Unknown Tourist dead, Dive Master ill - Ambergris Caye, Belize

[boulderjohn]

This is from the Undercurrent article quoted a few pages ago, and I would like to know why this is true:

It's more likely that the nitrox mix is suspect. This can happen especially if the nitrox is made
by partial pressure blending (when neat 02 is added to a tank and then
topped up with air) rather than de-nitrogenizing air through a membrane system.

Does gas mixing this way throw off the analyzer?

I could be wrong here, but I understood this to suggest that the mix was wrong and then was not analyzed. I have been on more than a few boats in tropical locations where I was expected to accept what I was given and had to ask for an analyzer.

If that is what they mean, then yes, it is much, much easier to screw up a nitrox mix with partial pressure blending than with a membrane system. Such screwups should be caught during analysis.

 

[hiker11]

The above discussion raises some questions for the novice. 1) for partial pressure mixing is the tank emptied first then the O2 and Air are added or is the mix form the previous dive in place and the O2 and air added on top? 2) 50% mix was mentioned above, I thought nothing over 36% was used? Does the 50% number refer to something else?

thanks,

 

[inquis]

The above discussion raises some questions for the novice. 1) for partial pressure mixing is the tank emptied first then the O2 and Air are added or is the mix form the previous dive in place and the O2 and air added on top? 2) 50% mix was mentioned above, I thought nothing over 36% was used? Does the 50% number refer to something else?

thanks,

1. Either. Top up requires the filler analyze & punch a few numbers into a calculator.

2. Nitrox 50% is a common gas for accelerating mandatory decompression stop diving. Pure O2 is as well. The first level nitrox course lets you use up to 40%, but that starts being a concern (CNS toxicity) as a bottom gas at 82 fsw (which is pretty shallow). As such, 36% (95 fsw max operating depth) and 32% (111 ft MOD) are far more common.

 

[TrimixToo]

The above discussion raises some questions for the novice. 1) for partial pressure mixing is the tank emptied first then the O2 and Air are added or is the mix form the previous dive in place and the O2 and air added on top? 2) 50% mix was mentioned above, I thought nothing over 36% was used? Does the 50% number refer to something else?

thanks,

Some shops always start from scratch, for a couple of reasons. (a) It's simpler. (b) They don't know who filled it last, necessarily, so they know the fill is *their* fill and believe they are less exposed to liability.

I don't, because (a) it's noisier, (b) it doesn't really complicate things by much (Multi-Deco's gas blending functions are quite good and I know where to fudge them to save time and have mixes come out right), (c) it saves a bit of money (especially for trimix!), and (d) I am generally the one breathing what I mix, so any liability I care about would literally die with me.

All this is really academic for you unless you plan to learn to blend for yourself. I highly recommend taking a course from someone experienced. Lots of handy hints and tips don't necessarily make it into the books.