First Time Nitrox

[BullDiver]

So I got my nitrox certification before my dive trip because I was planning on doing 4 dives in one day and I thought it would help extend those last couple of dives. Conditions weren't great so the first 2 dives were canceled. So now were down to 2 dives, the first one was a wreck dive 70ft to the sand, and the second was a reef dive max depth of 50ft. Being a new diver still working on my air consumption, I knew for a fact I would run out of air before getting close to NDLs even on air for these dives. But I asked the guy at the shop what he suggested and he said he always suggests Nitrox. So my first question is, for these particular dives, would I have been better off diving air and saving a couple of bucks (we did 1hr surface interval because there were people in our group on air).

Secondly, I was analyzing my tanks with the analyzer at the shop and the first tank I analyzed came up at 33.5%. I asked the DM if I should consider it 33 or 34 (just for confirmation since this was my first time using nitrox) and she said "It doesn't matter either way, you can even put 32 if you want." I knew it didn't matter much for our dive plan if it was 32 or 34 since we weren't going to be anywhere near our MOD, but is that okay for her to say that or am I being paranoid about it? (I wrote 34% on the tank and put 34 in my computer for the dives).

 

[Storker]

I asked the DM if I should consider it 33 or 34 (just for confirmation since this was my first time using nitrox)

If you'd paid attention during your nitrox class, you wouldn't have needed to ask that question.

Spoiler

You always round up

 

[uncfnp]

Question 1. You would not "have been better off" but yes you could have done the dives on air (as some others did) and "saved a few dollars." I personally would have still dove nitrox because I like the extra edge from the NDL it gives.

Question 2. For MOD I round up but for NDL I round down. For the log, just put the actual reading.

 

[tbone1004]

@Storker has the wrong spoiler in there imho. You use both. You use 34% for the MOD, but 33% for the EAD when doing it by tables. In that situation I would set my computer at 33% and calculate the MOD for 34% and use that as my depth limit.

regarding whether you should dive air or nitrox, if doing 4 dives a day, at 70ft I'd use nitrox, 50ft I'd use air. Depends on the dive and what I'm getting the benefit from. I have the advantage of not caring about NDL's, so if I go into a mandatory 3-5 minute deco stop, I can chalk it up to being overtly cautious on my safety stop

 

[Storker]

@Storker has the wrong spoiler in there imho

PADI doesn't agree with you.

In that situation I would set my computer at 33% and calculate the MOD for 34% and use that as my depth limit.

My computer doesn't accept fractional percentages. Round up or down? Ox-tox risk is more serious than risking overstepping my bottom time ever so slightly by assuming my gas is 0.5% richer than it really is.

Besides, like the OP, I'm usually gas limited, not saturation limited.

 

[Bubblesong]

How did you feel after diving? There is a rumor that some people feel less tired.

 

[tbone1004]

@Storker i don't give a flying what PADI says, they're 100% wrong. You ALWAYS round to safety. PADI is telling you to round to safety for toxicity, but away from safety for decompression. Very surprising considering how liberal their tables are.
With O2% that is up for MOD, down for EAD. You manually calculate the MOD based on rounding up, and put the rounded down number in your computer for decompression calculations. It may not matter in recreational diving, but it can change a surprising amount of deco in technical diving.

@Francesea that rumour has evolved from people diving nitrox as if they were on air so they have less nitrogen loading. If you dive nitrox to the NDL's then if you feel different it's a placebo since you have the same theoretical nitrogen loading in your tissues

 

[Storker]

It may not matter in recreational diving, but it can change a surprising amount of deco in technical diving.

We're in Basic. This is rec, not tec. And PADI's basic nitrox course is for rec divers, not tec.

The OP didn't even know whether they should round up or down. At that level, rounding up is safer than rounding down. In rec, most - if not virtually all - divers fly their computer, and with most rec computers, you have to round the numbers.

 

[tbone1004]

We're in Basic. This is rec, not tec. And PADI's basic nitrox course is for rec divers, not tec.

The OP didn't even know whether they should round up or down. At that level, rounding up is safer than rounding down. In rec, most - if not virtually all - divers fly their computer, and with most rec computers, you have to round the numbers.

and in rec they should still be teaching to round to safety. To me that's their same cop out make it easier not safer bullsh!t as when they teach you to use your drysuit for primary buoyancy control. It's not safer, it's just easier. Telling you to always round the nitrox percentage up is not safer, it's easier. No tech computer I know of allows you to put fractions of a percentage in either *rebreathers aside obviously since they're measuring*, but you should still round to safety. That means manually calculating MOD, and using a different value to enter into the computer for decompression calculation, not taking the "easy" way out by rounding everything in the same direction which is more dangerous, made especially risky with how liberal PADI's dive tables are

 

[Bubblesong]

@Storker i don't give a flying...

Wasn't the complete quote "...at a rolling donut"?

Well I agree about safety rounding so that is the direction I am taking.