Almost every diver has heard the phrase "Turn your tank all the way open and then back off half a turn.". A lot of divers still do it that way because that's how they learned it.
But why do you reverse this half turn (some also speak of a quarter turn)? And above all: Should you really make this turn? Let's take a closer look at this.
How it started
Back in the days, cylinder valves were not built as precisely and reliably as they are today. Therefore, it often happened that cylinder valves were damaged when they were opened completely. That's why the half turn was introduced back then so that the cylinder valves lasted longer.
Half turn back - Still do it?
The material and the valves have constantly improved and the risk of not being able to close a bottle is rather small. And if this is the case, it doesn't matter too much, since there is no situation in scuba diving where you have to turn off your bottle underwater.
But what can happen is that the bottle is not turned completely open and then turned back half a turn, but exactly the other way around. So first fully closed and then half a turn open.
Especially when things get a little hectic on the boat and the buddy is not that experienced, this error can easily happen during the buddy check. A half-open valve is usually no longer recognized on the surface. Only when descending, when the air becomes denser, does breathing become more difficult and, in the worst case, not enough air can flow through the half-open valve.
That's why there are actually only two positions for your cylinder valve: fully open or fully closed.
Here you can also find a great video on Youtube: