tank pressure.

Is this correct so far?

I dove unbalanced first and second stages in the past. Sure enough, they became harder to breathe below 60 ft. I always assumed this was due to depth, but overlooked that of course tank pressure would also be lower at that point in my dive. I was shore diving and it took a bit of time to reach 60 ft from shore, which means tank pressure would have been lower.

 

[formernuke]

It's a term that is sometimes used on different ways.

I'm getting one that I called blocked but manufacturer calls unbalanced. This means it doesn't adjust to compensate for depth. Result is it will only work to a certain depth and the flow rate will not adjust, so if I breathed off it it would get harder to breath off as I go deeper.

 

[Nemrod]

I've been told many times by very experienced divers that unbalanced first stages are not good at depth.

From what I understand, all first stages are "balanced" with respect to ambient pressure. At a given tank pressure, all first stages will deliver constant IP no matter the depth.

If a first stage is "balanced", it will also deliver constant IP no matter the tank pressure.

Is this correct so far?

I dove unbalanced first and second stages in the past. Sure enough, they became harder to breathe below 60 ft. I always assumed this was due to depth, but overlooked that of course tank pressure would also be lower at that point in my dive. I was shore diving and it took a bit of time to reach 60 ft from shore, which means tank pressure would have been lower.

The IP drop you will see on an unbalanced piston first stage such as the Mark 2 will be the same at depth as you would measure on the surface. An unbalanced diaphragm increases as tank pressure falls.

This is a more complicated question than may appear. I have been of recent quite smitten with the new Scubapro Mark 2 Evo. Using this unbalanced, simple, reliable first stage with a high performance second stage like a G250 I do not find any notable difference in perceived WOB over my Scubapro Mark 17 Evo and G260s or several other regulators I have at my disposal. I have done over 100 dives on my Mark 2E and G250 set this (now last) year inlcuding the Red Sea, Bonaire, twice to Cozumel, Keys, some deep and cold dives in TRL and I have purposely run the tank low down to less than 600 psi and all was well. Would such a first stage be my primary choice for dives consistently below 100 feet or practical sport diving depths (talking non-tech), probably not. But I have had the Mark 2 Evo and G250s to near 150 feet in very cold water with no issue.

I would not want to use an unbalanced first with an unbalanced second stage as my go to and general purpose regulator. A balanced first with an unbalanced (downstream) second or an unbalanced first with a balanced second stage I would be okay with.

 

[inquis]

Yes, almost all compensate for ambient pressure. ("Almost" because ones suitable for O2 use with mCCR "deactivate" such compensation, usually called "blocked".)

A "balanced" reg will also compensate for the upstream pressure (tank pressure in a first stage, IP in a second stage). An unbalanced first and unbalanced second stage combo will have noticable variation as the tank pressure changes. An unbalanced first and balanced second will have less variation, probably still noticeable, but perhaps acceptable. (Whether that's bothersome or not is an open question.) I believe a balanced first and unbalanced second would have still less variation, but I suspect a higher price than the other mixed combo. Both balanced will, of course, be very consistent from beginning to end of the dive.

 

[formernuke]

Right but this manufacturer calls the ones we call blocked unbalanced, shrug

 

[inquis]

@formernuke - for the OP's benefit, which is the unusual manufacturer again?

 

[formernuke]

NEX, I'm confirming with them in a second. And yes it's for a mccr

 

[L13]

Blocked (unbalanced to ambient pressure) first stage regulators are specifically designed for CCR's, specifically ones that use a constant mass flow orifice (otherwise the mass flow through the orifice would change with depth).

 

[formernuke]

Blocked (unbalanced to ambient pressure) first stage regulators are specifically designed for CCR's, specifically ones that use a constant mass flow orifice (otherwise the mass flow through the orifice would change with depth).

Which could result in ppo2 going to high, very bad.

 

[Bigbella]

I've been told many times by very experienced divers that unbalanced first stages are not good at depth.

That is a bit of a myth and brand dependent.

My first fifteen years or so of diving, commercially and otherwise, were on Poseidon Cyklon 300s, with unbalanced "compensated" 2305 first stages -- and they provided more than ample breathing at depth; the best that I have ever used below 40 meters, until, perhaps, the XStream, in the early 2000s. If anything, some users found that deeper water breathability preferable to the shallows.

They did run at a higher IP than that of any other brand, 12.5 bar (181 psi) at one time; but were generally set, some years later, a bit lower, to around 11 (160 psi). The 300 IP was always set at a nearly empty tank, at about 20 bar (290 psi).

I still have a few and use them on pony and stage bottles; simple to service and quite popular on the second hand market; parts remain available -- and have found them all but bulletproof . . .