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February 2009    Download the Entire Issue (PDF) Available to the Public Vol. 24, No. 2   RSS Feed for Undercurrent Issues
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A Must Have for Nitrox Divers

oxygen analyzers that met our requirements

from the February, 2009 issue of Undercurrent   Subscribe Now

A divers life used to be simple. After filling your tank, you inhaled and if it was clean air, everything was OK. If it had an oily taste, you felt sick during the dive. If it was contaminated with carbon monoxide, you died. The deeper you went, the less time you had, and you had to consider how much nitrogen you might have absorbed. That was what decompression tables were for. That much hasnt changed.

Then along came Nitrox. By increasing the amount of oxygen in air, the effects of nitrogen dissolving in the body at depth are reduced, which means, compared to air (Nitrox 21), longer dive times for any depth. If the no-stop times are kept the same as for air, an increased level of safety is introduced.

Increasing Nitroxs Oxygen Level Comes With a Price

If you breathe oxygen at a partial-pressure greater than, say, 1.6 bar (thats equivalent to pure oxygen at more than 1.6 atmospheres), there is an increased risk of oxygen poisoning. But arguments rage as to what the safe level is. After the statistical evidence for 1.6 was put forward at Planet Tec in the mid 90s, the training agencies, ever mindful of litigation, set a maximum of 1.4 bar. With ordinary air, that would give it a maximum depth of 175 feet. However, Bret Gilliam once held the record for the deepest air dive to 475 feet. Its been broken since. I watched Rob Palmer do a sequence of air dives to 390 feet, and he got away with it for five of six dives -- he died on that sixth dive. So we can only really say that oxygen poisoning is accumulative and affects each individual differently.

Because air is often supplied alongside
Nitrox, even air divers need to analyze
their tanks to be sure they’re filled with
air and not mistakenly filled with Nitrox.

Since Nitrox contains increased oxygen, the amount being up to whoever mixes it, we need to analyze it ourselves to be satisfied we have the right mix for our dive plan. Because air is often supplied alongside Nitrox, even air divers need to analyze the gas in their tanks to be sure that its air and that they havent erroneously been filled with Nitrox.

Analyzer Calibration

A Nitrox analyser has an oxygen sensor that generates a voltage that varies according to the percentage of oxygen present, and gives a readout on a voltmeter calibrated in percentages. An analyzer reading is affected by air pressure (weather) and the aging of the sensor cell, so before each dive, it needs to be calibrated at 20.9 percent O2 with clean air passing around it. If you calibrate using pure oxygen instead of air, the sensor readings will be three times more accurate.

Recently, I was on a liveaboard with a diver who was a gas analyst from Siemens. He merely checked his tank each time to see that he didnt have air. He told me that the analyzer on board was so inaccurate, it was only good for telling whether you got air or something else, but nothing more. He just checked to see if it went over 21 percent but didnt bother with a final reading. Furthermore, he said, the little oxygen analyzer that I had held so much faith in was almost totally inaccurate. I was shocked. So what was wrong with the simple equipment that was available to us?

The problem stems from gas flowing over the sensor. He pointed out that the gas had to be stationary or controlled to a known rate. By simply increasing the flow, the apparent oxygen content increased. Cracking open a tank valve just a little and holding the cell over it is evidently not accurate enough for him. Temperature also makes a difference, and gas depressurizing from a tank can be very cold.

Some analyzers use the direct-feed hose to the BC, using the regulator first-stage as a flow control. It takes time to get a reading and this seems unpopular with divers impatient to get in the water. Divers will always look for the most convenient method. I have witnessed Nitrox being analyzed by holding the sensor in the mouthpiece of a regulator while the purge button was pressed.

But manufacturers have come up with designs to get a stationary sample of gas at the same ambient temperature and pressure with which the analyzer was calibrated. Some restrict the flow of gas over the cell while others attempt to capture a stationary sample. It seems that the bit of plastic tube that interfaces between the tank valve and the sensor is very important.

If your Nitrox is made by the older partial pressure blending system, you need to know your analyzer is exact, because there you may inadvertently get very high percentages of oxygen, if by mistake.

Oxygen Sensors

Depending on how much work an oxygen sensor does, its performance declines. Even when you are not using one, it will still be working if the sensor is surrounded by air. So sensors tend to have a shelf life of two years (some manufacturers now claim up to five years), provided their gas-barrier packaging has not been opened. They should provide a millivolt reading between 10 mv for air and 60 mv for pure oxygen. Used sensors become unpredictable when they reach the end of their useful life. While it does no harm to keep an analyzer in an airtight container, some experts tell me this adds little to the lifespan of a sensor because there are so many other factors to consider.

