Question Faber LP85s "perfect trim" even though positive when empty?

[rverma]

Context:
I dive recreational sidemount on Vancouver Island in Canada so I'm in a drysuit year round. I'm currently using "vintage" LP72s that, according to specs found on scubaboard, are 0 lbs with valve when empty. This tallies with my experience diving them - the tails get floaty.

I can fill my own tanks and typically do a modest overfill on my LP72s to about 2800-3000PSI (180-200 bar) and feel pretty comfortable with this. Recently, I was cautioned against overfilling vintage tanks such as the LP72 (mine are circa 1976) due to thinner steel walls, though he did say that 3000 PSI wasn't too risky with appropriate valves and burst discs.

However, I just finished AN/DP with TDI and am considering getting some bigger tanks. Used steels are hard to come by in my area, so I'm between Faber LP85s and Faber HP100s since I can easily get them new.

I'm stuck looking at the spec sheet and I'm not seeing how people conclude that the Faber LP85s are "perfectly trimmed" throughout the dive when they are 2 lbs positive with valve when empty. Are they perfectly trimmed after adjusting them when they approach empty... just like AL80s? Or are they referencing older Fabers with different bouyancy characteristics?

Perhaps folks are referencing Worthington LP85s when they talk about being perfectly trimmed throughout the dive since those are -1 lb or so when empty. But they're also -7 lb when full, so taking them off and moving them around would be rather difficult? I tried my Worthington LP108s in the pool yesterday and they're -2 empty and -10.7 full. I could not hold one in front of me without it pulling me headfirst to the bottom of the pool. It seems unlikely to me that a Worthington LP85 or Faber HP100 at -7 lb and -8.4 lbs when full are much more maneuverable than the LP108 I tried to manhandle.

Questions:
1) Other opinions on filling vintage LP72s to 3000ish PSI regularly?
2) Are new Faber LP85s with positive buoyancy when empty really "perfectly trimmed" at all times without moving them?
2a) If not, is the option of cave filling them why they are so highly regarded compared to AL80s or HP100s?
3) The Worthington LP95 has very similar buoyancy characteristics to the LP108 and appears to have been widely used in caving with significant overfills. Does anyone still use them and how do you feel about unclipping them and maneuvering them?

Thanks all

 

[Scared Silly]

You need to understand the context on the comment. One is not diving the cylinder alone but with a regular which reduces buoyancy. I have not seen the "perfect trim" comment. But I dive Faber LP85 as doubles for their buoyancy characteristics. All total; regs, SS backplate, can light, etc I am not over weighted. That is "perfectly weighted" as I can swim my rig up without additional buoyancy.

 

[Bowers]

As for the fills on the the 72's.. I personally don't like to go above 3000 because they are so old, but know of guys that do it. I usually shoot for 2800 and that gives me basically the same gas as an aluminum 80 but with better buoyancy characteristics.

My experience with 85's (my favorite tanks for cave and tec) is that they do get butt floaty if you breathe them down. I have for a while now been rigging themm like aluminum cylinders but using stronger loop bungee. If using 3/8" bungee for the loops you can have the boltsnap on the band more towards the front of the tank and let the tank "roll up to you" for a secure and streamlined fit. Then with a sliding dring simply move it down/forward when needed.

95's can work but are wide and short so I sold mine years ago and upgraded to 85's (and 50's for light stuff). I cave fill new lp tanks so they give me gobbs of gas with great buoyancy characteristics.

On that note, I would venture to say that when you are referring to comments about 85's having great characteristics, that most of those comments are in reference to the fact that they are close to the necessary weight needed for neutral in a drysuit with light undergarments. With aluminum 80's I need lots of lead if diving dry, while with HP steels I am overweighted unless wearing thick winter undergarments. but with the 85's I am perfectly weighted for my 150g/ms undergarment that I like to use in Florida caves.

 

[rjack321]

I would just get a larger tank. LP72s are a 2250psi tank with thin walls. Mine are for sure tail floaty in salt water.

Faber lp85s are slightly less tail floaty to me, although the butts will still rise when full of mix or when less than full.

Under what circumstances are you removing tanks? I have yet to need to super-person any SM tank in the PNW period. If you are planning on doing that you either need mix in a large tank or if diving air nothing bigger than an AL80 or a lp72. The gas weight will always pull you face forward.

 

[Bowers]

another note, my lp 50's with a cave fill are SOOO much nicer to carry around and will hold as much gas as your 72s.

 

[inquis]

I've seen spec sheets -- both with and without the footnote about including a valve -- listing the same positive buoyancy, so definitely something amiss there. FWIW, I've directly measured my ~3 yr old, empty HDG Faber LP85 buoyancy at -3.7 lb with an Atomic M1 reg, jubilee band, and lower boltsnap in fresh water. (It would be about -2.9 lb in salt water.) The "without valve" better matches my observations. (You might say it's the HDG coating, but I've also measured non-HDG/epoxy LP50s well below their specified buoyancy as well.)

 

[tmassey]

I've seen spec sheets -- both with and without the footnote about including a valve -- listing the same positive buoyancy, so definitely something amiss there.

As someone who has put some time into creating a list of tank specifications, I can agree that what we see in print often has very little resemblance to what we might experience in the water. I’ve seen tanks manufactured by the same company (Faber), but distributed by a different company (OMS) be listed with noticeably different characteristics. Putting together that list really caused me to exhale about worrying about the details of a particular tank. Any type of precision, even within a few pounds, just isn’t consistently accurate.You just have to dive it and figure it out.

As for Faber LP 85s, the reason why I think their characteristics are outstanding are twofold: they are narrower, longer and lighter on the surface than other tanks of similar capacity, which gives them much better buoyancy and trim characteristics for me personally. And because I only dive them as back mounted doubles, I find adding the extra weight of the tank bands evens everything out nicely. Plus maybe a third reason: technical divers rarely dive a set of doubles down to near zero, so we probably rarely get to the part where they would start to become noticeably floaty.

I can’t say as much about their characteristics for single tank divers, especially when those tanks are at 500 psi or below.

 

[Rilelen]

Plus maybe a third reason: technical divers rarely dive a set of doubles down to near zero, so we probably rarely get to the part where they would start to become noticeably floaty.

I can’t say as much about their characteristics for single tank divers, especially when those tanks are at 500 psi or below.

I suspect it’s this. I rarely end a dive with cave-filled LP85s below 2000psi and almost never below 1500psi. I find that AL80s in sidemount get quite floaty quite quickly, while LP85s (at least at my end pressures) do not.

 

[rverma]

Thanks for the answers! Unfortunately I don't have immediate and easy access to LP85s so I was relying on the spec sheets and comparing them to known tanks that I have used and their associated specs. Although I had considered the weight of the first stage and rigging, I didn't really consider that the tanks would rarely be close to empty in an actual tec dive.

I'll see if I can source some LP85s to try.

Under what circumstances are you removing tanks?

Personally, I'm not removing tanks at all in normal diving and have no need to do so. I'm not squeezing into restrictions doing wreck penetration or caving, but part of the training I did (combo AN/DP and solo with TDI) the instructor had me unclip tanks and hold them in various orientations, presumably to demonstrate the dexterity of unclipping/reclipping and buoyancy control while performing this task. No problem with LP72s. Impossible with LP108s.

 

[inquis]

the instructor had me unclip tanks and hold them in various orientations, presumably to demonstrate the dexterity of unclipping/reclipping and buoyancy control while performing this task.

Tip for LP85s (or larger): when tidying up the long hose at the end of the dive, consider leaving the bottom clipped for this reason (releasing just the top).