DIVER TESTS
WELL AND TRULY TESTED
A bit different this month: we invited an experienced open-circuit but first-time rebreather diver to try out the Hollis Prism 2. Over to ZOE AGER-HOWDEN – while STEVE WARREN looks deeper into simple dive accessories
REBREATHER HOLLIS PRISM 2
FOR SOME YEARS I HAVE REGARDED closedcircuit rebreathers as over-complicated and too technical for my needs, even though I’m aware that they would allow me to spend longer in deep, dark and murky waters.
However, the introduction of the Hollis Prism 2 gave me the opportunity to find out more about the CCRs and to give one a try.
The Prism 2 is relatively new to the UK but its antecedents stretch way back. The original Prism was invented in this country back in the early 1990s.
The initials of Peter Readey’s Incredible Steam Machine gave the unit its name and it won its aficionados. In 2007 Readey sold the rights to Hollis in the USA.
Hollis’s new and improved Prism 2 appeared in 2012 but it was only in late 2019 that the unit finally gained CE approval in the UK, so you could say it’s been quite a slow-burning affair.
The unit, very similar to previous versions apart from details of threads and labelling to suit it to European consumers, is distributed by South Coast-based Rebreathers UK, but before the expected launch events could be held in the spring of 2020, Covid-19 put in an appearance and the plans were disrupted.
Only recently was the launch programme able to resume, and I went along to NDAC, the inland site in Gloucestershire, to find out more about CCRs and try out the Prism 2.
Rebreathers
The closed-circuit rebreather is a tool for particular jobs. It’s easy to achieve a three-hourplus dive at significant depth once freed of the restrictions imposed by open-circuit on the amount of gas you can carry.
The gas efficiency of a rebreather means that you need only small cylinders, which is why CCR divers often carry 3-litre units rather than big twin-12s.
Used gas flows back into the loop rather than being exhaled into the water, and passes through a scrubber that removes the carbon dioxide content, allowing you to use it again.
Another advantage is that a rebreather calculates the optimum gas mix for your depth, so you breathe what it deems to be the best gas for most of your dive.
It does this by mixing the two gases in the cylinders, one on each side of the unit.
Usually the cylinder on the right will carry oxygen and the other the diluent.
With open circuit you would choose the most efficient gas for a particular depth, but should you be diving an upright wreck, say, you might well not be diving the most efficient gas at all times throughout the dive.
The rebreather’s automatic adjustments help to reduce your decostop time and narcosis build-up.
The Design
My initial look at the Prism 2 suggested that everything appeared sensibly ordered, all packed into a compact lightweight and apparently robust unit, branded with Hollis’ smart red and black colour scheme.
A more detailed inspection of the unit showed that thoughtful attention had been applied throughout – to everything from having red O-rings for high visibility to the positioning of the stainless-steel stand, engineered to ensure that the unit can stand safely and independently without adding excessive weight.
A radial CO2 scrubber is standard and 3-litre cylinders are mounted on each side, although it’s worth noting that the P2 can take bigger or smaller cylinders to suit your needs.