Loose rules in Fort Lauderdale, FL

[MXGratefulDiver]

Wow, what a read. Apparently there are "loose rules" in way more places than Florida ... and from some of the responses it appears that while some folks may have gone through a nitrox class, they really didn't understand what was being taught.

As with all things, nitrox use involves trade-offs ... one must consider not just PPO2, MOD, and oxygen exposure limits, but also conditions. Those tables in the book are a good starting point, but things like temperature, current ... pretty much any condition that's going to increase stress or exertion ... should be factored into the decision about what mix is appropriate. Dives that involve complex navigation, overhead, or any condition where a momentary lapse in awareness could be potentially fatal should also factor into decisions regarding appropriate choice of mix ... and when it's appropriate to consider the use of helium.

It isn't as simple as choosing a PPO2 level and assuming that everything below that is somehow "safe" ... or for that matter, everything above it isn't.

It's good to have rules, and training is intended to provide them. But without understanding why those rules exist, and applying some thought to the trade-offs they impose, you really are just blindly following a formula and assuming that it will keep you safe. Without understanding why you're applying that formula, it's not always a valid assumption ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

Although difficult if the shop has a highly variable banked mix, you can make your own blends using a tank that has air breathed or purged down to the appropriate PSI and then topped off with nitrox. Can also be done the reverse with nitrox topped off with air. I have done this a few times to make 28% when I was short on time or didn't like paying for a full nitrox fill when I didn't need it.

I did it just last week. Knowing that on Saturday I was planning a dive to 120 fsw I breathed a set of HP100's down to about 2300 psi and asked my dive shop to just top them with air. Ended up with roughly EAN30, which is about what I had wanted ...

... Bob (Grateful Diver)

 

[stuartv]

Although difficult if the shop has a highly variable banked mix, you can make your own blends using a tank that has air breathed or purged down to the appropriate PSI and then topped off with nitrox. Can also be done the reverse with nitrox topped off with air. I have done this a few times to make 28% when I was short on time or didn't like paying for a full nitrox fill when I didn't need it.

I do that all the time. Really, if Best Mix is, say 30%, using 26 or 28 just doesn't make that much difference. So, I will often start a weekend with 30% and then just get topped up with air for the next day. Maybe top up with air again the next day, so the mix is even leaner, then top up with banked 30% the next day, rather than paying for a complete fill of 30% (so still ending up with a mix that is less than 30%).

As long as my mix is within 5% or so of Best Mix - and has an MOD (@1.4 ppO2) that is at least as deep as I'm going to go, then I am fine with it.

Wow, what a read. Apparently there are "loose rules" in way more places than Florida ... and from some of the responses it appears that while some folks may have gone through a nitrox class, they really didn't understand what was being taught.

As with all things, nitrox use involves trade-offs ... one must consider not just PPO2, MOD, and oxygen exposure limits, but also conditions.

On the basis that this thread is only talking about recreational sport diving and using only Nitrox (or Air), what "conditions" would ever cause you to prefer something other than Best Mix for the planned dive?

I mean, I can see where, if you have Trimix as an option, there are dives where conditions might be so benign that you'd stick with Nitrox one day, but a different day with the same planned profile you might choose Trimix based on conditions. But, when I only have the blend of Nitrox to choose (implicit in this thread and sub-forum, I think), I would always choose Best Mix, no matter what the expected conditions are. The conditions would only possibly sway me to just not dive - but not to prefer a leaner (or richer) mix.

Well, except I might use a leaner mix than Best Mix just because an air top-up is cheaper. But that's a cost consideration, not conditions.

 

[boulderjohn]

if Best Mix is, say 30%, using 26 or 28 just doesn't make that much difference. So, I will often start a weekend with 30% and then just get topped up with air for the next day. Maybe top up with air again the next day,

I do something like this frequently. I call the blend I get after a few topoffs "slightrox."

 

[CptTightPants21]

I do that all the time. Really, if Best Mix is, say 30%, using 26 or 28 just doesn't make that much difference. So, I will often start a weekend with 30% and then just get topped up with air for the next day. Maybe top up with air again the next day, so the mix is even leaner, then top up with banked 30% the next day, rather than paying for a complete fill of 30% (so still ending up with a mix that is less than 30%).

This is not exactly what NW and I were talking about. You are just getting a nitrox tank filled with air which gives you a diluted mix wherever...I like boulderjohn's "slightrox". NW and I are talking about the deliberate calculation so we know exactly what the mix will be when we top it up--it won't just be leaner or richer, it will be exactly what we wanted it to be.

On the basis that this thread is only talking about recreational sport diving and using only Nitrox (or Air), what "conditions" would ever cause you to prefer something other than Best Mix for the planned dive?

Depends on factors. If I have 32 or 30 in a tank (or the shop banks 32 or 30) and my dive is only at 100ft. I am not going to go through the hassle of getting 34%, I'll just use what is easier. For shallower dives, there is plenty of leeway on best mix.

