Overfilling LP108s?

[Addison Snyder]

Was thinking about getting a new set of LP108s, for both sidemount and singles. At the moment I've got 4 LP85s and overfill them regularly to a settled pressure around 3500psi. If I were to get LP108s, would I also be able to (somewhat safely) overfill them by about the same ratio? Asking because I've read a bit into the history of the LP85 overfilling in Florida.

 

[Manatee Diver]

As long as you don't have a vintage tank, cave filling LP tanks to 3,600ish psi cold is normal in cave country including the 108s. Just make sure that your valves don't have 4,000psi burst discs that should come standard with LP tanks.

 

[tbone1004]

@Addison Snyder the 108's are no different than the 85's from a construction stand point. Both meet the criteria of 3aa engineering standards and what you are comfortable doing on the 85's will apply to the 108's. I would however recommend you get FX133's, particularly the hot dip galvanized ones as they are vastly superior tanks and are identically sized to the 108's

 

[rjack321]

The fx133s are a bit heavier than the lp108s which are lighter in the tails

 

[Protondecay123]

Was thinking about getting a new set of LP108s, for both sidemount and singles. At the moment I've got 4 LP85s and overfill them regularly to a settled pressure around 3500psi. If I were to get LP108s, would I also be able to (somewhat safely) overfill them by about the same ratio? Asking because I've read a bit into the history of the LP85 overfilling in Florida.

I debated getting Lp108's versus HP133's. And went with the 133's. The HP weighs less than a pound and a half more. But the buoyancy in a twinset stays negative as long as you can follow the rule of thirds. Here is a photo of the specs.

 

[Addison Snyder]

@Protondecay123 Reading your chart, would the LP108 (top row) also be mostly negative following thirds? 8 lbs from empty to full, so using 2/3 of your gas would mean you end the dive with each tank a third of a pound buoyant. Usually not happening if you've got flow pushing you on the return trip. Considering that those tanks are going to be overfilled, the buoyancy when filled will be higher by a pound or two (lets say filling to 4/3 service pressure (3520psi), so 8lbs*4/3 = 10 2/3, so the filled buoyancy would be around 7.6 lbs negative, etc). I guess in conclusion, it seems to me that the buoyancy characteristics between a HP and LP tank aren't too significant when cave diving.

Buoyancy aside, what other reason would I want high pressure tanks for? Many places outside of Florida are barely capable/willing to fill HPs past service pressure, right?

Correct me if I'm wrong on the weight calculations above, I know some more advanced stuff comes into play with gas types and pressures and stuff. Also, if it wasn't inferred already, I'm gonna be cave diving with whatever I get.

 

[Addison Snyder]

And since I'm on the topic, what's the background and truth to the saying I've heard that LP tanks are supposed to be 'capable of 10000 cycles to hydro pressure'?

 

[king_of_battle]

@Protondecay123

Buoyancy aside, what other reason would I want high pressure tanks for? Many places outside of Florida are barely capable/willing to fill HPs past service pressure, right?

Buoyancy aside some places outside of FL are less willing to cave fill an LP tank. I've never had a problem getting HP fills on HP tanks.

 

[MrVegas]

I dive LP 108s as singles and actually really like them. However, as indicated above, if you are taking them outside of Florida, you will be lucky sometimes getting them even filled all the way to 2640 without asking for them to be topped off. Of course, you might also be able to get 2 used LP108s for the price of one new HP133.

 

[Protondecay123]

@Protondecay123 Reading your chart, would the LP108 (top row) also be mostly negative following thirds? 8 lbs from empty to full, so using 2/3 of your gas would mean you end the dive with each tank a third of a pound buoyant. Usually not happening if you've got flow pushing you on the return trip. Considering that those tanks are going to be overfilled, the buoyancy when filled will be higher by a pound or two (lets say filling to 4/3 service pressure (3520psi), so 8lbs*4/3 = 10 2/3, so the filled buoyancy would be around 7.6 lbs negative, etc). I guess in conclusion, it seems to me that the buoyancy characteristics between a HP and LP tank aren't too significant when cave diving.

Buoyancy aside, what other reason would I want high pressure tanks for? Many places outside of Florida are barely capable/willing to fill HPs past service pressure, right?

Correct me if I'm wrong on the weight calculations above, I know some more advanced stuff comes into play with gas types and pressures and stuff. Also, if it wasn't inferred already, I'm gonna be cave diving with whatever I get.

Well Addison, one thing as you well point out is that the differences are minor between the 108's and 133's. Living where the fill stations are sticklers for pressure rating because of potential *litigation*. So the HP's worked out better from that perspective.
Honestly the LP - HP (85 -100, and 108 - 133) has proponents on both sides. The weight savings for the LP is less than 4%. But an HP tank is a slightly stronger tank with higher pressure rating, slightly thicker chromoly steel. They has slightly more negative buoyancy characteristics particularly the closer you get to empty. 108's twinned up and empty have almost 6 lbs of positive buoyancy.HP's here are less hassle to get filled to 3400 psi. The galvanized HP's keep the maintenance low as well as @tbone1004 pointed out. But the internal dimensions 108 and 133 are almost exactly the same.
Regardless of your choice, enjoy!