NB: This post is probably not very interesting to non-divers.
Background
Believe it or not, I have recently been (gasp!) doing some rebreather dives at recreational depths, such as building a photogrammetry model of the Yukon and model of the Corsair south of Pt Loma and I have a few other projects at that depth I’m working on. I would normally dive these depths with a set of doubles since I would just dive regular air; however, I want to essentially not worry about bottom time so I dive them with my rebreather.
All of my “lean” bailout tanks have less than 21% oxygen due to the depths I usually dive — hence, they make no sense for deco on dives in the 120-130 foot range and I don’t necessarily want to carry a second cylinder for deco with Nitrox 70% or O2.
Therefore, my plan was to have a dedicated “pony” size cylinder for recreational-ish depth dives filled with Nitrox 32 (or something close to that). I don’t need a big cylinder since I usually only accumulate 10-15 minutes of deco (which would be 15-25 minutes on bailout).
The Plan
I had originally planned to buy an Aluminum 40 cubic foot tank (AL40) for this purpose. That would be the “normal” thing to do.
I called up my local dealer (no, not that kind of dealer) — DJ Mansfield at Beach Cities Scuba. DJ checked stock and noted that they only had bright yellow AL40 tanks and not any brushed aluminum. Argh.
But, he said that they did have some new Low Pressure 50 (LP50) tanks in stock — which are actually a hot commodity right now as they are being used for all sorts of interesting things (like back-mount doubles for diving a Dive Rite Chest Mount Optima). They are a bit expensive (especially in times of Covid) but I figured “why not?”
The LP50 is about the same size as a AL40 (the AL40 is on the left for non-divers):
A comparison of specs between the two tanks is below (note: all specs are without a valve and without gas):
| Faber Steel LP50 | “Typical” AL40 | |
| Length | 25.2 in | 24.9 in |
| Diameter | 5.51 in | 5.25 in |
| Weight | 18.9 lbs | 15.3 lbs |
| Service Pressure | 2640 psi | 3000 psi |
| Capacity @ Service Pressure | 48.7 cu ft | 38.8 cu ft |
| Buoyance (full / empty) | -2.4 lbs / 1.24 lbs | -0.7 lbs / +2.8 lbs |
However, the LP50 has one significant advantage: gas volume. And it isn’t just 40 cubic feet vs 50 cubic feet.
The real advantage is that you can realistically fill the LP50 past the “service pressure” which is 2400 psi and it has the “+” rating for an additional 10% or a total of 2640 psi. At 2640 psi, you get about 50 cubic feet. At 3,000 psi, you get about 57 cubic feet. And I know people (cough, cough) who fill these tanks to waaaay past 3000 psi.
So, let’s say that you get a brand new LP50 delivered with Nitrox32 at 2640 psi and then you just happen to have a transfill hose and you happen to connect it to an HP100 tank filled with air and you just happen to open the valves on both of those tanks for a short period of time?
Well, you then get something like Nitrox 31 with almost 60 cubic feet of gas — in a nice small package which is perfect for recreational-ish depth dives.
Not only that, but the difference in deco time with Nitrox 32 vs Nitrox 31 is negligible but I get a good amount of extra bailout/deco gas. Also, I generally have a direct path to the surface on these dives so I don’t mind pushing the PO2 levels at max depth and I would personally be comfortable taking a Nitrox31 bailout tank to 150 feet (as long as you don’t accumulate too much deco).
So, there you have it — almost 60 cubic feet of gas in an AL40 sized package without too much additional weight.
The LP50 is a great tank. Obviously, it gives you the extra capacity over an AL40. The only downside is that it just doesn’t sling as comfortably. I never notice an AL40, but the weight a an LP50 is noticeable. Not a big deal, it is slightly less comfortable.
I’ve only used mine a couple times so far and I didn’t notice it that much. It is definitely heavier but I’ll take that tradeoff for the extra gas all day long.
I prefer an AL40 or AL72 for how they trim. As you pointed out though, with an overfill you’ve essentially got 50% more gas than a 40! If you want a compact package, indeed, the LP50 is hard to beat. Enjoying your blog, thanks!
Thanks for your comments, Ryan!
Yes, the primary reason for the LP50 is definitely the additional gas and I like the safety margin if that is my only bailout tank.
I like the AL72 tanks and have used them in the past. I currently have a bunch of steel LP72 tanks for deeper dives. Once again, they are really heavy (but not too bad with a drysuit/undergarments/etc.) but I can easily get 80+ cubic feet into a relatively small package (vs AL80) at the expense of weight / negative buoyancy.
So far, my favorite tanks are the Worthington steel & carbon fiber tanks that I wrote about here: https://wreckedinmyrevo.com/2021/05/29/carbon-fiber-twins-or-maximizing-bailout-gas-efficiency/
They have amazing weight, buoyancy and gas volume characteristics. However, they are not DOT certified so you need a “friend” to fill them and they need to have a compressor that can fill to 4500 psi to get the full benefit.