I have the opportunity to pick up a pair of 50cu ft tanks (NOT HP) for a great price. I would like to build a double 50 and the guy is offering me a double manifold for a few bucks more. The problem is I don't know anything about the manifold. From what I have been able to research, it is a DACOR 3000 psi manifold. It looks to be brand new, is in the original package. Obviously I will have it serviced before diving on it, but should I buy the manifold?
Mahalo!
Hello Budprop
Two key things here 1) know what you're dealing with and 2) know what you want from the twinset as it relates to your diving goals.
There are a number of tanks in the 50cf range that have been used for doubles over the years. Some are better than others. Check the tank dimensions to be sure you know what you're getting:
a) Some early 50cf twinsets used 6.9" diameter cylinders with a 3AA1800 stamp (1980 PSI with the "+") with an actual capacity of 52-53 cf at 1980 PSI. These are bulky, heavy (at 22 pounds each), and floaty (about +3 empty each). If vintage diving is your thing then maybe these tanks are right for you, otherwise, avoid them.
b) There are also some very early repurposed fire extinguisher, aviation, or industrial cylinders out there, usually with a 3A (not 3AA) stamp before the pressure rating, and usually with tapered valve threads. They are extremely heavy, and valve parts, service, compatible valves, and air fills are problematic to obtain. Avoid these unless you're running a museum.
c) The more desirable steels for smaller twinsets are the 5.5" diameter ones. I believe these have been made in 40, 45, and 50 cf capacities over the years with the difference being the length. All weigh around 19 pounds per cylinder. Again know what you're getting. Capacity aside, some taller divers find the shorter cylinders to be awkward.
So, manifolds and bands. In general the vintage cylinders will use vintage manifolds with a metal-to-metal connection between cylinders, no isolator valve, and a single yoke outlet. Vintage equipment enthusiasts still use those however they do not provide the reliability and safety of a modern manifold. That said, the 6.9" diameter cylinders can be set up with modern manifolds and bands as long as the threads on the cylinder aren't the old tapered ones. Most 5.5" cylinder twinsets use modern, o-ring, dual outlet manifolds. Some will have a short crossbar with no isolator valve and use bands with a shorter center-to-center distance. Others use a standard crossbar leaving more space between cylinders to accommodate it. If you're dealing with a manifold that's still in a box, you do want to be sure it matches the spacing of the bands you have.
If you are getting the doubles because you like the way they trim out, any manifold will work. If you're using your regulators in tech configuration, you'll need the dual outlets. If you are depending on full redundancy, then you'll need dual outlets and an isolator valve.
