setting to nitrox to reduce over conservatism on dive computers

[DaleC]

Here's a question.
About a month ago I spent 3 hours between 5-15 feet assembling a boat lift for someone. What was my decompression obligation and how would I meet it in a practical way. Remembering, every dive is a decompression dive.

 

[kr2y5]

Here's a question. About a month ago I spent 3 hours between 5-15 feet assembling a boat lift for someone. What was my decompression obligation and how would I meet it in a practical way. Remembering, every dive is a decompression dive.

Interesting question.... on Buhlmann ZHL-C with 10/50 gradient factors, you would be doing a 1-minute stop @ 10 feet ;-P.

 

[DevonDiver]

I've come to the thread late, but I'll answer the original question:

Yes, I have done this, with caveat.

Firstly, I won't endorse this as a solution for anyone else. Modern algorithms are calculated for a designed performance and intended to keep you safe. It isn't all about manufacturer liability. If a manufacturer aims to create a 'safe' dive computer, then algorithm conservatism is a very good solution to that.

As an instructor, I dive demanding schedules with students who are unlikely to be using the same computer as me. For the rare recreational-level course I teach (basic wreck or sidemount), I use my old 1st-Gen Suunto Vyper. Over repetitive deepish dives, it often provides significantly less bottom time than my student's computers. I need to maximize training time - so I experimented with setting my computer as per different nitrox mixes to better emulate/match what my student's computer were providing in bottom-time.

So, sometimes I will put my computer onto 22 or 23% EANx, for a air dive. This matches closely what some other manufacturers/ computers are giving.

The caveats:

1) All these dives, I have already planned on decompression software - and I have a bullet-proof understanding of the parameters I am working within. I carry back-up plans, on wetnotes, for all of these dives... and even with the 'adjusted' settings on the computer, I am diving within them.

2) All of my recreational dives end with a minimum ascent procedure (not a traditional safety stop). This is a minimum of 9m/1min, 6m/2min and 3m/3min. If compared against calculated dive plans, this more than covers the off-gassing needed well above and beyond and discrepancies caused by the rather inelegant manipulation of my clunky old computer.

3) My ascents are all performed to a high technical-diving standard, with proper ascent rates, stable buoyancy and no saw-toothed profiles. I am fit, well-hydrated, no other significant DCS pre-disposing factors and I dive very frequently (daily). I have dived for over 22 years and never had a DCS symptom from a recreational/no-stop dive (and only one unconfirmed 'pain-only' possible DCS following a particularly significant multi-week schedule of technical diving).

 

[Hatul]

One of the dive guides in Cozumel was doing this a few years ago because the computer did not allow him to do all the repetitive diving on air, and he could not afford Nitrox. He set his computer to 24% but used air. It's not recommended.

 

[kr2y5]

(...) So, sometimes I will put my computer onto 22 or 23% EANx, for a air dive.

Just wondering why not simply dive EAN23... you would only need to put about 2 cuft of O2 into an AL80 before you top it off with air, and that would be only in the ballpark of $1 extra cost per tank.

 

[Billy Northrup]

There are plans in place for minor incursions into NDL, but I wouldn't make that part of my plan. I would rather see someone just follow the instructions the computer gives than fudge it.

Why is it the guy that complains about buying Nitrox is the same guy you never see tip the boat and run up a $100 bar tab?

 

[redacted]

When you perform such unproven modifications, how do you know that you have not exceeded the "acceptable risk" under some conditions for all dive computers (and algorithms)?

 

[DevonDiver]

Just wondering why not simply dive EAN23... you would only need to put about 2 cuft of O2 into an AL80 before you top it off with air, and that would be only in the ballpark of $1 extra cost per tank.

1) Custom mixes aren't that available...and significantly increase the logistics of running my courses.

2) My students normally pay costs for gasses, but those are course dictated.

3) $1 per tank doesn't sound much, but I complete 000's of dives a year.

4) Here, it costs 250php (~$5.70) for a nitrox fill (charged PER cylinder, not cuft). Multiply that by sidemount doubles (500php/$11.40) over 000's of dives per year.

5) The cost of 2x EAN23 fills, compared to (free) air buys my lunch here in the Philippines.

6) I'm an independent-instructor, chartering through dive centers, in the developing world, so I don't blend my own gasses.

7) I've done hundreds of dives this way, with no problems, and it's supported by proper planning (all parameters inside custom-cut dive plans), so why?

8) I could "just" put the clunky old Vyper into gauge mode and dive the tables, with an algorithm chosen and shaped to match whatever computer my students turned up with. It still wouldn't change anything on the actual dive profiles... and the bottom would still be dictated by the student's computer. But then I'd just lose a back-up option, and some other occasionally convenient info provided by the computer working in computer, not gauge, mode.

If I worked in the USA, I don't doubt that many things would be significantly different... especially my income and what I charged for courses. But here in the Philippines, I eat rice with a fried egg for breakfast... and $1...or $12... per dive supplementary expense out of my income is a significant consideration.... especially for something that has no real benefit.

The caveat, as before, is that I am proving to my satisfaction that there's no benefit; bottom time on 'fudged' 22-23% on my computer versus bottom on 21% on a different model computer or cut-tables... and a much more robust ascent profile that significantly over-compensates for any fudging on a recreational/no-stop level dive.

When I'm inclined to pay for some extra 02 on the grounds of 'conservatism', it's gonna be stuck in a bottle at 100% and breathed during my stops from 6m to the surface. I do this, also, when my diving schedule is particular heinous and unrelenting...

 

[Tigerman]

I am not sure why a person would buy a computer that doesn't match up or doesn't give you the results you want. Why would you want to trick it and use it in a way it wasn't designed to work. Simple solution, buy a computer that uses the algorithm you prefer. Otherwise what good is it? I can use a big screwdriver as a pry bar but it isn't the right tool for the job.

No, you should always do the right too for the job and the right tool is always a HAMMER never a screwdriver...

 

[dmoore19]

Or an adjustable wrench, which can also be used as a hammer.

No, you should always do the right too for the job and the right tool is always a HAMMER never a screwdriver...