Overfilling LP108s?

[Addison Snyder]

Cool, thanks for the clarification.

 

[boulderjohn]

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.

They are not significant in any case.

Your buoyancy change during your dive is wholly dependent upon the weight of the gas you used. Your cylinders will swing from one weight to another with that loss of gas, but it does not matter whether or not that swing passes the number zero on the buoyancy scale.

Your buoyancy throughout a dive is based on Archimedes principle--if your total weight and volume (including cylinders, other gear, thermal protection, your body--everything) is equal to the total weight and volume of the water you are displacing, you will be neutrally buoyant. Some parts of your total package are positively buoyant, and some parts of your total package are negatively buoyant. It is the total package of those things that determines your buoyancy. At the beginning of your dive, your total package will be negatively buoyant, so you need to add a certain amount of air to your wing in order to increase your total volume to compensate for your negativity. As you dive and use air, you need to dump some of that air to compensate for the weight loss.

All cylinders will become more buoyant as the dive progresses. At the end of the dive, some cylinders will be part of the package that makes you negative, and some cylinders will be part of the package that makes you positive, but in either case, the change in weight and the resulting need to change volume is wholly dependent upon the loss of the consumed air.

 

[rjack321]

@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.

The lp108s are the preferred tanks for SM (in caves). Both are negative in freshwater, but the 108 slightly less of a brick and they don't want to hang as low with nitrox in them. In FL getting them filled to 3600 is easy.

The 133s work slightly better with trimix in them because they are more negative and the tails don't want to float up quite so bad in SM. Both still want to stand on their valves upside down when drained, the lp108 just starts to do that sooner as you breath down nitrox, or almost immediately with mix.

For entry to intermediate level cave diving (nitrox no stages) in FL, I would get the 108s. If you were debating getting tanks for that you'd keep full of 15/55 for Eagle's Nest I would say the 133s.

 

[kelemvor]

I dive (worthington) lp108's. Both as single tank backmount (open water) and as sidemount doubles (cave). Love them. As others have said, 3600psi is the standard fill in much of Florida (but not all). 3200 is still an overfill and that's what I've had in the keys and on the east coast. I assume that's because the tank fill people didn't realize they were LP tanks, and just filled them to match aluminum.

 

[boulderjohn]

I dive (worthington) lp108's. Both as single tank backmount (open water) and as sidemount doubles (cave). Love them. As others have said, 3600psi is the standard fill in much of Florida (but not all). 3200 is still an overfill and that's what I've had in the keys and on the east coast. I assume that's because the tank fill people didn't realize they were LP tanks, and just filled them to match aluminum.

I have both Worthington LP 108s (doubles) and Worthington LP 85s, and my experience with them in south Florida has been all over the place, and in the shops where they have been filled multiple times, it largely depends upon the individual doing the filling.

In one shop that I used many, many times for the 108 doubles, I once found them filled to about 2300. When I asked about it, the filler said the limit of 2400 was plainly marked on the cylinder. I explained the meaning of the +, but he was not convinced and had to confirm it with someone else. He finally grudgingly filled it to about 2600--still short of the standard 2640 fill. In that same shop, another employee, himself a cave diver, routinely filled them to 3600.

That shop's manager told me he could not risk the lives of his employees by asking them to fill past the rated pressure. On a day when he himself filled my LP 85s to 2500 (hot), I asked if I could at least get them filled TO the rated pressure, he said I was just splitting hairs. On that same day, I dived with those underfilled 85s with two friends who rented AL 80s from that same shop, and those were filled to 3300 (cold). That was the shop's standard fill for AL 80s, because they wanted the customers to be satisfied. So every day I dived with my friends, they had more gas in their AL 80s than I had in my LP 85s.

I had been using the shop because I was doing my trips with their boat, and it was very convenient to leave the tanks with them for fills between dives, but I decided to forego convenience in favor of a good fill. I called another shop in town and asked their policy, and they said they would fill them to 3000. That is the shop I have used ever since. Sometimes I do get fills to 3000, but it is usually more--often up to 3600. (It is quite something to do a shallow reef dive on a boat that has a one hour limit on your dive with 3600 in your LP 85--you surface with a lot of gas left in that tank.)

