Diving Bottom Stages Pros/Cons

After coming back from a recent trip with 100 bar of 21/35 (about 1500psi in old money) and having no top-up facility at the local LDS, I thought about diving a bottom stage.

The Pros:

Obviously cost, A fill of one Ali 80 as opposed to gassing twin 12s.
Min Gas is preserved if I descend on the twinset and switch to the bottom stage at depth. Anything goes wrong with the stage then I can ascend freely.

Cons:

Extra Stage to carry
Possible (very unlikely) gas switch to the deco 50 instead, but diving in a team. Also using bottom stages.

For the record the dive is 44m and runtime less than one hour (boat rules)

Have I missed anything?

Rob McGann, the dive-ops manager of the Truk Lagoon Dive Center has a phenomenal SCR --possibly 5-7 litres/min in a 11L's twinset (double AL80's)-- this "mutant aquaman" must have a set of gills somewhere! I always needed to take an extra 11L stage to compensate for my 12-15 litres/minute "hoover vacuum-cleaner" consumption at depth 45m & deeper when he's guiding the dive.

Just the usual comment & reminder from a tech wreck penetration perspective -always drop your stage/deco cylinders near your entrance/exit point, or if on a traverse through the hull, at a location that's easy to find (i.g. On top of the Bridge, or right next to a mooring upline tied-off to the wreck).

And of course, carrying an additional stage cylinder along with two deco bottles of 50% and O2, you should be utilizing a DPV/scooter for traveling & primary locomotion.

It was my training that, if you are using a bottom stage, you start on it, because it's your most fragile gas source (not firmly attached to you, only one regulator).

I was also trained never to put bottles down on an OW or penetration dive, because you can never be sure you will get back to them.

They can't even fill your twins with air? By my back-of-an-enveloping, that would give you something like 21/18, which still gives you an EAD of 34m on the 44m dive if you consider O2 narcotic, a tad less if you don't. Not crippling, unless you're doing complex penetrations in a really filthy environment. (OK, I used V-Planner. My mental maths isn't that good.) My biggest concern about filling twins with 21/35 and then diving a stage, apart from the initial extra expense, would be leakage of that precious trimix out of the twinset while it's sat there not being breathed...

Start the dive, as TS&M says, on the stage and (a) you're getting rid of your most vulnerable bottom gas source first and (b) there's no risk of a bad switch. Not, obviously, that there's a risk of a bad switch if you're using properly marked cylinders and solid procedures... You don't need to preserve minimum anything in your stage if you've also got a twinset on your back. Although the constantly having to service your stage reg and clean the oxide out of the tank would probably get a bit wearing if you suck the thing dry every dive.

I know conventional wisdom says you should be scootering if you have three slung stages, but I've swum twinned 80s and three slung 80s without it significantly increasing my SCR or giving me CO2 problems. Course, I don't as a rule swim very fast...

Kevrumbo:

Just the usual comment & reminder from a tech wreck penetration perspective -always drop your stage/deco cylinders near your entrance/exit point, or if on a traverse through the hull, at a location that's easy to find (i.g. On top of the Bridge, or right next to a mooring upline tied-off to the wreck).

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Wasn't that one of the contributing factors in the deaths of Chris Rouse Sr. and Jr.?

rongoodman:

Wasn't that one of the contributing factors in the deaths of Chris Rouse Sr. and Jr.?

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Extreme deep-air narcosis at over 200'/60m; being trapped/entangled inside the wreck; inability after egress to find their deco bottles while sharing dwindling backgas supply. A tragic series of cascading mishaps that tactically overloaded them, until the only option left was an emergency ascent to the surface without performing any deco profile whatsoever. . .

If you have to, take out your wetnotes and draw a sketch of the wreck and the location of where you dropped your stage/deco bottles complete with compass direction & ships reference points/description (i.g. "NE heading from Stern, on top starboard side bridge next to ship's external engine telegraph & helm").

GrimSleeper:

They can't even fill your twins with air?

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Yes, but I want a standard gas

GrimSleeper:

My biggest concern about filling twins with 21/35 and then diving a stage, apart from the initial extra expense, would be leakage of that precious trimix out of the twinset while it's sat there not being breathed...

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That one is a bit of a myth.

Not filling the twins though. Just filling a stage, twins would stay with only 100 bar for the dive

GrimSleeper:

I know conventional wisdom says you should be scootering if you have three slung stages, but I've swum twinned 80s and three slung 80s without it significantly increasing my SCR or giving me CO2 problems. Course, I don't as a rule swim very fast...

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Not sure why everyone keeps thinking I have three stages. It would just be bottom stage and 50%

I would dive that stage and not look back. We are all grown ups and know the rules and risks. Maintain your reserves, and manage your resources. No biggie. If you are diving with a team or buddy it would add another layer of safety, not to mention if you are going to deco on a upline there would be a strong case for making a dash to the line in an extreme case to get resources from someone else on the line.
Eric

Kevrumbo:

always drop your stage/deco cylinders near your entrance/exit point

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rongoodman:

Wasn't that one of the contributing factors in the deaths of Chris Rouse Sr. and Jr.?

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Kevrumbo:

inability after egress to find their deco bottles

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That's what Ron is taking about

Ste Wart:

After coming back from a recent trip with 100 bar of 21/35 (about 1500psi in old money) and having no top-up facility at the local LDS, I thought about diving a bottom stage.

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I used to do this all the time when I dove OC even on the deep dives where we were carrying 120', 70' and O2 deco cylinders. For the deeper stuff it was so we had enough bottom gas to meet rock-bottom requirements. For the shallower dives it was for gas management and so that we only had to fill the stage at the end. For those dives we usually never even touched our back gas.