Let me try to clarify some things for you and answer your questions for you.
Originally Posted by junko
OK, whoa, maybe I'm totally misunderstanding things then. I was under the impression that when I got a Nitrox fill I would get to analyze the contents myself using the shop's analyzer after they finish the fill. Is this wrong?
Logistically, you being there right when the tank is filled is virtually impossible unless you want to go hang out at the fill station all night (restricted areas anyway). There are three stations now that fill tanks for every shop on the island. They work around the clock filling tanks, are regulated, properly trained and do follow proper procedures and protocol. Inspections, testing and cleaning are performed routinely.
Shops here do not fill their own tanks for numerous reasons which I won't go into here...but mainly for the costs involved (electricity, getting the compressor to the island, additional and properly trained staff, licenses for this activity, etc.) The list of reasons is very long actually.
The way it works is like this:
The shop places the order for their nitrox fills the day before. The fill station fills the tanks and the mixer analyzes the tanks once filled and marks them. After the tanks cool, a different person at the fill station analyzes the tanks again and puts his sticker on them. The tanks are then delivered to the shops/piers etc.
Depending on who you are diving with and where you are staying, you may only be able to analyze your tank on the boat. The crew loads the tanks on the boat in the morning and then picks up the divers at the shops designated pier(s) or at the respective hotel piers. This is why either you or the shop must have an analyzer on board. The tank should be analyzed while the boat is not moving (at the dock) preferably. It is your responsibility as a certified nitrox diver and as the diver using the tank to personally verify the contents of the tank. You should calibrate the analyzer and actually see the reading on the analyzer. If the contents are not within 1% you simply change your computer setting or the plan for your max depth. This is not a big deal in Cozumel because most likely the majority of your dives will be above 100ft. So if you have to change your computer to 34%, no big deal. If it comforts you any, I/we have never had a tank here be off by more than 2%, ever and that is based on thousands of fills over the years. In that case we simply changed the computer settings. It wasn't enough to affect the dive plan.
It is my opinion that the boat should have an analyzer and I should have an analyzer. It is my practice to check at least one tank with both if the boat has one. If things are pretty close, then I feel better. I have been on boats where their equipment was not as well taken care of as I would like (but not in CZM). If there were not an instrument that I trusted to use on the boat, I would dive air instead. The most differental that I have found is 2.1% 02 in CZM.
dnhill
This is also a good idea. If your first reading is way off, check with a different analyzer if available. As junko pointed out, a personal analyzer is just as likely to fail as a shops analyzer. After seeing how poorly many divers take care of their own equipment, the shops analyzer is likely to be much more reliable and properly maintained.
On a tangent--is it common for people to use Nitrox in Cozumel? I've heard that some of the dives there are "deep" by rec diver standards...
Maybe it's not worth the bother/expense even using Nitrox there then? I was mainly thinking of using it as a safety buffer and to try it will the coursework is still fresh in my mind, but 40-60 feet isn't exactly high risk for the bends, and I assume I will most likely end up with an air diver buddy on the boat and therefore my bottom time will be limited to "air time" anyway.
Actually it is becoming more and more common. However, as Jeff pointed out, most shops will not allow you to dive it on the first dive unless they know you well as a diver. Most first dives are wall dives and the potential to exceed planned depths and depth limitations for nitrox is there. Additionally, we typically select dives the day of the dive as a group on the boat. For example, if you have nitrox planned for your first dive and we want to go to Maracaibo, we can't because this dive is much too deep for nitrox.
Regarding your comment that 40 to 60 feet is not high risk for the bends. I could go on for days about this comment, but I will just say that you can get bent at 20 feet as easily as you can get bent at 100 feet. You can get bent on nitrox and you can get bent on air. Does using nitrox reduce your risk for DCS? This is the topic of much debate, but in my opinion, it is certainly better than diving straight air for repetitive dives.
My recommendations and some guidelines to help you decide whether to dive nitrox anywhere are these:
If you are planning three dives a day, at least one tank should be nitrox.
If you are planning four dives a day, at least two should be nitrox.
If you are diving repetitive dives over several days or more, nitrox should be used on your second dive each day (36% if depth allows).
If you are planning to dive the day before you fly (provided that no-fly times permit) nitrox should be used for both dives (if the first dives allow).
Nitrox should be used as a buffer to add conservatism, not to extend bottom times necessarily. Diving nitrox on nitrox tables or computer settings is no different in theory than diving air to air tables or computer settings.
Diving nitrox as a safety buffer is better than diving air if you have the option. I don't personally think it makes me feel better or to have more energy, but it does equate into less nitrogen absorption, period.
Safety rules should still obviously be applied. Multi-level profiles when applicable, slow ascents, graduated deep stops if applicable, safety stops (the longer the better), adequate surface intervals (longer is better).
Posted by DandyDon
Your only choices last January when I was there were 32% and 36%. 32% has a Soft bottom of 110 feet, and I wouldn't like to see a diver with less than 50 logged go deeper than that, even if the guide did.
Yes, these are the standard mixes, however I routinely order 28-30% when we have divers with exceptional air management and plan to do all of Punta Sur, including the throat. I also typically limit divers on 32% to 100 ft and divers on 36% to 80ft for safety reasons and to allow plenty of contingency room in case they drop a little deeper...another reason we don't typically allow nitrox on wall dives for divers we don't know yet or who do not demonstrate good buoyancy and depth control.
As far as a diver with less than 50 dives...that number is arbitrary, and this will take me off on a separate tangent, but one I think is worth discussing (in another thread).
Short answer is that I've seen divers with 25 dives who had much more skill and grace underwater than divers with 100 dives. They may not have as much experience in an emergency situation, but who is to say a diver with 100 dives has either or that they will handle the situation any better. So, the number of dives an individual diver has really means nothing to us until we have actually been in the water with each individual diver and observe their attitude/behavior/actions both above and below the surface.
It should come as no surprise that very often the arrogant divers who boast about their experience level/how many logged dives they have and the ones who insist on being put on the "big boys" boat are the ones who become complacent, can't manage their depth/air, and have trouble in the water and/or panic in emergency situations. Don't get me wrong, many are also true to their word.
Our motto is "Show us what kind of diver you are, don't tell us."
OK junko...I am sorry this got so long winded...but I hope it helped explain some things and the protocol for diving nitrox in the Cozumel environment.
"Deep" generally means Punta Sur and the like, at approx 120ft for a couple minutes max. If this is what you mean by deep, then there ya go.
