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

[crispix]

I'm going on a two-day liveaboard trip in a few weeks. The boat does not have Nitrox but will provide steel 85s or refill my own tanks. Tentative dive plan is 4 dives on the first day, 3 dives on the second, all within rec limits but I don't know the sites or depths. Let's assume 65ft dives, +- 10ft.

I'm going to bring my two tanks, LP95s with EAN32. The boat won't have an O2 sensor and I don't have my own. I always measure at the shop.

The goal is to maximize the safety benefit of Nitrox since my buddies will all be on air. I don't expect to get any extra bottom time. Here's my tentative plan:

Day 1: use the boat's tanks for dives 1 and 2. Use my first Nitrox tank for dive 3 and set my computer to 32%. Dive 4 (night dive): top off my Nitrox tank with air and set my computer to 21% for the night dive#4 and pretend it's an air dive. (I tend to be good on gas consumption so topping off my Nitrox after a dive usually gives me a 26% mix but I won't count on that without an O2 sensor. So it's for added safety only.)

Day 2: use an air tank for dive #1, use my second Nitrox tank for Dive #2, set computer to 32%. For dive #3, top up my Nitrox with air and set the computer to 21%, using remaining nitrox as a safety measure.

Any suggestions? What would you do different? I suppose dive #1 on day two could be a third refill of my first Nitrox tank, still set to air on my computer, but that tank would be at 23% at best. Sill, I guess that's consistent with the goals of this little trip, and 95cf is better than 85cf, right?

 

[Graeme Tolton]

If you arent going past 75', you have nothing to worry about. Set your computer to air for everything but the full nitrox tanks and you are set. Do that and stay above 130' for the trip and your golden.

 

[Searcaigh]

First question, are your tanks O2 clean for filling by blending or does your Nitrox supplier use a membrane system?

Reason I ask is that you plan to fill your tanks with air after using Nitrox, but I see that you mentioned that you have done this before.

If your Nitrox supplier blends then you should really have the tank O2 cleaned before the next Nitrox fill.

Your plan for using your Nitrox though looks fine to me.

 

[NAM001]

why bring this up when it has nothing to do with the question. he is asking about nitrox management.. not tank cleaning and filling proceedures

First question, are your tanks O2 clean for filling by blending or does your Nitrox supplier use a membrane system?

Reason I ask is that you plan to fill your tanks with air after using Nitrox, but I see that you mentioned that you have done this before.

If your Nitrox supplier blends then you should really have the tank O2 cleaned before the next Nitrox fill.

Your plan for using your Nitrox though looks fine to me.

 

[promocop]

Way too much trouble. Forget you tanks and just dive the boats AIR. Why hassle it and confuse you and your computer. Not that big of a difference

 

[oly5050user]

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.

 

[HowardE]

I would dive the boat's tanks, and not even bother bringing my own at all. Boats with compressors on board are notorious for pumping water into the tanks. More than "contamination" with "non-o2 compatible air" I'd be worried about getting water in my personal tanks. Rust on the inside of a tank will probably cost you more to fix than the benefit of nitrox for 2, maybe 4 dives.

If the fills are included in the price of the charter, then save your gas, and just dive what they give you. If you're doing 7 dives over 2 days, what is 5 minutes of extra bottom time going to do for you?

 

[DevonDiver]

As Searcaigh mentions, the risk of polluting your O2 clean tanks might not be worth the benefit for topping up. That's certainly worth checking up on.

If it were me, I'd set my computer for air and ensure my dives remained within the MOD. There's no benefit for setting to 32%, as you're diving with non-nitrox divers.

If you do top-up, just keep the computer as per air. You won't know the exact mix, so retain the MOD for the previous (undiluted) mix as a precaution. Unlikely to be exceeding that MOD on repetitive dives anyway...especially night dives..

---------- Post Merged at 07:34 PM ---------- Previous Post was at 07:32 PM ----------

why bring this up when it has nothing to do with the question. he is asking about nitrox management.. not tank cleaning and filling proceedures

Because the OP may not have considered the issue - and it's pretty decent of Searcaigh to volunteer the information as a reminder. It could save the OP a little grief and money... let alone safety.

Sometimes there are factors to be considered beyond the initial question posed.

 

[knotical]

Like oly5050 I’d use the nitrox for each day’s first (presumably deeper) dive.

On air, we usually do our most aggressive (deepest) dive first allowing shorter surface intervals before the next (less aggressive) dive. This is another way of saying our overall exposure to DCS risk tends to be less when we do the more aggressive dive first. Similarly, if you do deeper dives on nitrox, you reap the maximum benefit – with a big caveat: - assuming you don’t exceed your MOD.

As an exercise run some simulations either on your computer or tables, doing deeper dives first or later, and doing nitrox dives first or later, on deeper or shallower. And do a series of dives of the same depth with nitrox on earlier or later dives. Watch what happens to your minimum surface intervals.

 

[RK05]

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