THE MOVE AWAY FROM PLAIN AIR to nitrox as a breathing gas for divers has had a marked effect on the way in which dives are planned and executed.
The benefit comes from the lower inert gas fraction of the breathing gas. This reduces the requirement for mandatory decompression, at the expense of the increased oxygen content limiting maximum depth.
For most divers on most dives, the trade-off is a no-brainer, because the theoretical maximum depth is usually irrelevant.
The effect on a multi-dives-per-day, multi-day diving holiday is just as marked. On air, a week’s Red Sea wreck safari would see me end the final dive of the week with 30 hours or more to total desaturation, according to my computer.
Nitrox reduced that time to the low 20s, which was reassuring because it meant that I had off-gassed before flying home, always desirable.
Using a closed-circuit rebreather with air as the diluent, which is effectively a machine for blending and delivering an optimum nitrox mix for the current depth throughout the dive, reduced my total desaturation time even further, to the low teens of hours. So I’d off-gassed by the time the second beer arrived.
Of course, the tricky bit is measuring either the oxygen fraction or nitrogen fraction.You need only to measure one, because you can work out the other by simple subtraction, and in a rare display of general agreement the dive industry has settled unanimously on measuring oxygen fraction as the way to get the job done.
Vandagraph has been making oxygen analysers for divers for the past 25 years. Its first unit was the original VN202, which has now been completely redesigned and remanufactured to bring it bang up to date, and it is offered alongside the Tek-Ox, an all-in-one hand-holdable analyser.
Both units work in exactly the same way and consist of the same three parts. First, there’s a lo-tec blue tube with a non-return valve, so gas can pass through in one direction only. Next comes an oxygen-sensor cell and, finally, an electronic control-box and display screen.
The blue tube, which Vandagraph calls the Quick-Ox, is important, even if it is the most lo-tec part of the unit.
Watch people checking their nitrox mix on the average liveaboard and you’ll see them place the oxygen analyser against the pillar-valve, then crack the valve open. The result is a blast of gas that can damage the sensor, which is why all analysers have some sort of restrictor in the gas stream. The blast also delivers gas to the sensor at relatively high pressure, so the analyser over-reads the amount of oxygen in the mix.
Vandagraph VN202 Mk11 control unit with on-off switch, calibration control and read-out.
The correct way to do it is to open the valve first and get a slow, steady flow of gas, then position the analyser.
I’ve often seen liveaboard guests measuring oxygen concentration two or three percentage points higher than I do, simply because they haven’t been shown how to measure it properly.
Vandagraph’s blue tube allows gas to flow harmlessly past the sensor, and collects a sample that is held at ambient pressure to measure. You get a more accurate read with less potential for damage, even if you do it wrong.
The Tek-Ox comes with just the standard tube that push-fits to a spigot on the bottom of the unit. The VN202 Mk2 offers either the Quick-Ox or a package with an extra adapter that can be screwed into a DIN cylinder-valve.
That Quick-Ox tube is probably the biggest difference between the Vandagraph units and the commonly seen Analox analyser, and is interesting because on my last Red Sea trip divers had a choice of an Analox or a spanking new Vandagraph Tek-Ox, and largely went for the Analox first because they didn’t like the blue-tubey thing.