2 Tank Dive, Nitrox for Both?

[ChuckP]

Our experience doesn't usually have the second dive that shallow. If it is, use air for it.

A second dive that shallow is snorkeling or shore diving - heck on a putzing around shore dive my average depth is deeper than that almost

 

[PBcatfish]

Assuming that I had good confidence in my ability to identify which tank had which mix, I would dive nitrox on the 90' dive & air on the shallow dive. If I had any doubts about telling the two tanks apart, I would bring just nitrox for both.

By the time you pay for transportation to a resort, lodging, etc., the extra $8 doesn't really show up on my radar.

On dives to 30', with no other dive after it that day, I see no advantage to nitrox. On dives much deeper than 90', you do need to watch your O2 partial pressure when using nitrox, especially if the mix is rich. Those would be my constraining considerations.

 

[ChuckP]

Assuming that I had good confidence in my ability to identify which tank had which mix

If we're Nitrox certified, we know how to test tanks - and in these tough economic times, I'd certainly be testing tanks.

 

[yle]

Do folks use Nitrox on the first dive, then switch to air for the shallow dive? Is it better to just use Nitrox for both, even if there's a not a benefit for shallow dives? I plan on diving 6 - 7 days.

My two cents...

Our typical Cozumel trip includes a dive plan just like yours, and we usually use one nitrox, one air each day. Our dives are usually around 100 ft max for the first, 60 max for the second.

Conventional wisdom might lead you to believe that it would make sense to use the nitrox on the first dive, but I've run the numbers both ways and found out that overall dive time is greater when using air for the first dive and nitrox for the second. I was surprised to see this, but if you do the calculations (I just used tables...) it's right there. Essentially the nitrox helps extend your bottom time for the second dive to just about where you'd want it to be. Of course this is assuming your second dive is in the 60 ft range. If you're actually diving 25 to 40 feet, the numbers change.

Also, using air for the first dive keeps you from worrying about MODs.

As for testing tanks... when we've used nitrox in Coz we have rarely been offered an analyzer or logbook. Seems to be standard practice that the shop orders nitrox and trusts that they're filled correctly. I always bring my own analyzer and test my tanks (and my mom's... gotta take care of mom.) They're usually correct, but I have found some of my "nitrox" tanks were filled with air. Why? I'm guessing that since the people filling tanks on the island deal with thousands a day, there's bound to me a mistake every now and then.

 

[BRT]

My two cents...

Our typical Cozumel trip includes a dive plan just like yours, and we usually use one nitrox, one air each day. Our dives are usually around 100 ft max for the first, 60 max for the second.

Conventional wisdom might lead you to believe that it would make sense to use the nitrox on the first dive, but I've run the numbers both ways and found out that overall dive time is greater when using air for the first dive and nitrox for the second. I was surprised to see this, but if you do the calculations (I just used tables...) it's right there. Essentially the nitrox helps extend your bottom time for the second dive to just about where you'd want it to be. Of course this is assuming your second dive is in the 60 ft range. If you're actually diving 25 to 40 feet, the numbers change.

Also, using air for the first dive keeps you from worrying about MODs.

As for testing tanks... when we've used nitrox in Coz we have rarely been offered an analyzer or logbook. Seems to be standard practice that the shop orders nitrox and trusts that they're filled correctly. I always bring my own analyzer and test my tanks (and my mom's... gotta take care of mom.) They're usually correct, but I have found some of my "nitrox" tanks were filled with air. Why? I'm guessing that since the people filling tanks on the island deal with thousands a day, there's bound to me a mistake every now and then.

So far I have not found a tank that wasn't within a couple percent of what it was labeled. I still test. We dive 2 tanks of nitrox. It avoids being pushed out of the water on either dive. The first dive is often nitrogen limited and a second dive with nitrogen built up in the body is often nitrogen limited if diving air. If we dive 32% we are always far away from NDL's and that is where I want to be at my age.

 

[Brett Hatch]

What the hell kind of operation are they running where they do not provide analyzers for customer use? That is a gigantic red flag. You breathe the wrong gas, you die.

 

[Barnaby'sDad]

So far I have not found a tank that wasn't within a couple percent of what it was labeled. I still test. We dive 2 tanks of nitrox. It avoids being pushed out of the water on either dive. The first dive is often nitrogen limited and a second dive with nitrogen built up in the body is often nitrogen limited if diving air. If we dive 32% we are always far away from NDL's and that is where I want to be at my age.

I got one tank that was off by 10%. I asked for 30% and got 40%. In the case of the place in question...2%+ off wasn’t uncommon (my dive buddy ran into similar discrepancies at the same place).

One other place...I got a pair of tanks that was 5% off. I asked for 32% and got 27%.

The second example was an honest mistake...new kid filling the tanks (accidentally filled with hyper air instead of the 32% bank).. The first one...his attitude was essentially that “you have to analyze them, so...”

Always test.

What the hell kind of operation are they running where they do not provide analyzers for customer use? That is a gigantic red flag. You breathe the wrong gas, you die.

One dive op that I used recently updated their website to say “if you’re diving Nitrox, you should have your own analyzer.”

 

[BRT]

I got one tank that was off by 10%. I asked for 30% and got 40%. In the case of the place in question...2%+ off wasn’t uncommon (my dive buddy ran into similar discrepancies at the same place).

One other place...I got a pair of tanks that was 5% off. I asked for 32% and got 27%.

The second example was an honest mistake...new kid filling the tanks (accidentally filled with hyper air instead of the 32% bank).. The first one...his attitude was essentially that “you have to analyze them, so...”

Always test.



One dive op that I used recently updated their website to say “if you’re diving Nitrox, you should have your own analyzer.”

I have my own analyzer. Actually I probably have 4 or 5 in working order. I agree, always test.

 

[Brett Hatch]

I got one tank that was off by 10%. I asked for 30% and got 40%.

That's utterly insane and irresponsible.

Edit to add: If you asked for 30%, you might well dive down near the MOD: 121 feet for 1.4 PPO2. 121 feet on 40% is 1.88 PPO2, which is well into the "can definitely kill you" range.

One dive op that I used recently updated their website to say “if you’re diving Nitrox, you should have your own analyzer.”

Sound advice, everyone probably should have their own analyzer. But not offering analyzers to customers is pretty far from OK in my book.

 

[Barnaby'sDad]

That's utterly insane and irresponsible.

Edit to add: If you asked for 30%, you might well dive down near the MOD: 121 feet for 1.4 PPO2. 121 feet on 40% is 1.88 PPO2, which is well into the "can definitely kill you" range.

Sound advice, everyone probably should have their own analyzer. But not offering analyzers to customers is pretty far from OK in my book.

Yup. The tank was fine for where I was going to be diving, so I marked it and took it. But yeah...for the average person that asked for 30%, that wouldn’t have cut it. The reason I tended to ask for 30% is that if you wanted ~32%, you had to ask for lower.

Agreed. The reason that I put the though is that their tone...given some of the other comments on their website...was along the lines of “not our problem.”