Nitrox on boat with air refill

[IVC]

I've recently got my OW and before I went for my first local boat dive to Catalina, I got Nitrox certification. The reasoning is that it's not a "diving skill," but a scientific one (my background is in physics, so I knew all the concepts already.) It all went smoothly and off I went to my first post-OW dive with Nitrox tank.

The trip was a three-tank dive with hot fills on the boat. As usual, it's "air only." Before the dive it occurred to me that I wouldn't know the correct mix (PPO2) after the first dive since there would be remaining Nitrox in the tank that would be topped off with air. I could do the simple calculation to figure out the PPO2, but I figured that someone would have an O2 sensor with them since it looked like a standard issue with anyone on multi-tank dives. It would be more accurate as well, since pressure gauge is not all that accurate.

Well, I was wrong. Not a single O2 sensor.

The simple workaround was to set computer for "air" and get more conservative no-deco time, EXCEPT that it would now underestimate my O2 exposure because there was more O2 in the mix due to the residual Nitrox. So, I asked the local DM (I believe he is officially a "course director") and he mentioned the same workaround of setting it to air, but no mention of exposure to O2. There was at least one much more experienced diver who was asking the same question and he got the same answer. Granted, it was only down to 90' and bottom time was not all that long, but it made me think about what else is taken lightly and whether it's a good idea to start carrying a calculator.

So, what would you do in such a scenario: (1) No big deal because it's pretty close to air and depth-time is likely non factor, (2) Get a calculator and get the best approximation to the true PPO2 based on pressures before and after the fill, (3) Abort the dive because a purist would, or (4) Something else.

(DISCLOSURE: The next purchase I made was an O2 sensor, so it won't happen again.)

 

[JahJahwarrior]

Calculator? Download one of the many apps for that. I use iDeco Pro.

Pressure gauges should be relatively accurate and precise for this stuff, but you can always round up for MOD, CNS clock purposes, round down for other purposes.

But of course, all nitrox divers should own a working o2 sensor. It's just one of those $70 per year expenses you have to put up with to scuba dive safely. Atleast the app is a buy it once and forget it deal.

 

[JohnnyC]

Use a blender app to estimate your nitrox percentage. I keep an O2 Exposure slate from IANTD in my wetnotes. For recreational depths and run times, you shouldn't come close to the limits if you're diving conservatively in the first place.

ETA: Dive your computer set to air, but estimate the high side for your O2 exposure, that way you're conservative on both fronts. Also easier than trying to do a bunch of math and enjoying your dive less.

 

[Bob DBF]

If I was one to worry, I'd dump my tank immediately before the refill. Since I'm not one to worry, I'd set my computer to air and dive. It really isn't going to make any difference unless your diving deep, and the boats I dive on do the deep bottoms first and work shallower as the day goes on, so no big deal.

No reason for a boat to have an O2 analyzer if they only fill with air.


Bob

 

[Max Speed]

You're not going to tox on OTUs while recreational diving. Your only concern would be max depth so to be conservative just make it the same limit as your original blend.

 

[Centrals]

A O2 analyser would be ideal but not really necessary in this case.
The O2 content is getting less on subsequent refill.
However, the "calculated value" of the O2 is required to reset the computer.

 

[Brandon]

You're unlikely to hit O2 exposure limits on a single day, three tank recreational trip. To do so, you'd either need to rack up so much bottom time you'd be into deco, push the MOD and run at high PPO2, have short SIs, or do some combo of the three.

Run the calcs yourself and you'll find that it's very difficult to hit O2 exposure limits on single tank dives with 1:30 surface intervals. It's a little easier to do with 1:00 SIs, but still unlikely that most recreational divers would hit them in a single day.

As for topping with air, just do the math: Adding air on top of 500-1000 psi of 32% is going to yield a slightrox blend somewhere between 22% and 25%. The third fill will almost certainly have you below 22%. Keep in mind that anything leaner than 28% has a MOD that exceeds recreational limits.

If the boat only had air available and there was no possibility of getting an O2 fill, I personally wouldn't stress about not being able to analyze on the 2nd/3rd dive. The math tells us that at most, you'd have a 25% fill... That's close enough to air that you may as well treat it as air- an extra 1-4% isn't going to make such a difference that it'd be worth calculating new tables for. Set the computer to air after the first dive and be done with it.

O2 exposure was unlikely to be a concern to begin with, but it'll be even less of an issue on the second and third dives.

I wouldn't have been the least bit concerned and have topped 32% with air many a time when that was the option that kept me diving.

 

[scubadada]

I would do a quick calculation for the 2nd tank based on pressures and round down to give me a little more bottom time. I would use air for the 3rd tank. Oxygen exposure is not an issue on 3 no-stop dives, my method would be very close

 

[archer1960]

You know your PO2 is less in the 2nd tank than the first one, so use the MOD from the first dive, and the NDLs from straight air, and you'll be good. My computer has a mode where it will calculate NDLs from air, and O2 exposure limits from EAN40, keeping me reasonably safe on both sides.

 

[geoff w]

Before I bought an O2 analyzer, I put together a simple Excel spreadsheet for EANx32 (which is what most dive shops will fill) with various starting and ending pressures, to just look up what the end mix would be when topped off with air, then printed it out, and took it with me on dive boats to ballpark the mix for my second dive. Being within one percent for the final mix is good enough. And treat subsequent dives after the second as air dives. Since you have a background in physics, do the math. You'll have a hard time getting anywhere close to O2 exposure limits on three recreational dives, especially as a new diver. As far as no decompression limits, on most So Cal dive boats, you'll have a hard time running out of bottom time as well. As long as you're within both those limits, one nitrox mix is as good as another within a few percent

If your tank was O2 clean when you got on the dive boat, you've got a bigger concern because it wasn't O2 clean once you got the first fill on the boat. And it wouldn't have mattered if the boat did nitrox fills or not