BackMounted Doubles and mixed gas - is that technical?

[Blasto]

What this thread really confirms, I think, is the value of technical training.

An experienced, skilled, solo qualified diver, with 200-500 dives and 4 years on this board, did something very stupid, improperly mixing up rec and tec-sourced elements in his planning, turning what would've been a safe and easy dive for a freshly minted Tec40 into something that almost belongs in Near Misses.

Throughout the dive and the discussion, he believed himself to have been very safe, and his hodge-podge of approaches to have kept him safer, rather than put him one problem away from casualty. He had everything he needed to do the dive with minimal risk - the only missing factor was the knowledge of the whys and hows of technical dive planning.

 

[beester]

Stupid question, perhaps: Are there situations where you would have an unbreathable gas on your necklace as part of a properly planned dive? (The described dive is definitely not one!) But how is this handled, for instance, when you dive hypoxic mixes and are on your travel gas during decent? What's on your necklace reg then, and how do you handle an air-share situation then? I'm not trimix trained, but curious. Hope to take normoxic soon...

Good question kafkaland (and nice name). Well from my perspective the impact of hypoxic mixes is overestimated and the use of it not really a big issue with some proper procedures. I'm talking OC here... with rebreathers hypoxia is a major concern.

You are very right that with deeper dives you can have a hypoxic mix in your backgas, rendering your main and backup reg (under your chin) unusuable in theory.

You have to remember tho that even with lean mixes the problem area is very limited, surface up to 30 ft depending on gas (12/65 is breathable from 20ft and even a very lean 10/70 is breathable from 26ft.) and it's the shallow part of the dive.

Procedures are:
- Never breath the hypoxic mix on the surface (duh)... we normally only put a reg in our mouth the moment we are descending.
- Do a proper gas switch to a travel mix on the surface before you descend (to make sure you are on a travel mix and not by accident on O² or another hyperoxic mix... there have been deaths of people descending on a "travel mix" which turned out to be a deco gas).
OR
- Just descend fast to a proper depth. A 70m gas (15/55) is breathable from less than 10 ft , a 90m gas (12/65) is breathable from 16 ft... I'm sure you can descend to 10-16 ft in 1-2 breaths... nothing will happen if you breath a hypoxic mix for 1 or 2 breaths. Just make sure you are not staying long in the hypoxic zone. Obviously this is for open water diving... in caves the situation can be totally different because you can have a long 10 ft zone and you will need a travel gas.
- If you have a situation requiring to donate gas... you can donate your travel gas and take your backup reg (hypoxic) and descend sufficiently (again 10-16ft) to make it breathable (again you will not pass out if you breath a couple of breaths, it takes a bit longer than that).
- During ascends you obviously also need to be carefull (not doing O² gas breaks on backgas for example if it is hypoxic for the depthrange).

In a sense hypoxic gasses (for OC) are not really a big issue... hyperoxic gasses always are (non analysed gas, wrong switch, etc).

Cheers

PS: RyanT made a good point. Basking Ridge, we might sound like a bunch of hungry sharks circling you.. but all critique is really posted with good intent. We just want to make sure you don't harm yourself. If you want to stick to your guns by all means, but at least we tried to warn you. ;-)

 

[uncfnp]

To the posters that are focusing on the danger of a high MOD "back gas," would your opinion change if the back gas was air and he slung an 80 of 40%?

 

[rongoodman]

At least that

To the posters that are focusing on the danger of a high MOD "back gas," would your opinion change if the back gas was air and he slung an 80 of 40%?

At least that way he could breath and donate both his primary and secondary regulators. IMO, carrying a hot gas below the depth it can be breathed requires training beyond recreational diving.

 

[luscioman]

I understand you are trying to save money. I personally would not like your setup emeegencies happen and ypu could have an issue. So I have 2 possible suggestions.
1. You said you paid 30 per tank to rent and nitrox what is just a nitrox fill? Norm is $7 to $15 per tank.

2. Fill your tanks both to 40% drain down and top with air and analyze just like doing a partial fill. You can you mix blender to get an idea. This is going from 40% to 30%.


Partial Pressure Instructions:
First, dump 1579psi of gas from the target cylinder to 1421psi. Finally, top off with 1579psi of air to 3000psi.

 

[Basking Ridge Diver]

To all - I have pretty thick skin so no offense taken - Or I would not have posted...

I have been diving since 1978 - I have more dives than profile but does it matter?

This is really a posting to understand views from others... I know my view - I asked really to get opinions... So no offense taken.

 

[northernone]

To all - I have pretty thick skin so no offense taken - Or I would not have posted...

I have been diving since 1978 - I have more dives than profile but does it matter?

This is really a posting to understand views from others... I know my view - I asked really to get opinions... So no offense taken.

Glad to hear it... We get quite passionate on such topics. Feeding frenzied even.

From the various comments and perspectives posted are there a few considerations you find most relevant? With your experience you've probably met all types.


Cameron

 

[Basking Ridge Diver]

So obviously - most are in the camp of carry your back gas all the same mix - as needed sling your deco or mixed gas separately.

Reasonable process and relatively risk free... Better folks than I have died breathing the wrong mix - so from a perspective of safety that seems reasonable.
From a perspective of taking AN/DP class - it is not in the cards just yet... Hope to be in the future.

 

[KDAD]

Did you consider putting the 40% in a AL 30 or 40 instead of in one of the back mounted 80s? If I was doing the dive like you were I would have had the air in both back tanks (better yet a mix like 28 or 30) and then the travel gas in a slung tank.

 

[doctormike]

You have lots diving experience, you have some of the gear, you have great wrecks like the Stolt that are fun with more bottom time. It seems like you are a good candidate to actually do the deco training and do this the right way. Why mess around with a non-standard and potentially dangerous approaches when you can get some training that will really open up your diving?

I didn't get tech trained to do very deep wrecks, I just wanted to be able to spend more time at moderate depths.

And if you REALLY like the idea of doing the top part of the wreck on a richer mix than the bottom to stretch out those NDLs, maybe constant PO2 is for you... Why settle for just two mixes when you can have a machine that will give you whatever you want?