How to Assemble a Set of Doubles
By: Dillon Waters
In selecting equipment for a planned dive, the technical diver’s first concern will be his supply of breathing gas, based upon the requirements for a particular dive, as well as a suitable reserve for contingencies. Technical diving may involve extended bottom times, perhaps at relatively deep depths, with additional time spent underwater during mandatory decompression stops. Sometimes a diver will be able to meet his gas supply requirements with a single high-capacity cylinder, though more frequently he/she will opt for double cylinders. In addition to volume, double cylinders offer an advantage to the technical diver in terms of redundancy. (TDI)
Assembling your own scuba gear can be a very rewarding and satisfying experience, but it is important to remember that there is no substitute for a trained professional. Until you are very familiar with the process, it may be wise to seek the assistance of one when assembling your double cylinder setup.
Step 1: Making sure you have the correct materials
- 2x Matching Cylinders
- Tank Bands (appropriate for cylinder diameter)
- 2x ½” Wrenches (or adjustable wrenches)
- Bolt Kit
- Tape Measure
- Backplate, Harness, and Wing
Step 2: Preparing Your Scuba Cylinders
If the scuba cylinders you will be using aren’t new, you may need to do a few things to prepare them before moving forward. If your cylinders already have valves on them, they will need to be opened to ensure the cylinders are free of compressed gas and then removed. If there are any stickers that may lie between the bands and the cylinder, they must be removed as well. Be certain that your cylinders have been appropriately inspected by a certified professional and have a current hydrostatic test before moving forward.
Step 3: Preparing Your Manifold
Make sure the manifold is broken down into its three components; one left post valve, one right post valve, and a crossbar with an isolator knob. Turn the lock nuts (one will be notched noting that it has reversed threading) on the crossbar until they are hand tight against the inside of the body. Then, inspect the o-rings on the valves and the crossbar; replace and coat with oxygen-compatible lubrication if needed. Be certain that your manifold has been appropriately serviced/prepared by a certified professional or the manufacturer before moving forward.
Step 4: Installing the Valves
Take one of your post valves and turn it clockwise into the threaded neck of your scuba cylinder. You should tighten the valve as firmly as you can by hand, making sure not to over tighten and crush the o-ring. Then, repeat the process with the second post valve and cylinder. Once both valves are installed, lay the cylinders down parallel to one another with the valve orifices facing up.
Step 5: Installing the Crossbar
Carefully place the crossbar in between the two post valves with the notched lock nut facing towards the right post valve (left/right posts will be referenced by their position when worn on the diver’s back). Then, slowly push the tanks closer together until the threads meet on both sides. It is extremely important to make certain the tanks are parallel to one another so that the crossbar sets correctly.
Next, begin to slowly rotate the crossbar, making sure there is even pressure on both sides, until you feel the threads connect. It is a common thing for only one side of the crossbar to connect – if this happens, just back the crossbar out and try again. Make sure both sides set evenly when threading.
Once the crossbar threads both tanks evenly, continue slowly turning the crossbar. This will bring the tops of the tanks together. If you feel ANY resistance while turning the crossbar, stop and gently tap the cylinders back together until parallel, then continue to turn. Do this each time you feel resistance.
Continue to thread the crossbar until about 1/8-inch of the threads are left showing on each side, or until the isolator can no longer make a full rotation around. This may differ depending on the tanks, bands, or crossbar you are using. You may need to slightly loosen or tighten when adding the bands.
Step 6: Installing the Bands and Bolts
Begin by disassembling the tank band bolts. Place a bolt in each band and then lightly secure it with a washer, a locking washer, and the nut.
Now, slide the first set of bands up from the bottom of the cylinders. Be careful not to stress or rotate the manifold and ensure that the cylinders remain parallel. Slide them all the way up until the top of the band aligns with the portion of the tank right before the curve of the shoulder begins. Next, add the bottom band just as you installed the first. Position the bottom band so that the center of the bolt head is 11 inches from the center of the top bolt head.
Once the bands are on, inspect the position of the cylinders inside the bands, they should be centered. If they are positioned or angled towards the inside of the band towards the bolts, then you will need to loosen the isolator while repositioning the cylinders. If they are positioned towards the outside of the bands, then the isolator needs to be tightened.
Before tightening the bands, place the backplate that you will be using on the tank bolts to ensure proper fit. If the bolt heads do not align with the holes in your backplate, you will need to adjust the bottom band.
Using your wrenches, loosely tighten the nut and bolt of each band, while periodically checking to ensure the isolator can still rotate freely and the tanks are still parallel. Tighten the isolator and reposition the tanks as necessary.
Step 7: Final Tightening
Once you are sure that the bands are in the correct position, you may tighten and secure them into place. Tighten the nut and bolts until snug and then turn once or twice more. Do NOT over tighten the bands – the cylinders will expand when filled and put stress on the entire setup.
Position the isolator valve knob in its final desired position – this may be straight upward or angled slightly forward towards the diver.
Tightening of the isolator bar is a personal preference. Many divers like to leave the locking nuts loose, allowing the isolator to have free movement to reduce the chance of damage if it comes into contact with anything during the dive. Many others prefer that the locking nuts be tightened to secure the isolator bar, keeping the isolator knob in place so that the location is always known when diving. Make your decision based on your personal preference and on the environment you will be diving.
Finally, add the wing nuts to the bolts for storage, then stand back and admire your craftsmanship. Your new set of doubles is now ready to be filled.
Krzeszowski, F. (n.d.). TDI Introduction to Tech: An Overview of Theory, Tools, and Techniques (Vol. 0816). International Training.