Doubles with Nitrox

[dougchartier]

I just got back from a great liveaboard vacation in the Galapagos, and I noticed that the nitrox fills on our boat were pretty inconsistent (e.g., 28-37% O2 between tanks on the same dive). This made me wonder what numbers a diver would use if he were diving doubles where each cylinder had a different percentage of oxygen.

What's best practice for this scenario? Use the richer number for MOD and the leaner number for NDL? Average the numbers?

BTW, I've never dived doubles and (regrettably) know little about it -- I'm just curious.

Thanks for sharing your knowledge!

 

[Rhone Man]

Doubles are usually connected, so the gas mix will ordinarily be the same in both cylinders.

 

[Thalassamania]

Doubles are usually connected, so the gas mix will ordinarily be the same in both cylinders.

My understanding is that it can take quite some time to mix. I'd be inclined to MOD on the richest mix and calculate my decompression on the poorest.

 

[TSandM]

Doubles are usually filled through only one post, so you get the same mix in both tanks. The only way to get different mixes is either to use independent doubles or to fill with the isolator closed, a situation which is quite dangerous.

If I somehow ended up, through a shop error, with different mixes in the two tanks, I'd do what Thalassamania said, AFTER I got through thoroughly discussing how this happened with the shop.

 

[Thalassamania]

TS&M, some of us dive independent doubles so the situation is not that uncommon.

 

[Scott L]

Doubles are usually filled through only one post, so you get the same mix in both tanks. The only way to get different mixes is either to use independent doubles or to fill with the isolator closed, a situation which is quite dangerous.

If I somehow ended up, through a shop error, with different mixes in the two tanks, I'd do what Thalassamania said, AFTER I got through thoroughly discussing how this happened with the shop.

With the partial filling method would not the closest cylinder to the post accepting gas receive a larger percentage of O2? If so, how long before equalibrum? I am probaly wrong though...

 

[Gombessa]

If I somehow ended up, through a shop error, with different mixes in the two tanks, I'd do what Thalassamania said, AFTER I got through thoroughly discussing how this happened with the shop.

Assuming this was caught before the dive (which I personally would only be able to catch at the shop through ensuring the isolator knob is open prior to analyzing, since I don't usually analyze both posts), I'd drain the tank and have the shop refill correctly, rather than dive different mixes. It seems like the situation where you'd know the mixes in both tanks, and be in a position to confront the LDS first, would afford you the opportunity for a do-over, instead of accepting it and going off to dive, right?

I've had shops try to hook up fill whips to both posts, as well as attempt to close the isolator knob prior to filling (both cases involved employees who had never dealt with doubles before), so I can't imagine this scenario is unheard of.

 

[dougchartier]

TS&M, some of us dive independent doubles so the situation is not that uncommon.

Thanks for the information, guys. Shows how little I know about doubles...

For my own benefit, what are "independent doubles"?

 

[Jorgy]

That seems like a huge difference....

What were tanks supposed to be filled with????

I tell my LDS what mix I want to match my dive plan......they get the mix to within +/- .5% of what I need.....

That range makes for big differences in MOD.....

M

 

[iztok]

With the partial filling method would not the closest cylinder to the post accepting gas receive a larger percentage of O2? If so, how long before equalibrum? I am probaly wrong though...

Why?

If you start from empty for example you fill to XXX PSI of O2 both tanks will have same pressure (assuming one post filling with open isolator valve). Now you top with air. So yeah pretty much you have the same mix in both.