No doubt, when you go diving, you will see at least one person when they are preparing for the dive, to open their tank fully and then turn the handle a quarter turn back. Generally, in my experience, this is more habitual and if you ask why they do this, I’m fairly sure the answer will be “because that is what I was taught in my course”. This then usually sparks a debate on the boat between divers about whether this is good practice or not.
Today, for various reasons, the quarter turnback is no longer recommended, the practice is outdated and should be avoided. We will explore the origin of the “quarter turn back”, why it was originally done and venture into today's best practice; answering the question "should you do the quarter turn back".
The origin of the quarter turn back:
Diving can be traced back many many years! There are several accounts of when and who invented the first self-contained underwater breathing apparatus (S.C.U.B.A). But 'modern diving' probably started around 1926 with the creation of the first open-circuit scuba system. This was famously made much more accessible with the invention of the “Aqua Lung” by Jacques-Yves Cousteau and Emile Gagnan in the 1940’s.
This system uses a combination of a demand regulator and a high-pressure gas cylinder. As with the regulator, the cylinder itself has seen much improvement over the years. In the early days, cylinders used a ½ inch taper valve which did not have an O-ring but instead, the threads were wrapped with Teflon. They had lower service pressures and were much simpler designs, therefore, more susceptible to damage. If the handle of these valves was opened fully without turning it back, it risked causing damage to the valve and could cause it to stick open so you would later be unable to close it. Today, technology has developed massively; modern valves are much more sophisticated and with more parts using a straight thread with an O-ring seal. This better-quality design means there is very little risk that the valve will become sticky or damaged if opened all the way.
The dangers of this method:
There are some tanks which have been designed to show whether the tank is open or closed by using a colour system; green is open, red is closed. However, this is very uncommon and generally, there are no markings on handles to indicate an open or closed position, so is easy to get it wrong. If the tank handle is turned back slightly, it can lead to confusion; is it a quarter turn from open or a quarter turn from closed? There have, sadly, been cases of divers who have turned the valve the wrong way, thinking they were turning it open and quarter turn back, but in fact, had turned the tank off and then only a quarter turn open.
On the surface, if your tank is turned fully off, you would notice pretty quickly (if you do a proper buddy check before you even get in the water!), however, if your tank has been turned only a quarter turn open, you would still be able to breathe at the surface, without noticing any issues. Therefore, the diver who tries to descend with their tank fully closed, will realize very near, if not on the surface, and be able to safely turn it on and continue. The diver whose tank valve is only slightly open, however, may not realize until they have reached significant depth. They may think they are in an out-of-air situation yet will actually have a full tank of air but not be able to breathe from it.
Why is this?
This can all be explained by Boyle's law, which states that, at a constant temperature, pressure and volume are inversely proportional. So, as the pressure increases, the volume decreases and consequently, the density increases. This concept is integral to diving with scuba systems specifically designed to allow us to breathe at depth.
As we descend and the pressure increases, the volume of air it would now take to fill my lungs will be more than what was at the surface and the density of that air will be greater. This means the opening through which the air travels (the valve in the cylinder) needs to be larger. If the handle is only opened a quarter, the valve is not completely open, so the now denser air will not be able to flow through.
Best Practice
Always open you tank completely
Be aware of others checking your tank
Always take a few breaths whilst watching the needle of your SPG
Cylinders should be 100% open or 100% closed
Comments