Nitrox Is it customary to reanalyze it at the dive boat?

[Northeastwrecks]

I understood that O2 sensors are sensitive to PP% but the construction of the sensor head is such that the reading is taken at very close to ambient pressure. The gas under test just flows across the sensor. So calibrating at ambient pressure is correct and sufficient. Altering the flow rate should have no significant effect.

The idea of having a cylinder of air to calibrate the sensor just adds another unknown variable.

While I don't disagree about the construction of the sensor, the flow rate does appear to make a difference.

When we teach nitrox, we show the students the effects of changing the flow. We can induce at least a +/- two percent difference by changing the flow.

 

[dweeb]

When you’re ready, you gear is ready. If that’s air you’re diving, there is no issue.

Do I disconnect my reg and analyze before every dive? Does every one of the twelve guests/divers and 3 DM on that liveaboard analyze every time? Becomes troublesome… All 15 tanks were just filled from the same source. Perhaps, it’s sufficient to analyze just one tank... That nitrox station has a built-in analyzer, and the tanks were just filled from it. Do I need to analyze at all? Paranoia? Why wouldn't I analyze air then if the same liveaboard, same crew were puming air?

I would. Think about it for a minute. The boat is pumping more than one kind of gas. A friend of mine worked every position on a liveaboard crew except cook, and he said he never worked so hard in his life. There are boat issues, guest issues, dive issues, and they all may pop up at any time, including while filling tanks.

"Now then, was it the white tank in the blue Mares BC on the left that was supposed to get 32%, or the white tank in the blue Mares BC on the right? Well, gee, I think it was the one on the left, but you know, what with scrambling because the anchor came loose and cleaning up after the seasick guest, and yada yada yada, I can't be sure, but these people want to get back in the water right after lunch, in about 10 minutes, and my instinct is it was the one on the left, and the sooner I get them in the water the sooner I can eat."

But hey, if you exceed MOD, it's only your brain that gets fried. Anyone who thinks they don't need to analyze their own tank won't be much worse off for a little more brain damage.

 

[dweeb]

I'm headed for a trip to Cozumel, and I wanted use Nitrox. This would be my first trip to use it, and when I asked about analyzing it at the boat, I was assured that it would be mixed by a certified center.

What part of the mixer certification course covers when you've lined up a bunch of similar looking tanks for filling, and know that the first two get 32% and the next two get air, and the next two get 28%, then two customers come in the door wanting to sign up for a charter, but one's forgotten his C-card, so you have to look him up on the web, and the phone rings a couple times, and someone needs to move something past the fill station, so they move those tanks around, and then....

It has very little to do with incompetence at achieving the desired mix in a tank, or with intentional malfeasance.
It's about getting the desired mix into the RIGHT tank, in a fill area where there's never enough room, never enough time, and never enough slack in demands on one's attention.

One guy asks for air, and you ask for 40%. Neither of you analyzes. He toxes, and you get narced and bent. Well, at least you're better off than he is. Maybe. "But hey, all our tanks are the same color, and all you gringos look the same."

Get an analyzer. Find out if someone else on the trip has one they can bring. Yeah, it might cost a little more, but if that's an issue, maybe you should find an activity where being too cheap can't get you killed, like bowling.

 

[nwdiver2]

It has to be analyzed by you when you pick it up from the shop or by you on the boat, you should have learned this in your class, shame on your instructor. On my boat I calibrate the analyzer to 21% then all those who are diving nitrox check their tanks at the analyzing station. My bank only does 36% but I still have everyone analyze and sign off their tanks. If the boat is supplying the nitrox they must (in my mind) give you the ability to analyze it.

People who bring their own mixes also have to right to use my analyzer on the boat.

 

[dweeb]

While I don't disagree about the construction of the sensor, the flow rate does appear to make a difference.

When we teach nitrox, we show the students the effects of changing the flow. We can induce at least a +/- two percent difference by changing the flow.

Some units take a static measurement, and flow isn't a factor.

 

[rcohn]

It's obvious that I analyze my tank when picking it up at a shop or any place when it is filled a la carte. But, I think, the original poster's question is referring to a different situation.

Imagine a liveaboard; let’s say 10 days, 5 dives a day, 12 divers, 3 divemasters. You assemble all your gear once. Then after every dive while you are off-gassing, having food/snacks, filling logbooks, doing whatever, dive deck staff refill the tanks, they don’t even need to take them off BCs.

When you’re ready, you gear is ready. If that’s air you’re diving, there is no issue.

Now, imagine that liveaboard has a membrane nitrox station, and instead of air they are pumping 32% nitrox.

Do I disconnect my reg and analyze before every dive? Does every one of the twelve guests/divers and 3 DM on that liveaboard analyze every time? Becomes troublesome… All 15 tanks were just filled from the same source. Perhaps, it’s sufficient to analyze just one tank... That nitrox station has a built-in analyzer, and the tanks were just filled from it. Do I need to analyze at all? Paranoia? Why wouldn't I analyze air then if the same liveaboard, same crew were puming air?

I’ve never been on a liveaboard with nitrox station. But, returning to the original question, what’s the custom?

I've done three liveaboards with nitrox. EVERYONE analyzes their tanks and records the result in the log before each dive. Fills do vary between dive to dive as much as 1 or 2 percent. More is likely when partial pressure fills are done.

Ralph