The purpose of the valve drill in a training scenario is to familiarize the diver with the proper use of the valves on their manifold and to prepare them to deal with some of the failures that might occur during the dive.
In addition, valve drills are made part of every dive that we do. This is to ensure several important things:
Valve Drills for Training
As with all skills, valve drills should be practiced regularly. This can be done as part of a regular skills practice dive, or it can be agreed to incorporate as part of a standard dive upon occasion.
Single Tank Drill
The valve drill on a single tank is pretty simple. You only have one valve - take a guess what you're going to do. :-)
It is important to leave your hand on the valve after shutting down so that you don't need to struggle under duress (i.e., no gas!) to get back to the valve to turn it back on.
Double tank drillThe following is the procedure for a valve drill on doubles for training purposes. It's always done in shallow water ( < 30FSW - ideally 10-20FSW ).
You always do an S-drill before doing a valve drill to ensure your buddy's hose is deployable if you have a problem.
Valve drill procedure:
reduces the risk that they will jam the valve in the open position, becoming unsure of the proper direction of movement.
Valve Drills for every dive
As stated earlier, we perform a valve drill on every dive to ensure valves are operational and in the correct position. If conditions warrant, we stop at about 15FSW during the descent and perform an S-drill and valve drill. If conditions do not permit, we perform the S-drill and valve drill on the boat. In both cases, the valve drill is not the full valve drill as described earlier in the Training Scenario section. This is an abbreviated version. The abbreviated version is:
Valve Procedures in an Emergency
Single Tank Valve Failure
The most common "failure" on a single tank rig is actually failure to have the valve on in the first place. There are several ways this can happen. The most common is for a diver to turn their gas on, check the pressure, then turn it off again. When they later check their SPG, it reads full, so they jump in. Two or three breaths later ... no gas!! I've also seen well-meaning divemasters accidentally turn people's gas off when they meant to turn it on. In this case, the solution is obvious (besides the even more obvious answer of verifying that your gas is on before jumping in .... ) - turn your valve back on!
In the case of an actual failure (e.g., regulator failure), in a single tank you don't have very many options (unless you have a dual-orifice valve with redundant 1st stages, in which case you treat it nearly the same as doubles). Your only option is to first secure a new gas source. Do not shut down your valve until you have secured a gas source (i.e., from your buddy). Once you have a new source, you simply reach back and shut down your valve. While this is not strictly necessary, the bubbles generated by a regulator/valve failure can cause a great deal of confusion, increasing the stress and task loading in a situation that is already stressful.
Double Tank Valve Failure
Probably the most common procedure taught by the technical diving agencies is that whenever you have a failure that causes you to start losing gas, you first shut down the isolator. The theory behind this is that you know you will at least have saved half your remaining gas. We do not advocate this approach.
If you go to the isolator first, you get no confirmation of what the problem is, which means you have to go to one of the posts anyway. So in every case you'll need to shut down at least 2 valves. If you go to the post first, in the most likely scenario (1st stage failure) you'll only have one valve to shut down. You solve the problem faster, get started on your exit faster, reduce the amount of bubbles blowing around your head faster (which may be bringing clouds of silt down on your head if you're in a wreck/cave).Effectively we're using the odds of failure. The types of failures that would force you to need to isolate would be:
The chance of a burst disc failure, tank neck o-ring failure or manifold o-ring failure are extremely small during a dive. The pressure in the tank is continually decreasing and ambient pressure is increased as compared to the start of the dive.
On the other hand, regulator 1st and 2nd stages have a number of o-rings and moving parts, which are constantly being used throughout the dive. Therefore by far the most likely failure will be a regulator failure (1st or 2nd stage) which can be solved by shutting down the post.
So if you can speed resolution in the most-likely case, why not do that?
In addition, you can usually identify the location of the failure. If the HP seat in the 1st stage fails, then the 2nd stage will begin freeflowing. So if your necklace reg begins freeflowing, you know you have a left post failure. Likewise, if your primary reg starts blowing gas uncontrollably, your right post has failed. If the failure is such that it is not delivering high pressures to the 2nd stage, and is instead just blowing gas straight out of the 1st stage, you can usually identify by sound which side this is coming from. Note that this even works with thick wetsuit hoods. Again, if you hear the gas blowing from the left side, it is likely a left post failure; gas blowing from the right side is likely a right post failure.
So the procedure we use is as follows: