Help deciding next tank purchases (yet another LP85 vs HP100 question)

[FlyingSeaMonke]

Hey everyone! Long time lurker, first time poster.

My wife and I are new OC Cave Divers. We currently own and dive HP100s as backmount doubles and find that we want to buy two additional sets of doubles to avoid losing our parking spot at caves mid-day. We currently rent LP85's which is really affordable up in cave country, but we prefer the convenience of just scooting up to the dive site w/o having to stop at the dive shop first to pick them up and on the way back home to drop them off.

We're trying to decide whether to buy LP85's or HP100's as our next set since getting cave fills on the LP tanks is not a large issue. Larger 8" tanks may be off the menu due to the weight capacity of my wife's legs, however I figure there's plenty of cave to see on 13L tanks + stages and we can always rent large LP tanks for rare bigger dives down the line.

So here's my question:
We've dove LP85 and HP100 doubles and are happy with both, but we see ourselves wanting to expand to learn OC sidemount and eventually rebreathers (likely backmount /w sidemount bailout) in the coming years.

I probably have a mild preference for HP100's since overpressure fills are hard or impossible to get down here and down in the keys, however my primary need for the new tanks is cave.

It sounds like HP100's may be a tad heavy for SM and possibly very heavy as BO bottles that stay full (ideally) the whole dive, but since I have no experience with either I figured I'd get your opinions on the matter. Are HP100's tolerable for SM and BO, or are they just too negative?

 

[passedoutwookie]

I dive with HP100 worthingtons sidemount and have no issues with a K2. My girlfriend had overweight issues when diving wet with them with her SMS75

I go back and forth on this question, but think I’ve settled on sticking with HP100s for the reasons you listed. I don’t live in cave country and the 100s are more versatile tanks to me for that reason. Capacity-wise once you factor in the Z factor, I don’t think you’re getting more gas with the pumped 85s than you are with 100s.

 

[runsongas]

tolerable for SM in cold water, but I don't see any advantage to HP100 if you live and dive in florida

 

[passedoutwookie]

tolerable for SM in cold water, but I don't see any advantage to HP100 if you live and dive in florida

Being able to get fills at shops that won't do cave fills would be the biggest advantage. They are more versatile tanks if you are planning on getting any fills outside of cave country or don't have your own compressor.

That said, I do have LP85s, LP80s, and LP95s that I use in cave country, but have HP117s, HP100s, and HP80s for use both inside and outside of cave country. If I wasn't fortunate enough to have both, I would stick with the HP tanks since they are more versatile in regards to fills. Just my $0.02.

 

[runsongas]

getting 3400 psi+ on hp100s isn't guaranteed, i constantly would still have to ask when picking up to get it topped off before when i used to leave them at the shop. and now i just give up and accept i will get something like 3200 psi after cooling. its not worth having to leave the tanks overnight and I just ask they fill it slowly while I wait. and there still is a good chance at most places they will still do 3000 psi for a LP85 even if they won't cave fill to 3600 psi. at that point its pretty much a wash with hp100s.

 

[FlyingSeaMonke]

tolerable for SM in cold water, but I don't see any advantage to HP100 if you live and dive in florida

We’re typically diving dry in caves. My wife is slightly light with LP85’s and steel plate and still needs lead due to her thicker undergarment. I’m a tad heavy in HP100’s with an AL plate and close to perfect with LP85’s and a steel plate.

My concern with sidemount is it seems like the bottles being the source of negative buoyancy would cause them to ride low or cause issues with bottle manipulation (advanced sm). I guess this may be mostly what I’m asking as I really know nothing about SM right now.

getting 3400 psi+ on hp100s isn't guaranteed, i constantly would still have to ask when picking up to get it topped off before when i used to leave them at the shop. and now i just give up and accept i will get something like 3200 psi after cooling. its not worth having to leave the tanks overnight and I just ask they fill it slowly while I wait. and there still is a good chance at most places they will still do 3000 psi for a LP85 even if they won't cave fill to 3600 psi. at that point its pretty much a wash with hp100s.

I’m very lucky to have a local shop full of tech divers so I get 3500-3600 fills on my hp tanks cold. I asked nicely, but it sounds like that won’t happen with LP’s.

 

[FlyingSeaMonke]

Capacity-wise once you factor in the Z factor, I don’t think you’re getting more gas with the pumped 85s than you are with 100s.

I guess I should clarify. I view the 100’s (12.9L) and 85’s (13L) effectively the same tank in terms of capacity.

SPG inaccuracy is probably a bigger factor than true volume when gas matching.

I’m purely asking about buoyancy characteristics for OC SM and SM bailout and if it’s important enough to pick the less versatile tank.

Also as a side note: I’m specifically talking about the new HDG faber hp100’s vs HDG faber lp85’s

 

[runsongas]

the hp tanks riding low can be remedied with the drop rings or even a buttplate with rails if you prefer. if you are looking at trying to unclip and push them through portholes and other restrictions, you will want the LP tanks.