Need some help with a depth and time

[Blackwood]

Ahh yes, the difference being cave diving and open water diving. Since we are not in an overhead environment thats when we turn and deco is done on the way back. Cave fills also helpful but not everyone does them.

While you may turn after using 2/3 of your gas, it's not "the rule of thirds."


FWIW, I had 320CF of breathing gas on me this weekend without overfills (double 100s, 80CF stage, 40CF deco). I mention it because your post regarding what fits in a set of large doubles doesn't really have anything to do with how much gas most open circuit divers planning 300' dives will bring (at least not those I know).

 

[DCBC]

I'm not really sure I'm following you on what you want to be accomplished by all this. BTW, 300 psi is approx 640 FSW (1 FSW= .4454545 psi plus 14.7 psi atmospheric).

If your looking at the theory, the length of time a dive to 450 FSW would take would depend upon: the mixture, the BT, the tables and the ascent protocols that you choose to follow.

I don't see how the dive length matters to the structural integrity of the camera housing. It either is strong enough or it's not.

 

[rjack321]

Ahh yes, the difference being cave diving and open water diving. Since we are not in an overhead environment thats when we turn and deco is done on the way back. Cave fills also helpful but not everyone does them.

Too bad your 1/3rd remaining isn't enough to get you home to 1 ata.

Seriously none of this has much to do with pressure testing camera housings.

The OP wants to overpressurize by some sort of guesstimated safety factor. Try if he can to actuate the buttons while pressurized to ensure those don't leak. And simulate dynamic water movement as cave_diver notes if possible.

Many/most plastic housings are only rated to 200ft.
The AL housings have deeper ratings but there's a point where the spring tension required to keep buttons from pressing at depth is so great that the buttons aren't usable on the surface. This ends up being the limiting factor for most housings.

 

[The Chad]

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.

When I'm talking about 260 cuft of gas, I'm talking about backgas (what your carrying on your back) Because in the orig. post he was talking about time at depth (300') with a .4 sac you end up around 43min....I'm not saying you should/could stay that long.

As far as your comment about "what fits in a set of doubles...." Gas planning has everything to do with OC divers planning a dive to 300'. The 260cuft would basically be cave filled 108's @3200psi Then you have a travel gas since your backgas is hypoxic, and 2 deco gases. We can couch dive this all day...but I think he got what he was looking for.

 

[Blackwood]

We can couch dive this all day...but I think he got what he was looking for.

I think was he was probably looking for is posts from people who regularly do 300 foot dives, and how long they stay at depth.

As such, I shouldn't be posting at all, but what's the fun in that?

 

[cfenton]

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.

When I'm talking about 260 cuft of gas, I'm talking about backgas (what your carrying on your back) Because in the orig. post he was talking about time at depth (300') with a .4 sac you end up around 43min....I'm not saying you should/could stay that long.

As far as your comment about "what fits in a set of doubles...." Gas planning has everything to do with OC divers planning a dive to 300'. The 260cuft would basically be cave filled 108's @3200psi Then you have a travel gas since your backgas is hypoxic, and 2 deco gases. We can couch dive this all day...but I think he got what he was looking for.

I was going to leave it alone at wow, but...

Using the rule of thirds in the way that you are suggesting seems pretty arbitrary. Would you use that gas plan at any depth? With any tanks? I prefer a more deliberate approach to gas planning. We'll often use thirds (or something similar) on an wreck penetration. But, but before we calculate thirds we're going to subtract our reserve for the ascent. Regardless of the dive objective, on a 300' OW dive I would want a lot more than 85cu.ft. as a reserve.

 

[The Chad]

Thanks for the "shake of shame". If you want to discuss this further, may I sugguest a new thread? There is nothing arbitrary about it....it's not my desire to write a long dissertation or plan the entire dive. Thats what training is for. The OP asked a question and I simply made the point that not a lot of people are going to be at that depth very long. Not sure if he's trying to figure out a market for a camera housing he is making or not. Wreck penetration has different things to consider but gas planning is the same. What you would want is your deal, and at this level anyone can opt out at any time for any reason...no questions asked. I was on the same boat with a guy that solo'ed to 430' obiously not on OC...would I do it? No, but its not my place to judge either.

 

[cfenton]

Thanks for the "shake of shame". If you want to discuss this further, may I sugguest a new thread? There is nothing arbitrary about it....it's not my desire to write a long dissertation or plan the entire dive. Thats what training is for. The OP asked a question and I simply made the point that not a lot of people are going to be at that depth very long. Not sure if he's trying to figure out a market for a camera housing he is making or not. Wreck penetration has different things to consider but gas planning is the same. What you would want is your deal, and at this level anyone can opt out at any time for any reason...no questions asked. I was on the same boat with a guy that solo'ed to 430' obiously not on OC...would I do it? No, but its not my place to judge either.

Points taken.:blush:

I've just never heard of anybody using the "rule of thirds" in the way that you are describing. It doesn't seem logical to me. It seems to be missing the entire point of the rule. But, as you say to each, their own.

 

[Blackwood]

Points taken.:blush:

I've just never heard of anybody using the "rule of thirds" in the way that you are describing. It doesn't seem logical to me. It seems to be missing the entire point of the rule. But, as you say to each, their own.

It can kinda work out since your decompression obligation is related to how long you stay, and how long you stay is in turn related to how quickly you consume your gas.

 

[cfenton]

It can kinda work out...

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.