THE APEKS QUANTUM DIVE COMPUTER is one of a number of computers made in Japan and sold under various brand names. It has proved popular because of its ease of use when setting up and when reading it under water and, not least, because of its competitive price.
The Quantum X is the latest version, which not only can now be used with three alternative nitrox mixes during a single dive, but features an electronic compass display too.
The unit is activated by two wet contacts. You can test these by wetting them with the fingers, when the word “Water” should appear on the LCD. Otherwise, at rest the instrument displays time, day and date.
It automatically detects atmospheric pressure and adjusts according to altitude, but you can fudge this by adding safety factors (1 and 2) that select the next altitude level up. The computer is operated by a combination of three buttons.

The algorithm is said to be a Swiss model, probably Buhlmann ZH-16, modified by Randy Bohrer. In side-by-side comparison tests we have found this to be suitably cautious, and also we noticed that it will not shed the last of the shallow (3m) stops unless you are at that depth.
The ascent-rate indicator is variable between 16m and 8m/min according to depth, and this ascent rate is included in the decompression schedule.

With this Quantum you can set nitrox at between 21 and 99% for any of the three mixes that you might want to use during a dive. You can also set a max PO2 from 1.0 to 1.6 bar.
Button B is used to change settings, button A to move to the next field and the Mode button is used to save the settings and exit. Setting it in gauge mode renders it simply as a depth-gauge and dive-timer.
For dive centres that might rent these computers out, there is a reset button on the reverse side. You can also set depth and dive-time alarms should you so wish.
I found it a little tedious scrolling through settings that I didn’t want to change, simply to get to the one I did need to alter.

Display Legibility
The displayed figures are large and easily read. Nitrogen loading shows up as a graphic on the left side until you are into decompression-stop diving, while the oxygen limit is shown by a graphic building up on the right. There’s a PO2 alarm activated by passing the maximum operating depth (MOD) for the mix set.
Should the oxygen level (OLI) reach its limit (I found that I would have to go well beyond my MOD for that) an OLI alarm flashes.

By pressing and holding button A, you get the time of day. Hold this button down for more than two seconds and it will show the maximum depth achieved on that dive and the current water temperature.
Pressing button B will cause it to enter compass mode and display an animated compass rose, a direction arrow, the bearing in degrees and the time of day.
Pressing and holding button A allows you to lock in a bearing, and turning through 180° displays the reciprocal bearing for the return swim. Pressing and holding button A will clear the bearing set.
You need to calibrate the compass whenever you change geographic location.

In The Water
A little dive profile graphic of the dive so far shows on the main display. This proved useful in that it gave me an instant picture of how I had been behaving.
Deep stops (optional) are indicated only once you’re into deco-stop diving, and the dive is deeper than 21m. The deep-stop timer runs for one minute. It shows the depth of deco-stop ceilings, the stop time and the total time required to ascend.
Pressing the mode button firmly for a few seconds allows you to scroll through the different mixes preset using button A, but it won’t allow you to choose a mix if you are below its MOD. Naturally, you should switch actual gas mixes breathed to match what you choose on the computer.
A safety stop is indicated and timed through three minutes once you get up to 6m. If you descend below 6m it stops, and picks up where it left off once you get back to 6m or above.
Only if you descend to 10m or more does it start to recount from three minutes again.
I had high expectations of this computer, but I was to be disappointed. I took it on the first dive just to get familiar with its display and in-water functions, and it appeared to be perfectly OK, with the compass displaying like a dream.
By the second dive the Quantum refused to tell me anything more than that I was in water, and the time and date.
That was not particularly useful, and by the time I got out of the water after the third dive, it had died on me completely.
I guessed that I had got a “bad” one, and reflected that such circumstances are the reason I am so often seen wearing a number of computers at the same time during a dive.
I would have tried changing the battery, but the plug was in so tightly I was afraid of damaging it while struggling to free it.
The performance of the Quantum X was impressive on the dives for the period during which it was functioning.
I’ll report back if any reason is established for the test unit’s sudden demise.

All other three-nitrox-mix computers available today are gas-integrated.

PRICE £329
ALGORITHM Swiss, modified by Bohrer
FAST ASCENT WARNING Variable by depth
IN-WATER DISPLAY Time-to-surface, depth, duration, deco time & stop depth
DECO INFO Stop depth, stop time, total ascent time.
DIVE-PLANNING For no-deco-stop diving.
LOGBOOK Yes – with dive-profile graphic
PC INTERFACE Using optional PC interface kit
MODES Nitrox and Gauge
BATTERY User-replaceable CR32032