Nitrox mixes that you use - 'best mix' or 'what is available'

[waterpirate]

I think people who carry their mixing machines and software on their backs are cheating! lol

Having access to my own continuous mixer did change my mindset in regard to " perfect mix " for a brief time while I was still ndl diving predominantly. It became a PITA as RJP outlined. When I became comfortable with an obligation and helium, it got to be more about speed and conveniance when it came to mixing.

In the mid atlantic, most diving begins at 75' due to needing to get away from inshore current/ vis problems so seeing anything banked higher than 32% is only for deco or blending, not diving. A divers progression through nitrox begins with extending the ndl to the max as their sac improves. This pushes mixes hotter so they can stay ndl. If a diver proggresses into tech 1 or simmilar programs, the gas back lashes the other way, into safety at depth via a low po2, but accepting a obligation as a way to add safety in the shallow stops.

Also up here due to mission driven diving, be it junkers or hunters we were mentored early about the dangers of pushing the table limits on a ndl dive when a heavy work load, or sac, or stress may be involved. It was another way to add a layer of margin.

When I started going down south I was shocked at the use of hot mixes by everyone when diving easy on reefs ect. By the same token now when I dive down south, I am viewed as diving a hotter mix because their is no mission for me, it is just a sight seeing tour with no rush.

Eric

 

[boulderjohn]

2. The idea of a 28% mix makes a lot of sense, and provides a PO2 of ~1.4 at the recreational limit of 130ft. I am curious – do any shops actually bank a 28% mix? In my case, the easiest approach is to dilute our banked 30% with air. But, it would be interesting to know if there are ops who take it a step further and bank it.

It isn't exactly banked, but Brian Kakuk in the Bahamas uses two standard mixes, 28% and 32%, for his guided dives. It depends upon which section of which cave is being explored on a given dive. There are sections that are consistently above the 32% MOD and sections that drop below it. He has a hard enough time getting oxygen on the island, and it is really hard to get helium, so unless divers want to make extraordinary efforts and pay a significant extra fee, they will have to accept the 28%.

 

[fdog]

First, THANK YOU to all who have commented, for a set of consistently thoughtful and informative posts! Second, I have a few follow-up comments and questions, for clarification.
<snip>

You are welcome. BTW, what on earth is happening with ScubaBoard?!? Civil discourse and all. Must be New Year's resolutions.

3. Several people appear to use a &#8216;fudge factor&#8217; and lighten the mixture a bit below a P02 of 1.4 if conditions encourage it (high workload, cold, etc.). Or, they blend for a depth a bit greater than the max planned depth, which accomplishes a similar result. Is there a particular reason for picking a specific PO2 of, say 1.3 (or 1.2, or any other value), other than it is just a bit more conservative in terms of the O2 content? I am not disagreeing with the practice at all, just curious about it.

For me, and the folks I'm gifted to be diving with, we are very risk-adverse. Downright conservative, actually. In this case, total risk comes from two sources:

1. Likelihood of a tox episode
2. Consequences of a tox episode

As a gross simplification, here's some ppO2 levels and how likely a tox episode is:

• 3.0 Very likely within 30 minutes even at rest
• 2.0 Likely within 30 minutes with a potentiating activity, such as very cold, workload, dark, or claustrophobia
• 1.6 Relatively unlikely when at rest
• 1.4 Unlikely in a recreational use
• 1.3 Very unlikely
• 1.2 None reported

Of the last two, I have only heard of 3 tox episodes at 1.3 over the last decade. I have yet to hear of a tox episode at 1.2.



Next, lets look at consequences.

• A tox episode within recreational diving limits, where not under a deco obligation, and the surface is relatibely colse (less than 100' or so): is survivable with a well trained buddy who manages the episode at the bottom, and ascends slowly with the victim. The ascent portion is not prolonged excessively because of proximity.
• A tox episode when at the bottom, under a deco obligation, where it's dark and with a hint of narc and my buddy may not be paying 100% attention: very bad. We all train to manage this, but let's face it, this would be a poor-outcome scenario.

Soooo...when we combine the risk and consequences...

Deco is easy. We dive at a bottom gas with a ppO2 of 1.2. Because the consequences are so bad, we would prefer to bring the chances of a tox episode to zero.

Recreational is a different story. Because the chances of a successful rescue from a tox episode are better, I'm willing to accept some risk. For me that level is 1.4 on benign dives with easy access to the surface, and a bit less (1.3) with some potentiating factors.



Not mentioned in all this is CO2 buildup, yet another reason to back off on high-workload dives.


All the best, James

 

[RJP]

Yeah, that's why I'd go with the 28% and the o-2, makes life easier.

If doing an unnecessary deco hang is your idea of "makes life easier" then more power to ya!

[thumbsup]

 

[Kevrumbo]

?...

2. The idea of a 28% mix makes a lot of sense, and provides a PO2 of ~1.4 at the recreational limit of 130ft. I am curious – do any shops actually bank a 28% mix? In my case, the easiest approach is to dilute our banked 30% with air. But, it would be interesting to know if there are ops who take it a step further and bank it.

....

The Truk Odyssey has a "customary" mix of Eanx 30 for its standard week long, 3 to 4 dives max per day itinerary diving the more popular but still recreationally deep Truk Lagoon Wrecks, and can blend lighter nitrox as well as 50% & 100% for deco gases. As of October last year, they've also just started banking 21/35 and 10/70 Trimix as well . . . (Helium blend supplied courtesy of the Truk Lagoon Dive Center at the Truk Stop Hotel).

 

[billgraham]

If doing an unnecessary deco hang is your idea of "makes life easier" then more power to ya!

[thumbsup]

Well I pretty much always do some extra, "unneccesary" deco. If you run out the tables, which I don't feel like doing right now, you'll see that the difference between 28% & o-2, and 30% & o2 for that 110 foot dive for 60 minutes is pretty much negligable. I'd rather spend an extra five minutes under the boat looking at fish than jackassing around with the mix, but that's just me.

 

[Buoyant1]

My local quarry has depths to 120', so not knowing our ultimate plan prior to arriving, which can be either a conservative 80-100', go deeper, OR stay above 80', I usually opt for 29% which would conservatively cover me in all situations. If we opt to go to another quarry which is about 2 hours away, and I need a fill, I would usually opt for 32% since the depths there don't reach anything close.

My source for gas doesn't bank, and only does custom mixes.

 

[eelnoraa]

I use 32% 99% of the time and I don't go below 100ft. For that 1% in the past, I have used 40%. It was when I was diving a well known site with a very specific dive plan. So my max depth is shallower than 75ft and longer dive time.

 

[AfterDark]

The one time I used nitrox it was a best mix 28% for a 130' dive. Diving local I can use the best mix as the LDS will do any mix you're qualified to use. Diving away from home I'd either use 21% or what the LDS had if approprite.