CAVE DIVING DOUBLES...Largest PRACTICAL sized tanks?

[Dive-aholic]

hit the gym Rob

LOL! No need. My RMV is less than .4 with smaller cylinders. In sidemount, even with larger cylinders, it stays less than .5. In backmount it shot up to .7 with 108s! :shocked2:

you also need to consider the amount of reserve gas you have, and think about gas as it relates to time at depth.

You might get just as far on smaller tanks (or maybe not), but you certainly have less gas to deal with issues.

True, but if your penetration is limited based on how much gas you want for exit and emergencies, then it's all calculated in there. And, fortunately, my RMV doesn't go up under duress anymore.

 

[PfcAJ]

I hope your buddy is the zen master that you are...

 

[Dive-aholic]

What buddy??

 

[PfcAJ]

That explains it...

 

[CamG]

I find simular findings as Rob with tank size relative to RMV.
I own both 95's, 85's, and have used 119's / 108's.
If I need more gas I carry a stage and or use larger tanks.

I used what I had till I could get the tanks I needed just that simple.
Never had to many issues adapting to what was available.
RMV, Distance per PSI, proper trim, gas reserve volume, dive plan gas requirements all factors we all consider before we dive.
These are all part of my dive planing process.

CamG Keep Diving....Keep Training....Keep Learning!

 

[Doc Harry]

I can barely climb stairs with twin banded 130s.

But then I'm old.

 

[DA Aquamaster]

The numbers vary a bit from brand to brand, but in general:

A Worthington LP 95 has a nominal capacity of 93.3 cu ft at 2640 psi, while a Faber LP 95 holds 95.1 cu ft at 2640 psi. At 3600 psi when doubled, that capacity is 254 cu ft and 259 cu ft respectively.

With a 3500 psi fill, you lose 7 cu ft from both, getting the Worthington LP 95 in the 245 cu ft capacity range AJ references. However, a 3500 psi comparison doesn't illumate the issue much as you'd round down to down to 3300 psi for dive planning anyway - using a tank factor of 7 to under estimate only 231 cu ft available with 77 cu ft "thirds".

In practice, with all of them filled to the same 3600 psi, double LP 95s and HP 119s are both in the 260 cu ft ball park, HP 130s are in the 275 ball park, LP 108s push the 300 cu ft range and LP 121s will be in the 325 cu ft range.

The issues are increased weight and drag on each and every dive. In that regard it makes more sense to pick tanks that best fit your expected dive plans and profiles and/or the tanks your other team mates have. It makes little sense to have too much tank and have to carry it and propel it through the water on all your dives when that extra capacity only gets used on a smaller percentage of your total dives.

Similarly, huge tanks won't provide any benefit in terms of extended bottom time or penetration when you are limited by "thirds" determined by the smallest tanks on the team.

In that case, it makes sense to go with more practical tanks like doubled 38 pound Faber LP 95s than it does to go with doubled LP108 at 46 pounds each or LP 121s at 50 pounds each. If and when you need extra capacity you can add a stage. When you don't need the capacity, you can enjoy the greater efficiency of the more moderate sized tank.

For example with Faber LP 95s at 3600 psi and an AL 80 stage, I now have 336 cu ft of gas (in tanks with a total dry weight around 109 pounds) compared to 327 cu ft of gas in LP 121 and no stage (in tanks with a total dry weight of about 100 pounds). So, I lose some efficiency with the LP 95s and stage compared to just the LP 120s on a comparable dive. On the other hand, I gain a great deal of efficiency on most of my non stage dives by saving about 24 pounds in tank weight (not including the excess gas I can't use when diving with LP 95 equipped team mates).

Bigger isn't always better.

 

[LiteHedded]

The numbers vary a bit from brand to brand, but in general:

A Worthington LP 95 has a nominal capacity of 93.3 cu ft at 2640 psi, while a Faber LP 95 holds 95.1 cu ft at 2640 psi. At 3600 psi when doubled, that capacity is 254 cu ft and 259 cu ft respectively.

With a 3500 psi fill, you lose 7 cu ft from both, getting the Worthington LP 95 in the 245 cu ft capacity range AJ references. However, a 3500 psi comparison doesn't illumate the issue much as you'd round down to down to 3300 psi for dive planning anyway - using a tank factor of 7 to under estimate only 231 cu ft available with 77 cu ft "thirds".

In practice, with all of them filled to the same 3600 psi, double LP 95s and HP 119s are both in the 260 cu ft ball park, HP 130s are in the 275 ball park, LP 108s push the 300 cu ft range and LP 121s will be in the 325 cu ft range.

The issues are increased weight and drag on each and every dive. In that regard it makes more sense to pick tanks that best fit your expected dive plans and profiles and/or the tanks your other team mates have. It makes little sense to have too much tank and have to carry it and propel it through the water on all your dives when that extra capacity only gets used on a smaller percentage of your total dives.

Similarly, huge tanks won't provide any benefit in terms of extended bottom time or penetration when you are limited by "thirds" determined by the smallest tanks on the team.

In that case, it makes sense to go with more practical tanks like doubled 38 pound Faber LP 95s than it does to go with doubled LP108 at 46 pounds each or LP 121s at 50 pounds each. If and when you need extra capacity you can add a stage. When you don't need the capacity, you can enjoy the greater efficiency of the more moderate sized tank.

For example with Faber LP 95s at 3600 psi and an AL 80 stage, I now have 336 cu ft of gas (in tanks with a total dry weight around 109 pounds) compared to 327 cu ft of gas in LP 121 and no stage (in tanks with a total dry weight of about 100 pounds). So, I lose some efficiency with the LP 95s and stage compared to just the LP 120s on a comparable dive. On the other hand, I gain a great deal of efficiency on most of my non stage dives by saving about 24 pounds in tank weight (not including the excess gas I can't use when diving with LP 95 equipped team mates).

Bigger isn't always better.

that's why it's a bummer when someone shows up with baby tanks.

and there's the issue of reserve gas to consider. on the vast majority of my dives I'm not touching backgas and bigger is just about always better.

 

[Rick Murchison]

Had a buddy who doubled up some LP 120's... that came out to right at 320CF with a cave fill...
But they are sorta big.
And heavy.

Rick

 

[ucfdiver]

I've always found this interesting...smaller tanks are less drag on the way in, so you get further in flow. With the same flow on exit, large tanks have more gas and more surface area to push against on the way out.

The logic in using smaller tanks to penetrate further escapes me.