Double Tank Rig

Two tanks, connected with a manifold, is the most common rig used for the bottom mix.

This tank configuration allows you to use two separate regulators, each with it's own on/off tank valve. Thus, in the event of a regulator failure, you can turn off the failing regulator and still have a working regulator to breathe from.

Because, the tanks are connected with a manifold, each regulator has access to all the gas in both tanks. As a refinement, some manifolds have an isolator valve in the center. This valve can be used to isolate one tank from the other in the event of a catastopic failure of one of the tanks e.g. a blown burst disk, or a blown main tank "o" ring.

Using two tanks, without a manifold, is not a safe way to dive doubles.

Absent a manifold, you must periodically switch your breathing from one tank to the other to insure that each tank maintains an adequate reserve for emergency use. Given the other tasks that require your attention during the dive, the task of periodically switching breathing from one tank to the other is a distraction to be avoided.

Setting up doubles properly for tech diving is both a science and an art.

Facing the front of the doubles, the primary first stage should be mounted on the right post since this is the post most likely to be turned off in an overhead environment. The gauges and primary inflator hose should also be mounted to this first stage to minimize hose congestion behind your head.

The secondary first stage should be mounted on the left post. The secondary inflator hose or dry suit inflator hose should also be mounted to this post.

The choice of which regulator hose should be the seven foot hose is a matter of personal choice.

It can on the primary regulator and worn tucked in the BCD belt and wrapped once around your neck. Or, the seven foot hose can be on the secondary regulator with the hose tucked in surgical tubing on the tank.

Note: Creativity is needed to get clean layouts with different types of regulators. Try rotating the first stages 90 degrees from their normal position. When properly laid out, all of the hoses should be within the silhouette of the diver.

Finish the assembly by putting mounting hardware on the tanks as needed to provide attachment points for your decompression tanks and accessories and surgical tubing to keep your seven foot regulator hose from dangling.

Note: If you fasten all of your decompression tanks and accessories to your harness, there is no need for any mounting hardware on the tanks. And, if you dive with the long hose as your primary, there is no need for surgical tubing on the tanks.

For additional information on which hose to use for your primary regulator, see the Appendix - Other Articles On Deep Diving - Short Hose - Long Hose by Karl Shreeves.