A valve drill is an incredibly important part of tech diving in doubles. The purpose of it is to prepare divers to respond to regulator or tank failures. A valve drill is an exercise to practice responding to failures of all sorts, not necessarily the exact thing you would do in the event of a failure. Common sense and thinking through the problem should still prevail.

There are two main approaches to valve drills. The “fix the mostly likely problem first” approach, or the “preserve air at all costs approach”. The “fix the mostly likely problem first”(or GUE) approach argues that the mostly likely failure would be your second stage primary regulator failing, and so their valve drill focuses on fixing that problem first. While that is correct, and that is the most likely failure to occur, if that isn’t the failure happening, you could potentially lose a large amount of gas while trying to fix the wrong problem.

The ”preserve air at all costs approach” starts off by isolating the manifold, so you’re only losing half the amount of air. From there you can correct the issue. If you did experience a second stage primary regulator failing, you then did waste a bit of time turning off the isolator before turning off your right post.

Either way it comes down to the specific diver. In any situation you should assess the best approach to respond to a failure, not solely rely on muscle memory. For a prepared diver, which method they practice shouldn’t really make much of a difference, as they should be aware of the reason behind why they’re shutting down each stage, and how it can resolve failure.

This post only addresses the “HOW” to do a valve drill. The specifics on why may come at another date.