Heat shortens a sensors life. Keep your analyzer out of the sun and away from any hot place. Other factors include the cleanliness of the lead anode and the exact composition of the electrolyte. Vacuum packing and freezing have inherent risks of damage. Some say that you should store it in an inert gas but this raises questions of wake-up time for the cell after it is brought out of storage. If you get a reading more than one percent different from your expectation, or are using a sensor that has been mishandled, do not use it to analyze the oxygen content of an unknown gas.

So should you trust your dive operator to analyze your Nitrox accurately, or should you carry your own analyzer? While you can trust most operators, today more divers have decided they want to know the precise amount of oxygen in their Nitrox, and tote their own - - and a good one at that.

I looked at several analyzers to see how convenient they were to operate. I also checked each against the same tank of Nitrox to see if there were differences in measurement. All the analyzers I tested were calibrated using the same tank of air, and all gave a reading of my sample gas within 0.5 of a percentage point of each other. Knowing the inaccuracy of decompression theory, thats probably accurate enough.

VN202 This provides a choice of methods for sampling gas. The sensor cell is at the end of a flexible cable, and can be plugged into a conventional Quick-Ox sampling tube. Otherwise, it can be connected to a DIN tank connection (not a pressure-reducing valve), complete with a not similar chamber to the Quick-Ox and a long exit tube. Sample by turning the gas on for five seconds and off for five more. It proved quick and easy to calibrate. (Approximately $245 with Quick- Ox and $255 with DIN tank connection; purchase on the manufacturers Web site www.vandagraph.co.uk)

Tek-Ox An analyzer designed to make use as convenient as possible. Simply hold the attached Quick-Ox sampling tube over the cracked tank valve, and let the gas migrate up inside the unit to its sensor cell. Then turn off the tank so that the trapped sample is static. I found it probably simplest to pull off the sampling tube and wave it around in fresh air to get an initial setting, being careful to avoid backdrafts from boat exhausts, which could give errors while under way. The reaction time was quick and the display is big. It was my favorite among those intended for individual divers. (Approx. $260; purchase at the manufacturers Web site www.vandagraph.co.uk)

Dynatron OxySpy This Swiss-made analyzer is similar in concept to the simpler-looking but more expensive Nuvair. Knowing that divers can be lazy, OxySpy is designed to be as convenient to use as possible. I thought the calibration procedure was a little confusing and also quite time-consuming, but it was very convenient to use once this was done. (Approx. $290; www.dynatron.biz)

Analox O2EII A popular compact analyzer often supplied on dive boats. Simply wake it up by pressing a button. Calibrate it in air by adjusting a knob to get the right percentage according to a compensation chart provided on the LCD.Then offer up the sampling hemisphere of the analyzer to the slightly opened valve of the Nitrox tank to be checked until the reading stabilizes. It may not be the most accurate way to do it according to the experts, but it is good for confirming an anticipated Nitrox mix. ($325; www.analox.net)

Nuvair O2 Quickstick This is merely a plastic cylinder containing the cell, a voltmeter, and the LCD readout with a simple hemispherical head containing a tiny orifice that you place firmly against the tank valve after cracking it open a little. It is calibrated for air (O2 20.9 percent) and supplied with a fine long-handled screwdriver. Alas, switching it off means taking the battery out, a cumbersome operation. ($250; www.nuvair.com)

Alpha-1 This tough little unit in an anodized metal box comes with a long hose complete with constriction in-line offering a connection that conveniently clips over your tank valve. It proved simplest to use on a known tank of air as a supply for calibration. It was a workmanlike answer to the problem of knowing what gas mix I had, and its metal construction will give it certain longevity. ($500; for sale at Andi Internationals Web site www.andihq.com)

A Must Have for Nitrox DiversOMS. Favored by many technical training agencies, this one comes with a DIN-connection regulator first-stage to operate as a pressure-reducing valve at the end of a hose. It also has an anatomical shape to help with holding it, an on/off switch and a rotating knob for calibration. I suspect most divers would just whip off the hose connection where it presses in at the sensor cell end, and wave the unit around in fresh air to calibrate it. Attaching it to a tank, I was able to get an apparently accurate reading that was quick, easy and free from user error. ($482; www.omsdive.com)

So which one to buy? All of the analyzers included here were adequate for the job. However, what separates them is not the core hardware but the method of gas-sampling. Only the Vandagraph analyzers with the Quick-Ox sampling tube attachment meet the requirement to analyze a stationary sample of gas. And of all these, Vandagraphs VN202 and Tek-Ox analyzers seemed the most convenient to use.

John Bantin is the technical editor of DIVER magazine in the United Kingdom. For 20 years, he has used and received virtually every piece of equipment available in the U.K. and the U.S., and makes around 300 dives per year for that purpose. He is also a professional underwater photographer.

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