However, I made the mistake of heading off across the world without the instruction manual, because I was trying to save weight in my bag.
My reasoning was thus: it's only a computer-watch - how difficult can it be to set up?
Was my face red I had to use it set for air, because I couldn't fathom how to set it up for a nitrox mix, let alone enter two different mixes to use during the dive. It's all about the four buttons being used individually or in pairs, with long, longer or short pushes.
Back in the UK, I started at page one of the manual and waded my way through it, before setting off on another dive trip to somewhere less distant.

I bet this computer is made for TUSA by one of Bob Holliss companies. Why do I say this?
The TUSA algorithm is based on Buhlmann ZHL-16c, which is actually the Pelagic Z+ algorithm developed by Oceanic with an eye on the European market for use with
its new range of computers.
Its a development of the ZH-L16 algorithm used by numerous independent manufacturers with a good track record for safety.
I added in the one caution factor available as an optional setting.

The Zen can be set to default to a worst-case scenario 79% nitrogen, 50% oxygen, but this will also set the alarms going if you pass 18m without setting the actual nitrox mix youre using.
I found it best to turn this feature off, because I was diving every day and the computer could remember the mix I used, a reliable 31.9% from a membrane system.
There seem to be no short cuts for anyone who hasnt been shown how to set this computer, but I systematically managed to set it for normal diving as opposed to gauge
or free-diving mode, with the right nitrox mix, a second mix, no default O2 50% setting, audible alarms on, a depth alarm at 40m, an elapsed time alarm at 45min, a dive-time remaining alarm, a PO2 setting of 1.4bar, wet activation, deep stop on, a safety stop on, a sampling rate of once every 15sec and a conservative factor.
After all this I felt quite proud of myself, not least because the instruction manual is very hard to digest. It is not possible to set an O2 percentage for Mix 2 that is lower than that used for Mix 1.

Display Legibility
Its quite hard to tell whether youre using the computer with gas 1 or gas 2, because this is indicated merely by the position of a little tank icon relative to the percentage displayed.
However, the figures displayed were surprisingly legible.

In the Water
I used the setting for Air, but the computer never got into deco mode during my dives
in Raja Ampat. No-stop time counted down at a rate that agreed more or less with the Suunto on my other wrist, while a little nitrogen-loading graphic built up round the rim of the display.
The ascent-rate indicator also built up as I ascended (in common with other computers using this algorithm). The Zen uses one of two ascent rates, according to the depth at the time.
The deep stop seemed to make no difference to the no-stop time displayed, and there was no protest if you decided to ignore it.
It was set to come on automatically on any dive deeper than 20m. If you go deeper than
the chosen pO2 setting for the nitrox mix in use, an alarm sounds, but in decompression mode the alarm triggers only if 1.6 bar is exceeded.
You can change gas mixes during a dive by pressing one of the four buttons for the right amount of time. The IQ-900 will allow you to change to a mix at a depth exceeding that mixs MOD (assuming that youre doing this because the tank with the first gas is depleted), but it sounds an alarm and displays the legend Do Not Change Gas. You can then switch the alarm off as you make your way up to a safer depth.

Planning & Logbook
It seems that the conservative factor, when applied, merely moves the pegging of the algorithm up another 915m of altitude, or twice that if you are at sea-level. This allows a degree of dive-planning using the two mixes set.

Comparable computers to consider:
Suunto D6, £525
Apeks Pulse, £320

PRICE £255
FAST ASCENT WARNING For variable ascent rate
NORMAL IN-WATER DISPLAY Total ascent time, stop depth/time, current depth with ascent-rate warning, O2 and N bar graph.
ALTERNATIVE DISPLAYS Temperature, time, maximum depth, pO2, fO2 set-point, gas mix.
DECO INFO Yes, with optional deep-stop
LOGBOOK One-touch access
PC INTERFACE Datalog interface optional
MODES Air, nitrox, gauge, freediving
BATTERY User-replaceable CR2430
DIVER GUIDE width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100%