Nitrox first dive, Air second dive?

[mainedvr]

Hello,

I am new to using nitrox and have a question regarding Nitrox and Air. My wife and I are going to be doing a 8-12 dives over a four day period (NC wreck diving so excited). From what we have learned nitrox has its plus and minus factors and limitations. While budgeting our trip we found that if we dive nitrox on our first dive and air on on second (in one day) we can save enough to actually do a few more dives. What we are wondering is are we getting the benefits of using the nitrox doing this? (were looking at decreasing our surface interval and hopefully getting a bit more bottom time, as well as help lessen the fatigue of diving repetitively)

Our reasoning was this "first dive is typically the deeper dive and we want to lessen our surface interval, the time between our second dive of the day and the next day will be around 18 hours so that should give us plenty of time between dives (so we it seems to us we planned out some mock times to see what our residual should be).

Hope this makes sense...its not that big of a deal but hey if we can get to spend some more time diving thats great...but we also want to be safe and have fun.

Thanks

[Brian Sharpe]

Lots of variables involved but generally speaking given the same dive profile you'll build up less residual nitrogen using nitrox than air and, personally, I dive nitrox almost all the time (I rarely go deeper than 100 to 120fsw and have never exceeded the MOD for EAN32).

Is the the 0-24 dives in your profile correct? What size tanks will you be diving? If you're diving AL80s you may be running out of air before coming close to your NDL, perhaps nitrox may be a waste of $$?

Wreck diving in NC is on my "to do" list for this year......

[herman]

I think your math and expectations are off somewhat for NC diving. First off, it is difficult to get in 3 dives a day much less 4 with the schedules we have, 2 dives per day is more realistic. Our wrecks tend to be 2 to 3 hours off shore, we do have a few inshore wrecks but getting on 2 trips a day is not easy time wise. Plan on a typical 2 tank charter to be 6 to 8 hrs dock to dock. You also need to plan on and expect rough seas, 4 to 6 ft waves are common. That said, it is unlikely that you will get in more than 2 dives a day so the cost of nitrox limiting your dives is a moot point. Another factor is your gas use. If your profile is correct and you have less than 25 dives, odds are you are going to be out of gas way before you get close to your NDL on air so the extended NDL of nitrox is not needed. Something else you should consider, our diving is not like Fla or the Caribbean, you are on your own. We do not provide DMs or do any hand holding at all unless you hire one on your own. You get a boat ride, a good dive briefing and you are on your own. IMO, a much better plan would be for you to back off on your dive schedule, plan at least one day of inshore diving and spend the saving on a good DM to help you learn about NC diving. That will make your experience much more enjoyable and give you a solid basis to plan another trip around.
Finally, you need a plan B. We dive in 4 to 6 ft sea conditons but there are years that half or more of our dives are canceled due to rough seas and trust me, if our captains do not want to go out, you really don't. We are all disappointed that we don't get to dive but it's the way it is here. You need to plan for the possibility of not diving the entire trip due to sea conditions.

[Bubbletrubble]

I think it would be wise to heed herman's words. Excellent advice.

Here's a link to a recent thread that discusses the thought process in figuring out when to use various nitrox mixes or air in a repetitive dive sequence. ClayJar made a couple of insightful posts in that thread.

[DivemasterDennis]

If your are doing 2 or 3 dives a day, surface interval shouldn't be a problem. You are correct that a nitrox dive first will increase your ndl time or decrease your surface interval, but probably not both, certainly not by an appreciable amount as to the surface interval. Remember that with nitrox you have shallower maximum depth limits, so stay within those limits. Finally, remember to reset your nitrox programmable computer to the gas you are using on each dive. That can be a pain if you are changing back and forth. You might consider having a nitrox day , then an air day and back and forth day by day rather than dive by dive.
DivemasterDennis

[FredT]

As a general rule you should do the dive with the deepest EAD first, and thien chase it with the lesser EAD dives in decending sequence. That said if you actually work out the physics, and maintain the "zero bubble" profiles and ascent rates many divers never seem to master, the sequence of dives is not very important. Blow that "zero bubble thing" and all bets are off.

I do agree 100% with Herman on the "Plan B," but I define it a bit diferent. PLAN B is the way back to the beach that does NOT involve the boat you went out on. This plan varies quite a bit depending on where you are diving. In a quarry it's not as complicated as 100 miles offshore in a high current area, but you need one for ANY environment you intend to dive in. "Swim to the beach" is sometimes an appropriate plan B, but it won't work diving the Graveyard.

[DandyDon]

To the direct question: If I were doing two, square profile dives on wrecks of the same depth and only had one tank of EAN, I'd use Nitrox on the second dive to make the adjusted air depth shallower.

For NC graveyard diving tho, as explained - it's a two tank day. I don't know that I'd agree with the expectation of running out of air before NDL even on 80s, but there are larger tanks available. For NC, Nitrox is really essential IMO - all dives.

[mainedvr]

Thanks for all the replies...When I said three dives we are considering doing a half day after a full day once or twice but had not yet decided as we are not aware of how things will transpire but will heed the advice of those more experienced and just stick to our two tankers. We do plan on diving with a DM at the very least our first day if not everyday as we are aware of our experience level we are confident in our abilites but also know it makes for a bit more relaxing dive with some one that knows the envrioment. So looks like nitox for all dives and at least a guide once, oh and thanks for the advice on rough seas and such.

[Wingwalker84]

Definitely go with what herman said. As an experienced NC diver, I can tell you that it is not like any resort dives. Herman brought up a good point with the DM, and actually I was hired 2 weekends ago to DM for a family from Virgina on the Indra and Hutton. The inshore dives are good places to start if you want to get into NC diving, but without a lot of experience I would not suggest going straight into doing the outer dives like the U352. Start inland and work your way out, and with the DM you should be fine. As for Nitrox, the inshore dives you won't need it, but on the ones further out, if you are good at saving air I would recommend it. Once again that will depend on you experience. Have fun!!