Doubling Faber FX40s

[Deep South Divers]

Anyone tried this yet?

My recent love-love relationship with my latest drysuit purchase (there's been many, but this one I'm marrying) has allowed me to exercise some weighting fine-tuning. I'm thrilled, as I always am, to further minimize gear size and weight.

Interestingly, Faber's FX80 turns out to have the exact buoyancy characteristics that I'd need to be able to drop that last bit of lead off my rig and overall end up having a rig nearly 10 pounds lighter - not to mention shorter - between the lighter tank and no more necessary lead.

...So despite the cost, I'm committed to making the switch. I'm aware of the fact that I will need to pay closer attention to the tank's coating than before due to the nature of steel vs. aluminum in salt water. Getting fills to 3500 psi is not a problem for me, as I own my own extensive fill station and compressors.

Faber recently introduced the FX40, though... 1 3/4" smaller diameter and even shorter than the FX80. To me, a short, lightweight tank works just fine - in fact, is preferable - as I don and doff overhead and never set my rig upright on a stand to don or doff. Yes, light is very good.

Doubling these mini tanks would impressively improve trim and clearance, saving some 4" overall to the "thickness" of a rig like this... Overweight me by about 2 lbs, which would be nice in case I wear a little thicker undergarment... Allow for a redundant first stage (not important in my job)... And still allow for 80 cuft of gas (I don't need - or want - more gas). Best of all, I'd have a LOT more clearance everywhere for getting up into nooks and crannies for doing my job. Doubled FX40's manifolded would be about 2 pounds heavier than a single FX80, but the advantages in streamlining, trim, buoyancy and size I feel would be worth that. It'd still be a rig weight savings of like 8 pounds over the single AL80's I'm using.

In short, it seems expensive to do compared, say, to keeping the load of AL80's I've got sitting around... But it seems like it'd dive positively badtothebone, and be the lightest, smallest, most compact 80 cuft of backgas I've ever had.

The closest comparison I can make is a pair of Faber LP50's, which have been praised many times as the nicest-diving tank setup bar none. The introduction of the FX40 would be a shorter, even lighter set... Same idea, just a new tank.

I bet an all-black "microdoubles" set would look awesome.

Has anyone done this yet? I'd be interested to know how the rig dove - and what bands and manifold you used and where you got them. I'd be interested in a manifold WITHOUT an isolator valve.

Also... What wing did you use with them? They may be small enough to dive with a singles' wing of the manifold and bands are small enough...

 

[sea_ledford]

I think you might want to re-run your weight calculations. When you say a set of double FX40s are 8 lbs lighter than an 80 are you speaking of wet weight or dry weight? Bands and manifold and another 1st stage will add about 7 lbs to the dry weight, and probably another 5-6 of negative buoyancy. If that is wet weight are you counting as full or empty? 80cuft of air weighs the same regardless of what you put it in, so the buoyancy shift will be the same. It'll just have a different starting point.

I did look at setting up a set of wee-doubles for a co-worker out of LP50's. It ended up being about the same weight/buoyancy as the HP 119's we already have for 10cuft more air and $777 more cost. I certainly wouldn't want to put on 60-70lbs of gear over my head.

Weights and buoyancy for the bands and doubles were estimates.

	Steel 50	AL 80	AL 100	Steel 119	double components		Doub 50	Doub 80	doub 100	doub 119
Pressure	2640	3000	3300	3442	bands	manifold
Volume	50	77	100	119			100	154	100	238
volume at 3000 psi	57	77	91	104			114	154	182	207
Diameter	5.51	7.25	8	8			5.51
Height	25.2	26.1	26.2	23.9			25.2
weight	18.9	31.9	41	42.5	1	5	43.7	69.7	88	85
Weight full	23	39	50	53	1	5	58	83	105	112
weight @3000	24	39	49	52	1	5	59	83	104	109
Bouy full	-2.43	-1.8	-3.9	-10.9	-1	-4	-9.86	-8.6	-13	-27
Bouy Empty	1.24	3.2	3.5	-2	-1	-4	-2.52	1.4	2	-9
cost	$244	$0	$0	$0	$149	$140	$777	0	0	0

-Chris

 

[Deep South Divers]

Good point!

When I said, "8 pounds lighter," I was talking dry weight, topside, and I was including the 6 lbs of lead necessary with the AL80.

Let's get more specific:

AL80 on rig = 31.5 lbs + 6 lbs of lead to make me neutral in my drysuit, in salt water, with a 6 lb plate and a 1 lb STA. Total = 37.5 lbs, not including gas (empty tank).