There was an interesting side story about the shop where most of the employees would not fill my doubles to the rated pressure of 2640. One day I came in and was told that they had found a tiny leak in one of my burst disks and had fixed it. I assumed that meant they had put a wrench to it and tightened it, but what they had actually done is replaced it--with, of course, a burst disk appropriate for an LP tank. I later took those doubles to Marianna, Florida for some cave diving, and I had them filled at Cave Adventurers. I am sure the employees in the shop that day will never forget it.

 

[kelemvor]

I have both Worthington LP 108s (doubles) and Worthington LP 85s, and my experience with them in south Florida has been all over the place, and in the shops where they have been filled multiple times, it largely depends upon the individual doing the filling.

In one shop that I used many, many times for the 108 doubles, I once found them filled to about 2300. When I asked about it, the filler said the limit of 2400 was plainly marked on the cylinder. I explained the meaning of the +, but he was not convinced and had to confirm it with someone else. He finally grudgingly filled it to about 2600--still short of the standard 2640 fill. In that same shop, another employee, himself a cave diver, routinely filled them to 3600.

That shop's manager told me he could not risk the lives of his employees by asking them to fill past the rated pressure. On a day when he himself filled my LP 85s to 2500 (hot), I asked if I could at least get them filled TO the rated pressure, he said I was just splitting hairs. On that same day, I dived with those underfilled 85s with two friends who rented AL 80s from that same shop, and those were filled to 3300 (cold). That was the shop's standard fill for AL 80s, because they wanted the customers to be satisfied. So every day I dived with my friends, they had more gas in their AL 80s than I had in my LP 85s.

I had been using the shop because I was doing my trips with their boat, and it was very convenient to leave the tanks with them for fills between dives, but I decided to forego convenience in favor of a good fill. I called another shop in town and asked their policy, and they said they would fill them to 3000. That is the shop I have used ever since. Sometimes I do get fills to 3000, but it is usually more--often up to 3600. (It is quite something to do a shallow reef dive on a boat that has a one hour limit on your dive with 3600 in your LP 85--you surface with a lot of gas left in that tank.)

There was an interesting side story about the shop where most of the employees would not fill my doubles to the rated pressure of 2640. One day I came in and was told that they had found a tiny leak in one of my burst disks and had fixed it. I assumed that meant they had put a wrench to it and tightened it, but what they had actually done is replaced it--with, of course, a burst disk appropriate for an LP tank. I later took those doubles to Marianna, Florida for some cave diving, and I had them filled at Cave Adventurers. I am sure the employees in the shop that day will never forget it.

Those stories make me sad. I'm sorry you had those experiences with whatever shop it was.

"Bad shop, go lay down!"

Was it recently or a long time ago? I've heard lots of stories about crazy things that apparently used to happen. Such as stories about shops that couldn't properly fill an HP tank, or shops that couldn't fill nitrox.

 

[boulderjohn]

Those stories make me sad. I'm sorry you had those experiences with whatever shop it was.

"Bad shop, go lay down!"

Was it recently or a long time ago? I've heard lots of stories about crazy things that apparently used to happen. Such as stories about shops that couldn't properly fill an HP tank, or shops that couldn't fill nitrox.

I made the switch 3 years ago. The original shop went out of business last year for unrelated reasons--I was still using them regularly for my diving, and I would still use them for diving if they were still in business.

 

[Manatee Diver]

@boulderjohn Sounds like PDC, though I seem to remember them being one of the few places to get cave fills in South Florida.

Anyways I will mention for @Addison Snyder that I keep a few sets of HP100s for my trips down to South Florida and the Keys. Getting caves fill down there can be hit or miss. If you have a good shop that will do it gladly great, but as a visitor I found it easier to just have a set of HP tanks. Heck I can get cave fills locally easily, I always fill my LP85s before I leave cave country.

 

[Addison Snyder]

I'm lucky to have a solid shop locally in Daytona (Spruce Creek Scuba) that cave fills whatever nitrox you want, and for 8-9 bucks after tax. Don't want them to up their price or anything, but I take mostly drained tanks there for fills where they crank it up to 3300-3600 psi for me . After cave dives (>2000psi), I'll sometimes top them off at places that charge by the cubic foot. That's also another (minor) reason I'm looking into bigger tanks. More gas for my psi (dollar).