Matching tanks-- lengths aren't the same

[Thadmn]

Hi All,
I bought 2 Faber steel medium pressure tanks from a shop on the east coast through the mail in August (same lot #). My intent was to get a nice matching set for doubles. I dove these tanks as singles through September. They appeared to be the same, but now I have assembled them into doubles and have found 1/4 " difference in length between the two. To be sure, I broke them down and measured them individually, standing them on their necks. I have reasembled them making sure the manifold is straight etc. I am using rubber in between the tank and boot on one tank to shim and make up the difference. I have contacted the owner of the shop, who says that this happens. I also talked to Blue Steel and they say that the length measurement is nominal and there are variations. I expect there to be differences, but 1/4" seems excessive (especially with the same lot #)! Does anyone else have any similar experiences or suggestions? ----Thanks!

 

[spectrum]

It's a random manufacturing varriance and not really a lot driven dimension. I think you can find examples of much bigger differences.

Did the seller know you were intending to pair them as doubles? If so they will usually give you the best matching pair.

Is 1/4" a big deal?

Pete

 

[SparticleBrane]

I would certainly say that 1/4" is a big deal, or at least a large annoyance (especially if you aren't using tank boots). If you order tanks in the future, specifically state that you plan to use them in doubles. If you do that they should measure them for you to get tanks that match.

Would you be ok if the cabinets in your kitchen were 1/4" off? That's a huge variation.

 

[WD8CDH]

At least it's not a sefety issue. You could manifold a 13 cf pony with a 120cf tank as long as they both have the same pressure rating.

 

[Thadmn]

Thanks Guys for your reply.
I consider it a big deal since I spent the money to specifically avoid this problem. I did specify that I was using them for doubles, but aparently they did not open the boxes and check the lengths. I am told that even with the bands installed, this can put stress on the manifold. I ordered them online from NESS. I have gotten good deals there before, and I'm sure that this is unintentional. Anyway, I'm stuck with them now.
I guess this goes back to the online vs local shop debate. Next time I will definately INSIST that they uncrate them and measure them before shipping.
The rubber shims I made seem to work OK.
Thanks again!

 

[WD8CDH]

When you manifold them togeather, make sure the necks are at the same level to minimize the stress on your manifold.

 

[Scared Silly]

Thanks Guys for your reply.
I consider it a big deal since I spent the money to specifically avoid this problem. I did specify that I was using them for doubles, but aparently they did not open the boxes and check the lengths. I am told that even with the bands installed, this can put stress on the manifold. I ordered them online from NESS. I have gotten good deals there before, and I'm sure that this is unintentional. Anyway, I'm stuck with them now.
I guess this goes back to the online vs local shop debate. Next time I will definately INSIST that they uncrate them and measure them before shipping.
The rubber shims I made seem to work OK.

Sounds like you and they did what most folks would have done assumed that the cylinders from the same batch would match. How come you did not measure them when you first got them and before you dove them? Seems that this was very much on the radar screen. It is an easy check to make all that is needed is a level and a flat surface.

Even with rubber shims I would be slightly bothered cause if you plunk down one cylinder will hit harder than the other. Hopefully you used shims made from hard rubber.

 

[DA Aquamaster]

It does happen. The tanks start as large round discs that are drawn in 2 or 3 steps. The long tube that results is then and heated while the end is brought back in to form the neck and the neck is then turned and the threads and o-ring groove are cut.

The turning process is pretty uniform, although shoulder shape can vary slightly. Where the lenghts become different is in the drawing steps where the lenghts may vary slightly from tank to tank on successive draws.

One of the major advantages of buying from a shop is that you can stand them side by side and match both overall lenght and shoulder profile. This is a LOT less likely to happen with an on-line purchase as the on-line retailer's order picker would have to unbox (potentially) a lot more than 2 tanks and then have to rebox them all. If an on-line retailer offerred this service at all they'd be justified in charging you $10 -20 more per tank for hand selecting the tanks.

As an industry issue, it would be much easier and more efficient for the tank manufacturer to quickly gauge them for overall lenght at the factory and then sort them and mark the shipping containers accordingly so that tank suppliers could ship them as more or less matched pairs to shops and customers.

I think shimming the short one in the boot is an acceptable solution that is not going to excessively stress the manifold. I hear lots of concern about the perils of stressed manifolds, but I have to say that the modern dual o-ring sealed adjustable lenght manifold is SOOOO much stronger than the old face sealed fixed lenght manifolds of 20 years ago and the current bands are also much, much better.

In my opinion, as long as you take reasonable care to assemble them so that the tanks remain parallel and do not mechanically stress the manifold by having the center to center dimensions of the lower band, upper band and manifold be so far off that the manifold is stressed, the tanks are not going to care if they sit cockeyed because one tank is 1/4" or even 1/2" shorter than the other.

As an aside, I agree that lifting a set of doubles by the cross bar is bad form. But in reality, while this was problematic with older face fit manifolds (that ironically often only had a center outlet with no valve knobs on the posts to lift it by) newer manifolds are strong enough that I don't see this as being a significant issue compared to old manifolds as long as the manifold and bands are properly assembled and the bands are suitably secure.

In short, it's possible to get too obsessive compulsive about things like this, and shimming them will be fine.

If you don't agree, feel free to ship them to me for proper disposal.

 

[Luis H]

As an industry issue, it would be much easier and more efficient for the tank manufacturer to quickly gauge them for overall lenght at the factory and then sort them and mark the shipping containers accordingly so that tank suppliers could ship them as more or less matched pairs to shops and customers.

That reminds me of the way they make skis. The last step in manufacturing a pair of skis is matching two skis that are close enough within the tolerances to be called a match pair (they measure dimensions, weight, spring constant, etc.). Skis are mass produced in fairly big quantities, but there is still enough variation were not any two of the same model can be called a pair.

As an aside, I agree that lifting a set of doubles by the cross bar is bad form. But in reality, while this was problematic with older face fit manifolds (that ironically often only had a center outlet with no valve knobs on the posts to lift it by) newer manifolds are strong enough that I don't see this as being a significant issue compared to old manifolds as long as the manifold and bands are properly assembled and the bands are suitably secure.

When you are talking about face seal manifolds are you referring about the US Divers or Sherwood styles that used industrial CGA type of fittings between the center manifold and the tank posts?

They are both standard metal to metal CGA fittings (different sizes) and my experience is that if they were not damaged or dirty they are very reliable. If they have any scratches or they are not perfectly clean they may never seal, but once sealed I have never heard of one failing. Have you?

I have a semi-vintage (1980’s) Sherwood manifold with double yoke outlets (one in the center) in one of my sets of doubles and I would not trade it for any other manifold. I am actually looking for a second one to put together a set of twin 50’s.

Yes, the only way to lift them is by the center manifold, but I never thought it was as bad with the CGA metal to metal type fitting. I did wonder about lifting my Poseidon tanks with the DIN fittings, but they seem to be OK also.

The connection between the center manifold piece on my Poseidon tanks to the two individual tank valves is with a 5 thread DIN connection. The manifold basically has two males and one female fitting (and a reserve mechanism in the center). The connection is rigid enough that you can pick up the tanks by the center manifold without any fear. Actually the system has no top band. Instead it has a rubber block for the shoulder straps. This rubber block is tightened between the tanks putting a lot of tension on the manifold.

 

[David K]

Does anyone else have any similar experiences or suggestions? ----Thanks!

I have two (2) sets of double PST LP104's, one pair is 1/4" taller than the other, if I mix them incorrectly they lean noticeably:-(

Had you considered buying one more tank, specifying the length so you end up with a matched pair (height wise) + a single?

David K