In the last installment of our tech-diving ABCs series, we’re looking at the letters X, Y and Z, starting with X-over (crossovers) and finishing with Z-knives.

We’re nearing the end of our tech-diving ABCs series with the letters U through W: U for understanding your limits and W for wetsuits vs. drysuits. Check out the ABCDsE through H,  I through L, M through P, Q through T and U through W as well.

X is for X-over

Though we know the letter designation is a bit of a fudge, crossovers are fairly common in technical diving, both at the instructor and recreational tech-diver level. They can happen between agencies, with a diver going from one agency to another, or when an instructor adds another agency’s curriculum to their array of courses.

There are benefits in both following one agency’s curriculum and crossing over between several. By sticking to one agency, tech divers become familiar with that agency’s materials, the course structures and, eventually, its teaching philosophy. Moving on to further levels of training, they will have a better idea of what to expect from the course.

On the other hand, moving between agencies helps divers understand different approaches to tech diving. For tech instructors, offering courses from several agencies is a way to attract more students, who may want to stick to ‘their’ agency’s courses.

Another relatively common form of crossovers in technical diving occurs between closed-circuit rebreather (CCR) units. CCR training is unit-specific, meaning you only qualify to dive the unit that you trained on. But, if divers change their mind and want to dive another brand of rebreather, they don’t necessarily need to go back to the beginning. Instead, a shortened version of the course, highlighting the differences between the units, is often available. This allows the CCR diver to learn everything they need to know about their new unit.

Y is for Y-valves

Y valves and H valves allow divers to back up their regulator. Shaped like an H or a Y, these valves have two outlets where a diver can attach a first stage.

You may never have seen one, as we don’t commonly use either any more as twinsets/doubles or sidemount configuration have become standard for technical divers, both in training as well as after their courses.

The main drawback is of Y or H valves is that the diver cannot back up any cylinder or valve-related issue by using them. Take, for example, a gas leak between the top of the cylinder’s neck and the actual valve — neither an H nor Y valve would help a diver conserve gas in this scenario. More drastically, a leak from a burst disk of a valve can empty a full tank in well under five minutes.

For those reasons, having more than one tank with one regulator each makes technical divers safer.

Z is for Z-knives

Z-knives, also known as Eezycut or Trilobite cutting devices, are among the most universally useful cutting devices for divers. Their popularity is partly due to their small size and the many options for mounting them. These small, flat cutters fit well on harnesses, computer straps or the outside of utility pockets. In fact, many divers carry more than one.

That said, there are plenty of other options for cutting devices. If there is a trend in this area of equipment, then it is toward smaller tools. How many cutting devices should tech divers carry? At least two, one that’s reachable with both hands, as it’s impossible to predict when and where you may get stuck underwater.

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