Two day dive trip, when and how to use limited Nitrox?

[DevonDiver]

The benefits of nitrox are well known. Maximising those benefits is cause for the discussion. The major point identified was that the OP shouldn't risk contaminating their nitrox tanks, if unsure of the re-fill source; to do so would over-rule any benefits.

The best way for the OP to identify benefit would be to simulate the dives (using nitrox at different stages) on their computer. That'd identify the optimum solution.

Failing that...quick and easy... is to identify which daily dive provided the highest nitrogen loading. Use the nitrox to decrease that loading. It'd probably be the first dive, assuming deepest depth and aim for maximum bottom time, but not necessarily.

 

[beaverdivers]

I would use nitrox on the first dives of the day as those dives are usually the deeper ones with limited ndl times. You would have less nitrogen loading this way that can be of benefit on 2nd/3rd dives of day.Then use air on shallower dives where ndl time is longer.
Would not bother getting personal O2 clean tanks filled on boat, possibly contaminating them. Just use the boats tanks after you are done with yours.

Ditto. As long as the first dive of the day is w/in the MOD of your mix.

Moreover, the #1 rule of diving nitrox is ALWAYS test your mix!

Why violate this rule?

 

[tphelps]

Since it is only a 2 day trip, you should not be at risk for pulmonary oxygen toxicity but you still should always track your oxygen exposure (so make sure to bring your nitrox tables). I would use the nitrox for the deeper dives where you will get the most direct benefit (usually done in the morning). As a technical diver, I would personally never dive on a cylinder that I do not know the exact contents of (given there will be no O2 analyzer to use for top offs). I understand that you are using the nitrox for added conservatism but I personally would not do that myself (I like to know what I am breathing, track O2 exposure accordingly). My biggest recommendation would be to make sure that you set your dive computer properly before every dive. A lot of people (including myself) prefer diving on nitrox but given your situation, I would not bother bringing my own tanks as you the ones provided by the liveaboard. If you do choose to top your tanks off (or even it you use their tanks), I always smell the air before I test breath the cylinder to make sure it is safe. Hope this helped, happy diving!

 

[ajduplessis]

I would dive first 2 dives on Nitrox and would not fill them thereafter on the boat. It sounds like a lot of hassle taking your own cylinders. If all else fail rather extent all your safety stop to 10min or until nearly empty. Sticking to controlled 25ft p/m ascent rate (slightly slower than normal) will also help and stick to flat bottom profiles.

Control bubbles with a slow controlled ascent
Stick to "flat" bottom profiles
Extend safety stops
Extend SI
Enjoy the diving and have fun

 

[CT-Rich]

although not a nitrox diver, it seems you would get the most benefit from diving Nitrox the first dives of the day. that would give you the most benefit since you last dive of day one will probably be at least 8 hours before the first dive of the second day. Using those tanks on the lead off dives will reduce your surface intervals for dive two on both days and give you the best repetetive dive time on the second dive even if you have to wait for your buddies. diving those tanks last would give you more of a safety margin at the end of the day, but I hope you aren't planning to push dives where you are going to have to worry about skating right up to the NDL. If you are planning on dragging the tanks along, get the benefits of the nitrox rather than a little more safety margin (that you should already be counting in).

 

[windapp]

Edit: so just saw someone else say this... Oh well

So I skimmed these posts and (maybe I skimmed too fast) but I didn't see anyone mention that you could benefit from using your nitrox tanks by simply refilling them and diving them for every dive. If you are diving for two days and have two tanks with 32ean in them simply dive one tank each day. The process goes something like this...( I will assume a 3 dive a day trip but works same regardless of how many dives you do each day)

First dive, set your comp so it knows u are diving 32 ean. Get the benefits

Second dive, since u have no O2 sensor ( go buy one!) refill the tank on the boat and dive it as air on your computer. U still have a higher % of O2 in your tank that u can benefit from. U now are getting an extra, conservative, benefit because u are diving a higher percentage of O2 than your computer is calculating so you have an extra safety margin.

Third dive, repeat steps for second dive... It takes so many fills to dilute the O2 back to straight 21% without emptying the tank completely that you should actually have higher then 21% for a long time. After 3 fills or so it will only be 22-25% most likely, but why waste the safety factor? Dive it and benefit from it.

For day two, repeat the process starting over with your second 32ean tank.

Obviously you are basically limited to your MOD for 32ean because you don't have a way to measure the O2 content for subsequent dives, but your deepest dive should have been done first so it shouldn't affect your dive experience anyways.

This is my normal nitrox diving method when I go out on multi dive one day boat trips. If you have an O2 sensor you can take advantage of the EAN % that is left in your tank on subsequent dives and get a little more bottom time but with that little extra O2 in the tank the benefit in bottom time is negligible and I believe you said you were diving with folks on air anyways.

Additional benefits would be...

1. Not having to change your tank out every dive.
2. Not having to readjust your set up if they have different size tanks than what you normally use
3. Knowing where the tanks on your back have been ( nothing against loaner tanks but I bought my own tank for a reason, you probably did as well)

Hope that helps, just my $.02 about it.

Happy Diving!
RK05

If you empty to 500 psi, and fill to 3000 psi, your O2 will be less than 25 percent if you started with 40% nitrox. Don't exceed 40m with that and you will never even come close to your mod. For recreational purposes, and for recreational depths, you can treat a topped up Nitrox tank as air if you empty it to 500 psi and fill to 3000. it is simple math. No guessing or risk involved.

However, it is highly unlikely that a compressor on a boat that doesn't offer nitrox will be filtered to offer hyperpure air, so unless it is time for your yearly O2 cleaning this could be a very expensive nitrox fill.

 

[crispix]

Just finished the trip and it was fantastic. I wanted to follow up and report what I ended up doing. After talking with someone who had been on a similar trip before, I decided to follow what was essentially HowardE's advice in post #7 above and just dive with the boat's tanks on air for the entire trip.

Reasons: I was one of the more experienced divers on the trip. While I was dry none of my buddies were in a dry suit. Cold, wet, newer divers results in shorter, shallower dive profiles. The benefits of nitrox were therefore greatly reduced. While my tanks are not O2-clean, I really took to heart the advice that it was best not to fill them on the boat because of the chance of water contamination, and that was a big part of my decision. Not having to lug my tanks around was also a factor.

So it all worked out and the trip was a blast. Thanks much for all the replies, it was very helpful. I think on a simliar trip with more experienced (and dry) divers, nitrox would have been advantageous, but I think I'd prefer to find a dive op that could fill EAN on the boat just so I don't have to lug my tanks and worry about contamination.