Nitrox Blending - Single Tank

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ystrout

Contributor
Messages
151
Reaction score
85
Location
San Diego
# of dives
25 - 49
Hi,

Here's the situation:

I only can bring 1 tank (not oxygen clean) and do 3 dives per day while on the island. First dive, I bring my own nitrox since my mainland shop fills from a Nitrox bank and doesn't require cleaned tanks. The next 2 dives where I fill on the island use regular air.

I used to just set my computer for EAN 32 on the first dive then air the next two dives but I was wondering if I should use a lower nitrox setting instead. I would assume this math makes sense, but I just want to make sure I'm not missing something...

*First dive (EAN 32) - Dive to 600 PSI, from 3K.
*Second dive (Air) - ((600/3000)*32)+((2400/3000)*21) = 23.2 EAN. Dive down to 500 PSI.
*Third dive (Air) - ((500/3000)*23)+((2500/3000)*21) = 21.3, so just 21.

I would cool down the tank by dipping it in the water for 30 seconds before doing these calcs. And of course know to change the weighted average base if my fill is +/- 3K PSI. Also, the dives don't go deeper than 90ish feet so no worry about PPO2.
 
Cooling down your tank has no effect whatsoever on the fO2 of your tank's contents.

Do you analyze? The only prudent answer for me personally is to analyze (as I was taught during my nitrox class) and set my computer accordingly. I never dive a tank I haven't analyzed myself unless I have very, very - very! - good reasons to believe that it can't contain anything but air.
 
The most correct answer is to bring your analyzer with you, and always analyze your tank before each dive.

You can do the math based on tank volume and working pressure/fill pressure and a known starting point, however that would be only an educated guestimate.

All that said, you should be fine diving within your MODs and diving conservative in the dive park :wink:
 
As others have said, you need to analyze the topped off mix. You can just drain the tank and refill so you breathing air.

An option for the subsequent dives is to use the starting mix MOD but dive an air profile. This is a conservative approach I would be comfortable with.
 
Hahaha. I have no idea why I didn't think to use an analyzer. It didn't even cross my mind.... Lol. Well there's an obvious answer to the question.

Thanks for the replies.

And to answer the questions. I do analyze my tanks when using nitrox, but not when using air. But I watch my tanks get filled and can see they're hooked up to the air compressor. I guess it's probably pretty smart to analyze them if you are going on a charter and using their tanks, just to be safe.

And I meant cool the tank down after the fill because if it showed I am at 3.3K PSI rather than the actual 3K (after tank gets to normal temp), it would throw off the calculation.
 
The most correct answer is to bring your analyzer with you, and always analyze your tank before each dive.

You can do the math based on tank volume and working pressure/fill pressure and a known starting point, however that would be only an educated guestimate.

All that said, you should be fine diving within your MODs and diving conservative in the dive park :wink:
Ya I've never had an issue. During spring and summer, I try to get to Catalina once a month. We used to do two dives, get lunch (and take wetsuits off), then do one more dive after lunch. This was kind of tough since we would be ravenously starving and overeat at lunch then hate putting on wet wetsuits at 3PM. So we started doing 3 dives in a row then get food/drinks after. We hold ourselves over with snacks and Gatorade.

This is more enjoyable, but puts us pretty close to our NDLs. But I guess just using a light blend of nitrox on the 2nd dive gives us some safety pad as long as we set our computers to air on those subsequent dives and stay within those NDLs.
 
guess just using a light blend of nitrox on the 2nd dive gives us some safety pad as long as we set our computers to air on those subsequent dives and stay within those NDLs.
Are you at all able to get close to the NDL on air if you limit your dives to min gas? I dive single tank, metric but pretty close to a 100 in your system. And on air, I used to reach min gas more or less at the same time I reached my NDL. On EAN32, I have some 50% extra NDL time when I reach min gas.
 
Are you at all able to get close to the NDL on air if you limit your dives to min gas? I dive single tank, metric but pretty close to a 100 in your system. And on air, I used to reach min gas more or less at the same time I reached my NDL. On EAN32, I have some 50% extra NDL time when I reach min gas.
I'm not sure what you mean by min gas. Minimum gas as in starting a safety stop at 15 feet with 750 PSI and ending the dive around 500 PSI?

If so, that's how we dive there and the 3 consecutive dives with air get me right around the NDL. If I remember correctly from the last time I was there (didn't use nitrox at all), we had around 5 minutes until our NDL at 50 or so feet. We could have reached this but went shallower to be safe. We also waited 10 to 15 minutes longer than we wanted to at the surface between each dive because I don't like getting too close to the NDL.

This is why nitrox would be beneficial for my dive profile there. But I've found that just using nitrox on the first tank then air the next 2 gives me plenty or margin.

On a side not, this is the only place I have ever needed to use the 'dive planning' function on my computer. I normally just play with it but don't need it because most of my dives are shallow and more spread out, but Catalina lets us do consecutive deep-ish dives without very long surface intervals. It's kind of fun to plan our SIs and next dives like that.
 
I think most people would just figure nitrox for the first dive and air settings for subsequent dives. If you're pushing NDL's to the point that you're wanting to take advantage of a 23 mix you may be asking for trouble. Why not relax and enjoy the surface interval? I was taught to do a min 1hr surface interval with nitrox. You're doing less aren't you?
 

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