Just starting in doubles

[Frackingawesome]

I am just getting started in doubles. But I have a question. Can I use 2 takes from different companies? For instance, I have an HP100 from Faber and a Genesis HP100. can I band them together or do I need 2 identical tanks? I know I can not mix tank sizes.

 

[tbone1004]

@Frackingawesome you can, but I really would not recommend it. The reason you don't want to mix sizes is because the lengths being different will make sitting quite annoying. Underwater it doesn't really matter, but sitting is going to suck. I'd try to find a mate to either the Faber or the Genesis. If the Genesis is an Asahi, then you'll be rolling to whatever side that bottle is on.

 

[Frackingawesome]

that I what I though. thanks

 

[drainaps]

Your trim will be difficult to keep unless you balance things out between the tanks with weights. I'd not do it, especially at the learning stages (I was particularly slow at getting comfortable with doubles after being quite proficient with a single tank, so any complications such as the one you have in mind seem like a bad idea to me).

 

[melanie.]

You can do whatever you want.

The tank length isnt the biggest issue - you can fix that with tank boots and spacers to make them level on the ground.

It's your trim. Different tanks float differently... even the same brand but different years can. So you might feel rolly because of the way the tanks get lighter through the dive.

That being said, if money is an issue, you can always try the tanks you have. Worst case you don't like it, and buy another tank so they match (which is what you were going to do anyways).

 

[Graeme Fraser]

Another thing to consider is whether the diameters are exactly the same, otherwise the cylinder bands might not tighten evenly. This could result in overstressing the manifold, particularly if there's a difference in length.

 

[TrimixToo]

Another thing to consider is whether the diameters are exactly the same, otherwise the cylinder bands might not tighten evenly. This could result in overstressing the manifold, particularly if there's a difference in length.

I used to think that scuba tanks were built to precise dimensions. Then I chucked a few in the lathe to cut them, face the cuts, and try to bevel them. (Think "porch bells" and "umbrella stands.") I was amazed at how much runout they had. You don't even need a micrometer to measure it; you can use a ruler. They all had to be beveled with a file.

IMO, the effective diameter of a tank under the bands depends on it's rotational position. That in turn is controlled by the essentially random thread starts for each tank and valve. There's enough compliance in the bands to let them be snugged up with some amount of mismatch. I'm sure that "some amount" varies by band manufacturer, at least. If they didn't snug up I'd flip them and try them the other way before giving up.

I have one mismatched set of doubles. Both tanks are Fabers! The height difference is just enough to be annoying once in a while. I could trim one of the boots to even them up, but (a) I'm too lazy and (b) I would inevitably reinstall them on the wrong tanks, or even on a different set of doubles, making things worse.

 

[ScubaSam]

Another thing to consider is whether the diameters are exactly the same, otherwise the cylinder bands might not tighten evenly. This could result in overstressing the manifold, particularly if there's a difference in length.

This is the exact reason given by my instructor when I moved from single tank to doubles.

Good luck, and welcome to doubles!

 

[Compressor]

Great explanations above. That's why many recommend 2 tanks from the same manufacturing batch which for us common divers means buying the tanks at the same time from one location.

 

[divad]

I have three sets of Fabers (how did that happen?) They are all "matched", but from different years. The older set is 1/2" taller at the neck than the other two sets so they get the lower profile Halcyon manifold. The other two (shorter) sets get the taller Thermo manifolds so they're all in about the same place.