FX80 on rig = 28.6 lbs, no lead necessary. All else equal. Total weight savings, topside, is 8.9 lbs.

Twin FX40's on rig = (14.8 x 2): 29.6 lbs, no lead necessary. 5.5 inch bands are about 3 pounds, and a non-isolation manifold is around 1 pound. Total 33.6 pounds. Total weight savings, topside, is 3.9 pounds... So yeah, I was a couple pounds off. Do you think I have underestimated my bands and manifold?

Of interest, the AL80 and FX80 examples above have equal buoyancy in the water within a couple ounces. The twin FX40's are about 7 pounds or so more negative than the FX80 (without lead) and the AL80 (with 6 pounds of lead)... Which is where I need to be.

Hm. That may answer my question right there. The dub-40's would make me 7 pounds overweighted, which in my opinion is marginally too heavy, especially on my back. Not much is as annoying when commercial diving as a tank(s) that wants to turtle... Even only 7 pounds' worth.

Both steel options are lighter topside than an AL80 when necessary lead is taken into account, even with bands and manifold for the 40's. The single FX80 saves more weight than the double FX40's, and costs about half as much. Both are smaller packages than the AL80, but the double FX40's take it one step further by being much flatter on my back - by about half the diameter of the AL80!

Lighter weight and smaller is what I'm after. The double tanks do it better at twice the expense as a single FX80, which alone is a pretty big improvement.

If I'm going to be honest, then I have to admit: There's also the cool-factor of BLACK tanks and a tiny set of doubles.

I doubt I'd ever dive with a second first stage on double FX40's. I don't even use a second second stage. Such is the nature of working 8 feet underwater all day.

 

[dumpsterDiver]

Then get a suicide bar off eBay for 20 bucs

 

[Deep South Divers]

A what?

---------- Post added December 1st, 2015 at 04:38 PM ----------

You mean one of these?

It would be a consideration, but I've never seen one that doesn't place the first stage perfectly to interfere with the back of the diver's head.

Besides, what are the chances that I'd find one the right size to go with 5.5" bands?

Frankly, the FX80 is looking better and better. That's a real shame, 'cause I'd be all about a set of baby doubles.

 

[sea_ledford]

Yeah, my coworker and I got really excited about building the wee-doubles. It just wasn't worth the cost and hassle. The plus sides were that it would have provided redundant regs (which you won't have, and could also be achieved with a Y or H valve) and they would have been less prone to turtling (theoretically) since the center of mass is closer to the body. That was really the only thing that had any sort of merit. It just wasn't a $777 problem.

Tiny black doubles would look badass though...

-Chris

 

[Deep South Divers]

Yeah, let's see...

$220 x 2 = $440 for FX40's
Bands = $150
Manifold = $100-$300. Let's say $200

Total then would be $790... If I could find the right manifold.

A single FX80 is $300.

An AL80 is free (I already have like 20 of them, and actually selling some off).

I really need like 2, but maybe as many as 4 tanks to dive with each day.

4 x $790 = $3160 for double minis
4 x $300 = $1200 for FX80's
4 x $0 = $0 for AL80's.

Suddenly that 6 pounds on my rig doesn't seem such a big deal.

I'll probably sell all the AL80's I've got and buy 2 FX80's and be done with it until I need more tanks... Then buy a couple more when I do.

 

[agilis]

I recently picked up one old Dacor doubles manifold and one old Scubapro doubles manifold from ebay. The parts were interchangeable, probably built in the same French or Italian factory back in the 70s. I combined the best parts and rebuilt one clean nicely functioning manifold. I use it with two aluminum 40s. I also got the 5.5 bands from ebay, total cost for bands, manifold, and some parts about $120.

The manifold is old school stuff, very simple, so the two 40s are, in effect, equivalent to one 80cf tank. I've only used the set up twice in a local inlet, after a few pool test runs. It's light, and very much better balanced on my back, comfortable, easier to put on overhead.

Those cheater bars are not a good idea because they are unstable, and the Thermo product was, when I checked, unreasonably expensive, especially for the shallow easygoing diving I do these days. The manifold I rebuilt functions exactly like it, but is less robust looking and can be disassembled.

 

[Deep South Divers]

Good info!

 

[divad]

I don and doff overhead and never set my rig upright on a stand to don or doff.

How nice for you......... Mr. Mike "Frisky" Nelson......