YOU MAY HAVE READ MY REVIEW of the Shearwater Predator. I first used one with the JJ rebreather. Later I dived with one I had borrowed for some open-circuit dives.
I probably omitted to tell you that JJ, no connection with the rebreather but the captain of the Truk Odyssey liveaboard, almost broke my arm to get me to leave it with him.
Of course, as usual I sent it back whence it came after I had finished with it.

The Petrel has the same big colour OLED display, but in a unit that is distinctly smaller – 40% smaller, according to the manufacturer.
It can be used in conjunction with any AA battery but of course the number of dives it will do will depend on which battery you choose. Shearwater recommends the Saft 3.6V but a li-ion rechargeable 3.7V or even a 1.2V ni-mh will do.
You could use a photo-lithium 1.5V (good for very cold water use) or even a cheap carbon-zinc battery, although that will accomplish only 10-15 hours of diving, so it’s not such a good idea. However, get stuck in some out-of-the-way place and you can still use the Petrel, provided you have some sort of AA battery available.
Screen brightness automatically adjusts to match ambient lighting. You get full information including stops and stop depths, plus time to the surface.
Information is displayed in colours to match its significance. For example, everything comes up in green unless it needs your attention, when it turns to yellow. If it’s urgent it shows in red, and if you need further prompting to act, it flashes in red at you.
It’s all unmistakable except for the fact that it all mysteriously appeared red in my photographs under water, although none of it looked red at the time!

Unlike some other technical diving computers, even I can understand this one. It uses simple adaptive menus operated by the two piezo-type push-buttons. Naturally it’s in either metric or imperial measures, and you can flip the screen to suit the way you mount it on your wrist or person.

The Petrel uses a choice of algorithms, including the now almost-standard Buhlmann ZHL-16C with variable gradient factors or the optional VPM-B that finds favour with many technical divers. On top of that it has user-adjustable safety factors.
It can be used with air, nitrox or trimix in either open-circuit or closed-circuit modes. In each case, it can dovetail with up to five gases, and it employs a simple two-pushbutton bail-out from CC to OC mode if required.
It can also be used simply in Gauge mode, unlike the Predator.
In addition, the Petrel has a tissue-saturation display that covers the 16 tissue models used in the algorithm. As a side-effect of understanding this display, the on-line demonstration found at gives a very good explanation of how gradient factors work. I recommend it.

Firmware updates come via Bluetooth, and you get free Shearwater desktop software with it. Dive log downloads are also done via Bluetooth.
The Petrel includes a dive log that will record 1000 hours, whereas the Predator recorded only 20 hours or so.

I gave the Predator 10 stars when I reviewed it for these pages. This left me with the problem of how to score the Petrel! It’s now available with a Fischer connection for CCR, too.

Suunto DX, from £995
Liquivision Veo, from £778
VRX OLED, from £945
HeinrichsWeikamp OSTC 2N, from £650

PRICE from £725
BATTERY AA (almost any)
ALGORITHM Buhlmann ZHL-16 C or optional VPM
DIVER GUIDE width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100%