Matching LP72 tanks for doubles

[guruboy]

Currently in the process of putting together an LP72 doubles set.

Visually I know the neck transitions can vary in both exterior slope as well as how they feel on the inside. Wall thicknesses might vary as well. I'll have to look up Luis' old post.

Does anyone have practical experience with matching these tanks as doubles? How much variation is there between years and manufacturers and is it something worth worrying about?

 

[guruboy]

As far as my situation goes, I can match manufacturer but they are about 5 years apart and the necks look different.

 

[lowviz]

Forget the necks.

Here is what I think is important:

Many years ago, at the Dutch Springs fill station, I watched some instructor swing his doubles down a short drop from the cement pad to the driveway. Perfectly matched set, cheapie bands. One cylinder hit first. We all heard the hissing coming from the manifold. Walk of shame back to the truck. It is all about bands, not necks.

Seems like you can't get anything from the Peanut King anymore. So I make my own bands. See pic. Ignore the rust, I do. Cheap-ass Chinese welding rods. I won't use them anymore. Two-inch marine grade stainless steel straps and chunky spacers.

A proper set of bands will protect your manifold.

 

[guruboy]

I have a set of highland mill bands so I think I'm covered there.

Just don't want to end up lopsided later in the dive.

 

[lowviz]

I have a set of highland mill bands so I think I'm covered there.

Damn straight.

... Just don't want to end up lopsided later in the dive.

Put your lobster bag on the other side...

 

[Luis H]

I agree with Lowviz.

It is nice for the two cylinders to be similar in height (for convenience when standing up), but I would not worry about any of the other dimensions. I use boots on my doubles so it is a bit annoying if they don’t stand up straight, but even that is not critical.



I have 18 steel 72’s (11 are PST and 7 are Norris). Only one pair of the PST are doubled up (most of my doubles are smaller, twin 50’s or twin 45’s).

There are differences between the steel 72’s, but most of them are close enough.

For example:

Their average volume capacity is about: 70.5 cubic feet, maximum 72.2 cu ft, minimum 69.6 cu ft, but the standard deviation on this 18 cylinders is only 0.68 cu ft.

I have also measured the buoyancy of 9 of them (I hope to do the rest this next summer).

I measure the weight by hanging them into a fresh water container and I also measure the air pressure in them before submerging them. I wrote an Excel spread sheet that calculates and adjust the weight of the air in the cylinder and it also calculates the weight in salt water.

The average weight in salt water is 3.6 pounds. The maximum 5.2 pounds and the minimum 2.6 pounds, the standard deviation 0.81 pounds. Two of the Norris cylinders are much heavier (in the water) that all the rest. The PST tend to be a bit lighter, in the low 3+ pounds in salt water, but my point is that in average most of them are within a pound of each other.

 

[guruboy]

Thanks, Luis. I tried looking at the data you posted in another thread but it was a little blurry. I think for the Norris tanks, the measurements were all pretty close even across a wide range of years.

 

[CuzzA]

I have an unmatched set of 72 doubles. 1970's Norris and PST? I haven't noticed any issues. Although, I'll note that I'm in Florida and would gladly trade a match to a match.

 

[Scared Silly]

This will not help the OP, but I tired to do the same some time ago. I found an original matched set of LP72s that had not even had a second hydro. One failed, other barely passed. Got a deal on a second set of LP72s. Great shape, but the hydro shop left water in them, toasted the liner, tumbling did not help. Gave up. But then found a set of Faber LP76s from another SB member. A bit heavy compared to my LP85s.