Need some help with a depth and time

[rjack321]

I believe you and I have similar training. I know I wasn't trained to use gas plans that kinda work out.

I don't do 300' dives either. But even at 230' my minimum gas (to get 2 divers from the bottom to the first gas switch, and cover the possibility of a lost deco gas) is about 130 cu.ft.

I realize that people execute these kinds of dives in very different ways (solo, travel mixes, different deco gases, etc.) but even so I would think that using a more deliberate way to calculate reserve gas than saying 1/3 of back gas (or bottom gas) would be prudent.

If you're on the bottom at 300ft with 12/60 and your first deco gas is 15/55 at 270 it all kinda works out

 

[Blackwood]

I believe you and I have similar training. I know I wasn't trained to use gas plans that kinda work out.

I don't do 300' dives either. But even at 230' my minimum gas (to get 2 divers from the bottom to the first gas switch, and cover the possibility of a lost deco gas) is about 130 cu.ft.

I realize that people execute these kinds of dives in very different ways (solo, travel mixes, different deco gases, etc.) but even so I would think that using a more deliberate way to calculate reserve gas than saying 1/3 of back gas (or bottom gas) would be prudent.

Yah. Personally I use rock bottom. I'm just saying that given the relationship between gas supply, consumption rate and available bottom time, and the relationship between bottom time and decompression obligation, it's not out of the realm of plausibility that starting your ascent at a given percentage of your total bottom gas supply will work to cover lost deco gas.

Whether it covers two divers to the first switch (a less bottom time-dependent quantity) seems less likely, but not everyone plans dives that way.


Also: camera housing (to stay on topic )

 

[cfenton]

If you're on the bottom at 300ft with 12/60 and your first deco gas is 15/55 at 270 it all kinda works out

Would you do you first gas switch at 90% of average bottom depth??

I'm just saying that given the relationship between gas supply, consumption rate and available bottom time, and the relationship between bottom time and decompression obligation, it's not out of the realm of plausibility that starting your ascent at a given percentage of your total bottom gas supply will work to cover lost deco gas.

Whether it covers two divers to the first switch (a less bottom time-dependent quantity) seems less likely, but not everyone plans dives that way.

OK, I will begrudgingly agree with both of you that it is possible to make it work. But, I would still not want to be at 300' with only 85 cu.ft. left in my back gas. Call me crazy. :wacko:

Also: camera housing (to stay on topic )

Yeah, sorry about that.

 

[rjack321]

Would you do you first gas switch at 90% of average bottom depth??

Hell no! But heck some folks add a 190ft bottle for not especially deep dives in the 250-ish range that are constrained by limited backgas (ie. 108s/130s are not available)

Anything's possible even if its suboptimal...

 

[Meng_Tze]

Oh brother, here we go....let me make this real simple. Decend to 300' going in your planned direction until you use 1/3 of your gas....turn around staying at that depth until you use the other 1/3 of your gas, you should end up about where you started. Once the 2/3 is used the dive is over and you start your deco/accent. The remaining 1/3 is planned for emergencies.

With only 1/3 of BG for an emergency while IN deco obligations from that deep; you are not going to make it if you actually need to use that gas. So no, it is not 'real simple'.

 

[landonnin1]

I was just asking for some avg. time people could stay at that depth I add in what I was doing with theses numbers so everyone would know I was not trying to dive it. All of you have added some good info for me and I an grateful for this. I know that the dive time has nothing to do with the structural integrity of the camera housing but if I could leave the housing at 200.45 psi for a day then I would know it would hold up but that would take allot of time to test the housings.I could run the vessel up to a higher pressure but if i can use a lower psi the less stress on the vessel. (the vessel is build to Handel the 300 psi ) with any thing lower the pressure the safer it is. that is was why I was asking.Most people that use the housings only dive to 300-350 max.thank you guys again for all the info
Matt Landon DM

 

[The Chad]

For what the orig. question from the OP was...yeah it's real simple. This was not an exercise in dive planning. But since theres such an interest in that, lets go ahead and start another thread so everyone can show why their way is the right way. New thread is "dive planning @ 300" And to think I almost forgot why I quit posting.

 

[Blackwood]

And to think I almost forgot why I quit posting.

To me the ability to have conversations progress is one of the great things about these collaborative boards. If I wanted a strict FAQ, I'd look elsewhere. I understand that using them conversationally may make things harder to index and locate, but on the other hand holding everyone to a specific topic within a thread is going to substantially decrease the quantity of discussion.

 

[cfenton]

Not to mention landonnin1 pretty much had his answer by post #9.

 

[landonnin1]

guys I am good with all of this info,cfenton you are right about the 9th post but I seem to keep checking back just to see what has been add because of where this thread is going

again thanks to all of you
Matt Landon DM