Deco dive with divers on different back gas

[SelfDiver]

The downside would be that one of the divers brought the wrong gas, so the other diver will have a very short dive. The ait diver would be cutting down the “activity” length by half, I wouldn’t dive with someone like that again unless we were good friends.

With a square profile, at 30 meters using 32% nitrox backgas and a 50% deco stage you can nicely max out your bottom time to about 45-50 minutes. Diver on air is a muppet who will need to cut the dive short, probably at 30ish minutes. This assumes taking into account a lost deco gas scenario which will be the main limit of the air diver (yeah teammate might be around with their 50% but try relaying on that in 2 meter viz).

What I would do is wave a good bye to the air diver and stay longer.

I am having trouble following.
Assuming both divers have the same SAC rate and tank size. They will both leave the bottom at the same time.
The air diver will have a longer decompression debt and longer stops on 50% starting at 20meters
As a team, they have two 50% tanks so a lost deco is highly unlikely. Even losing some bottom gas is not a big pain from 30meters because they are 1 minute away from deco gas.
Why would you break the team?

 

[girlwithbigtanks]

So we go from a pretty simple, barely-into-deco dive to full on hypoxic with all sorts of other issues. That is just an absurdity.

The "slippery slope" argument is a fallacy, and you are showing why. The fact that something makes sense in one situation does not mean it will make sense in another. That is why we have to use our minds and not follow dogma that might make sense in one case and not in another. Once you have become part of the dogma, though, it can be hard to see that. It took me a long time to clear that junk from my brain.

Where is one’s line in the sand for normalization of deviance? If the dive was planned and diver b shows up with air, maybe change the plan instead of working around the d’oh! I could only get air? Discuss with your buddy, hey my LDS was out of O2. I also don’t see gasses for a full on hypoxic dives suggested.

 

[Blackcrusader]

Also air is not used, especially for a 100ft deco dive.

Most ridiculous comment of the day. I use air for deco dives at 100 feet or deeper.

 

[FF / EMT]

Most rediculous comment of the day. I use air for deco dives at 100 feet or deeper.

He’s referring to DIR diving

 

[Celt]

People have too much gear, this should be moved to the basic forum. A 15ltr tank of air will give you 35 minutes at 100 feet with a 10 minute stop at 10 feet

 

[LFMarm]

Why do they have mismatched GFs? Does having different gases require you to use different GFs?

No, GFs are a personal preference based on risk appetite. Research over the past 10 years has shown that for a given runtime, some GFs are much less conservative than others. Having mismatched gases, may be a reason to have different GFs though – e.g., the diver on air may choose higher GFs to match the run time of the diver on EAN32 – this will increase the risk of DCS for the diver on air (all other physiological factors being equal).

Most recent research from NEDU led by Doolette shows that dives with higher PrT (i.e., Pressure, square root of runtime) require lower GFs in order to keep the risk of DCS fixed. Over the past 12 months I have started monitoring and planning for PrT and adjusting GFs accordingly. In my experience, the most dangerous dives are those with long duration and medium average depth – being not so deep can enable very long dive times with reasonable amount of bailout. Those with very shallow average depth are not problematic neither are the very deep ones because your run time is limited by bailout.

 

[Wibble]

The downside would be that one of the divers brought the wrong gas, so the other diver will have a very short dive. The ait diver would be cutting down the “activity” length by half, I wouldn’t dive with someone like that again unless we were good friends.

With a square profile, at 30 meters using 32% nitrox backgas and a 50% deco stage you can nicely max out your bottom time to about 45-50 minutes. Diver on air is a muppet who will need to cut the dive short, probably at 30ish minutes. This assumes taking into account a lost deco gas scenario which will be the main limit of the air diver (yeah teammate might be around with their 50% but try relaying on that in 2 meter viz).

What I would do is wave a good bye to the air diver and stay longer.

Maybe it’s a UK diver thing, virtually all boats I’ve been on are a bunch of independent divers diving the same wreck/wall/scenic location and all surface when they’re ready.

Probably something to do with conditions, most experienced divers are very used to surfacing alone and develop the skills to manage that without fuss.

The exceptions would be for novices where buddying up is the norm. However, by definition a decompression dive is not a novice dive and thus subject to planning and gas resilience skills.

So wave goodbye to the diver with the short bottom time and the longer TTS as there’s more wreck to explore.

 

[Wibble]

I am having trouble following.
Assuming both divers have the same SAC rate and tank size. They will both leave the bottom at the same time.
The air diver will have a longer decompression debt and longer stops on 50% starting at 20meters
As a team, they have two 50% tanks so a lost deco is highly unlikely. Even losing some bottom gas is not a big pain from 30meters because they are 1 minute away from deco gas.
Why would you break the team?

Why would you dive as a “team” and not independent divers?

 

[jborg]

Wow this thread became a shitshow even faster than I'd expected

 

[DiveLikeAMuppet]

I am having trouble following.
Assuming both divers have the same SAC rate and tank size. They will both leave the bottom at the same time.

As a team, they have two 50% tanks so a lost deco is highly unlikely. Even losing some bottom gas is not a big pain from 30meters because they are 1 minute away from deco gas.
Why would you break the team?

Because I don't dive in unlimited visibility with no currents. In the UK, good visibility might be 3-5 meters (10 - 15 feet) and rapidly drop to 1-2 meters (3 - 6 feet). It's tidal and there are fierce currents. It's pretty easy to get separated by accident. You always try to carry enough gas for yourself. Teammate is a welcomed bonus.

So when carrying a single deco cylinder, you need to account for a failure and keep enough back gas to clear your deco - or at least that's how I've been trained by people who dive in similar conditions. That's fine with 32% but gets ridiculous pretty quickly on air. So the air diver would be cutting the dive (and all money and time and effort spent to go diving) quite short.

No, GFs are a personal preference based on risk appetite. Research over the past 10 years has shown that for a given runtime, some GFs are much less conservative than others.

It's a 30 meter dive with a single deco gas with maybe 20 to 25 minutes of deco? Honestly if you switch gas at 21m, you will clear whatever deep stops you might incur on the way up no matter the GFLow and GFHigh will give you 3-5 minutes difference on the last stop... It's a very academic discussion unless one of the divers dives hardcore Buhlmann 99/99 or something funny like 10/95.

Maybe it’s a UK diver thing, virtually all boats I’ve been on are a bunch of independent divers diving the same wreck/wall/scenic location and all surface when they’re ready.

Probably something to do with conditions, most experienced divers are very used to surfacing alone and develop the skills to manage that without fuss.

I prefer diving with a teammate if the conditions are appropriate (good visibility, teammate with similar training / approach / mindset). But the conditions don't always make that possible or nobody might be available to go diving - Billy No-mates.