For some of the deeper stuff, best mix is more important to cut down on deco, minimize narcosis, etc. In New York, divers will routinely fill tanks with 28%-30% to "plan for the Oregon" at 130ft regardless of which wreck they are scheduled to head out to because they want to be prepared if the boat gets redirected.

 

[tarponchik]

Shops are going to offer the blends that make the most sense for the dive sites they go to. If you come to NC you are going to be offered EAN30 (some shops may have additional blends). Why? Because a lot of the offshore wrecks are in 115-125' to the bottom.

Can you explain the bolded? That seems to be a blanket statement. To me it's going to depend on the depths of the dives I'm doing. If the dives are going to exceed the MOD of the nitrox blend the shop is offering, then yeah you are pretty much forced to use air. So if you were going to a site that was 85' and you only have EAN32 and air to choose, you are going to go with air?

Maybe I just misunderstood you

The statement I replied to was "Have you heard of...it looks like your experience is limited." Hence the reply. I usually do not have the luxury to plan my dive according to the best EAN percentage possible.

I did some wreck diving in NC and MD, and the option was the same 32. This was some time ago, so maybe things are changing or I did not dive with the most advanced dive shops. Keep in mind that recreational diving in the tropics and technical or semi-technical wreck diving in US are very different. If your question refers to a situation where I can plan my own dive, I'd rather do 85' on 32 than on air; however, if I dive from a liveaboard with conservative DMs, this will be a total waste of money cause you dive with the group. Half of the group (including the DM) have air, half have Nitrox, and they dive the same dive profile and get back to the boat together. I've seen people doing this again and again, and never understood this.

 

[stuartv]

This is not exactly what NW and I were talking about. You are just getting a nitrox tank filled with air which gives you a diluted mix wherever...I like boulderjohn's "slightrox". NW and I are talking about the deliberate calculation so we know exactly what the mix will be when we top it up--it won't just be leaner or richer, it will be exactly what we wanted it to be.

I understand and I have done that as well, when the exact mix was more important to me. But, when I know there is a hard bottom of, say 120', and I have EAN30 in my tanks and they are somewhere between 1/3 and 2/3 full, I know that an air top up will leave me with something that will be okay without doing the exact arithmetic. And that is my most common situation.

If I were doing a deco dive and wanted an exact mix (say, +/-1%), I would just tell my guy at my LDS what I wanted and let him do the math, since it's going to be a custom blend and he'll have to do the match himself anyway. My shop is nice because, in that situation, they'll analyze my tanks and do the same calc I would and then only charge me for whatever amount of gas they put in. So, I just tell them the mix I want. That way, if it comes out wrong, it's on them to fix it. If I do the calc myself and give them the "recipe" then if it comes out wrong, it's on me to pay for more gas to fix it.

Depends on factors. If I have 32 or 30 in a tank (or the shop banks 32 or 30) and my dive is only at 100ft. I am not going to go through the hassle of getting 34%, I'll just use what is easier. For shallower dives, there is plenty of leeway on best mix.

For some of the deeper stuff, best mix is more important to cut down on deco, minimize narcosis, etc. In New York, divers will routinely fill tanks with 28%-30% to "plan for the Oregon" at 130ft regardless of which wreck they are scheduled to head out to because they want to be prepared if the boat gets redirected.

Right, that's why I didn't ask what you (he, actually) would actually use. I asked what he (or you) would PREFER. I understand that logistics and finances may influence you to a different choice in real life. The point was to ask when would "conditions" cause you to PREFER something other than Best Mix for your max depth.

 

[stuartv]

I did some wreck diving in NC and MD, and the option was the same 32.

Or maybe you just assumed they would not do a custom mix upon request?

I have only been out with two different operations in NC. They both bank 30%. I know for sure that one of them will do custom mixes and will also do 100% O2 fills (if you have an Adv Nitrox cert). You just have to ask. They also bank 36%, but you have to specifically ask to get that, too. If you just ask for "Nitrox", you'll get 30%.

 

[tarponchik]

Or maybe you just assumed they would not do a custom mix upon request?

I have only been out with two different operations in NC. They both bank 30%. I know for sure that one of them will do custom mixes and will also do 100% O2 fills (if you have an Adv Nitrox cert). You just have to ask. They also bank 36%, but you have to specifically ask to get that, too. If you just ask for "Nitrox", you'll get 30%.

Maybe. At least, they did not advertise.

 

[MXGratefulDiver]

On the basis that this thread is only talking about recreational sport diving and using only Nitrox (or Air), what "conditions" would ever cause you to prefer something other than Best Mix for the planned dive?

Many of us become accustomed to diving standardized mixes, such as EAN32 (almost always, in my case). For frequent divers, it makes dive planning and tracking easier ... particularly at sites and profiles you're familiar with.

Best mix is useful for some dives ... but standardized gas will often more readily keep you within a familiarity zone that just makes managing the dive easier. For some challenging conditions ... heavy current, low vis, it frees up some mental bandwidth for paying closer attention to the dynamics of what's going on around you.

And sometimes it boils down to nothing more than personal preference ...

... Bob (Grateful Diver)