Buoyancy Question

[TSandM]

I agree that your friend is just simply wrong.

I also want to point out that, if you miss enough pre-dive checks to end up in the water with your isolator closed, you do NOT want to open it! You do NOT know what is in the other tank -- it may contain enough helium or oxygen that, when mixed with the depleted contents of the other tank, you can get hypoxic or tox. Discovering a closed isolator is a reason to abort a dive.

 

[Paleface]

TSandM- That is an interesting point. I have learned so much from reading here and other places in the short time that I have been diving. I have just started reading the TDI manuals for advanced nitrox and decompression diving. I assume this would apply when diving trimix?? If I was diving nitrox and left my isolator closed does the mix not stay equal? I thank you ALL for your input and advice!!!!
Have a great Thanksgiving everyone!!!!! Keith

 

[dkktsunami]

You are correct and he is likely improperly weighted.

 

[rongoodman]

TSandM- That is an interesting point. I have learned so much from reading here and other places in the short time that I have been diving. I have just started reading the TDI manuals for advanced nitrox and decompression diving. I assume this would apply when diving trimix?? If I was diving nitrox and left my isolator closed does the mix not stay equal? I thank you ALL for your input and advice!!!!
Have a great Thanksgiving everyone!!!!! Keith

Unless you analyzed both posts before the dive, you wouldn't know the MOD of the mix in the other tank--it might be a left over rich mix from a previous shallow dive, and now you're on a deeper wreck thinking you have a weak mix in both tanks. You don't necessarily know when the isolator was closed, before or after filling.

 

[Hickdive]

Confirming that the isolation valve is open is part of the process of analysing the contents of a twinset. Though if you're the kind of person to miss that step out then aborting the dive is the best thing to do under the circumstances if you open it underwater and hear the cylinders balancing.

However, if you ensure that the isolator is open when you analyse (and you did check that too when handed the set over for filling or when you started to homebrew) then hearing the cylinders balance if you inadvertently left the isolator shut from transit and didn't re-open it isn't a major problem.

Having a single SPG on the left post and having your primary on the right post will also serve as a useful warning that the isolator is closed.

 

[TSandM]

If you get in the water and discover the isolator is closed, you have no idea what is in the tank you didn't analyze, assuming you didn't analyze both (which few people do). If the shop was filling Nitrox and put all the O2 in one side, and that wasn't the side you analyzed, you could have a potentially toxic mix. If they put all the helium in that side, you could end up with a seriously hypoxic breathing mix. Assuming pressures are equal between the two sides, there is not much mixing that goes on between tanks (Rick Murchison actually did an experiment to assess this). But as you breathe off the right post, you are drawing gas from both tanks in a roughly equal proportion. That mix may or may not be survivable.

There is a reason that flow checks are part of the pre-dive procedure for people diving doubles with manifolds.

 

[Hickdive]

Assuming the pressures are equal on either side and not much mixing occurs when the isolator is opened then whether it is opened on the surface or underwater still leaves you with the potential problem.

Unless you did the mix yourself or observed it being done and you know the isolator was open at the appropriate times then you need to analyse both posts.

Abandoning a dive underwater because you failed to do a simple thing like that and can't be sure that hearing the cylinders balance doesn't mean a dangerous mix strikes me as a waste of everyone's time and effort.

Getting it sorted on the surface before you dive makes life easier and safer.

 

[Paleface]

Thanks again. With my limited experience with doubles I never close my isolator valve,If you close the tank valves for transport it doesnt seem to matter so why close it. I will always check it but a shop would have to close it before they fill my tanks for me to have a problem.
My buddy was diving air in a local quarry and didnt open his isolator valve until after the dive so this was not an issue. Keith

 

[Paleface]

I'm not sure how he has his doubles set up because I have not dove doubles with him. That being said it would be very difficult for me to do a dive for any length of time breathing from my primary right tank and reading my spg on the left and not realize a problem. Keith

 

[IXΘYΣ]

...he didnt realize until after the dive when he went to turn off his isolator...

???? Why would he "turn off his isolator" after a dive? Secondly, if he didn't realize the tanks were isolated, he either didn't get alarmed at his exorbitant gas consumption (or his apparently stuck SPG), or he didn't monitor his gas consumption at all...
Except for shutdown during an emergency or the occasional functionality check, (and that should be taken care of during regular valve drills) the isolation valve should remain open. Open for fills, open for diving. And it should be checked open for fills; open for diving.

...I have 8 dives on my double 100's and I wear 4lbs in fresh,8lbs in salt water...

If you're a lightweight (like me) and wearing small doubles then that's about right, but I have in my mind that you're a man and you're diving double 100's, so your total weight (average guy) with your doubles and a drysuit & BP & wing is around 240... and if so then if 8 pounds is good in salt water, 2 will be right in fresh water. (Salt water weighs about 2.5% more than fresh, so your weighting difference for perfect buoyancy - same rig=same volume of water displaced - when you move from one to the other is 2.5% of your total weight)
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