Monday, 1 June 2009

Oxygen Analysers

I have been using Nitrox as a decompression gas for some years. Recently though, I started using Apex Quantum Dive Computers which enable me to gas switch to a high % oxygen (O2) mix whilst decompressing. For divers using Nitrox mixes its imperative that you know precisely what gas you have in your tanks to avoid errors in deco schedules. In the past I would have been happy to have the dive shop analyse the mix or I would have borrowed a friends O2 analyser, knowing that I was building safety into my dive plan by decompressing on Nitrox whilst my old computer was only reading air. I would be much less happy to do that now, and whenever I use Nitrox I check my mix myself to ensure its right.
The tool required to do this is the Oxygen analyser. For this edition I was sent analysers form Vandagraph in the shape of their TEK-OX analyser (Vandagraph) and from Analox I received the Analox O2EII analyser (Analox). Both are purpose built for the sport diving industry and claim to be water-resistant and shock resistant.
Both units whilst doing the same thing look slightly different and go about their business in slightly different ways.

Both units are around the same size but both look very different. The Analox fits nicely in the hand and is very ergonomic and easy to use. It comes in a cardboard tube and has a small lanyard attached to the unit. The Vandagraph unit is easy to use too but is a little more boxy. It does come in a protective plastic box, which is advantageous for divers. This unit has a large viewing screen and is also supplied with a "Quick-Ox" adapter, which is used to take the readings, and I will explain a bit more about this later.
Both units are extremely easy to use but both go about taking the oxygen samples in a slightly different way.
The Analox unit is simplicity itself. Switch on and then calibrate the unit, Crack the tank and place the restrictive sample dome hard against the tank valve. The reading will automatically take place as the opening from the dome leads directly to the oxygen sensor face. It’s therefore extremely important to only have a gentle flow of gas from your valve when taking a reading to avoid over pressurising the sensor and gaining a false reading. I will discuss this situation in more detail later. Once you have finished taking the reading the display will freeze displaying the oxygen content of your Nitrox mix.
The Vandagraph follows the same process up to the calibration stage and then the Quick Ox adapter is fitted to the unit. This is held against the tank valve and basically it is a system of valves that let gas reach the unit’s oxygen sensor at ambient pressure. This ensures that readings are always correct, as the sensor never becomes over pressurised due to high flow rates of the gas from the tank valve. (Have you ever been in a dive store and your Nitrox mix has been reading a bit light, only for the sales person to give the valve a quick blast to bring the reading up to the desired mix? This may be that the flow rate has been corrected or more likely the sensor becomes over pressurised and the higher partial pressure of oxygen in the mix now provides a false reading!). The Quick Ox adapter is a great piece of kit and its something Vandagraph are rightly very proud of. The Quick ox also has the advantage of diverting the flow of gas away from the sensor face and this is thought to increase the life of the oxygen sensor.
I tested the units on a number of Nitrox mixes over the course of a few months. Readings for both units were basically identical. I found the Analox calibration knob to be slightly less sensitive than the Vandagraph unit allowing me to get the O2 mix of 20.9% set slightly easier but there wasn’t much in it. The viewing screen on the Vandagraph unit is larger and easier to read. Both were a joy to use and suffered no effects from being stored in my dive bag apart from the Analox cardboard tube becoming very soggy. The Vandagraph unit was well protected in its plastic x-treme case. So if you fancy the Analox model I would recommend purchasing a protective case for it.
Both units boast a range of accessories such as blanking caps for storage when not diving. Battery life for both units is said to be around a year and oxygen sensors will last between 2 and 4 years depending on use and storage methods. Both units will auto turn off saving battery life.
Both units are water resistant so if you are unlucky enough to drop them in the sea they should be OK, at worst the sensor and batteries may be affected whilst the electronic components are sealed off in a separate compartment.
The Vandagraph TEK OX with protective case and Quick Ox adapter costs £155.00 + VAT @ 15%.

The Analox O2EII costs £170+VAT @ 15%
The list price for an O2EII Case is GBP 16.75 + VAT
Mike Clark

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