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I am pasting Jack Kellons O2 sensor file here for anyone who could not read the attached file he sent out. Dan ELECTRICAL OXYGEN SENSORS Of all the components in a closed circuit rebreather, perhaps the most critical are the oxygen sensors. With the exception of a few very expensive kinds of electronic oxygen sensing devices, almost all oxygen sensors used for rebreathers are galvanic fuel cells. The basic elements of a sensor consist of a lead anode, a gold plated cathode with a solution of potassium hydroxide as an electrolyte. The cathode is a convex metal disc plated with a noble metal e.g. gold, silver, etc. with numerous perforations. It is designed to facilitate continuous wetting of the upper surface and contains a small amount of electrolyte between the membrane and the cathode. When oxygen is diffused into the sensor the lead is oxidized into lead oxide, this reaction produces a small current between anode and cathode. Galvanic oxygen sensors are, in effect, batteries that generate an electrical current that is proportional to the abundance of oxygen molecules (i.e., oxygen partial pressure) exposed to the sensor. An increase in the exposed PO2 results in a proportional increase in the current generated by the sensor. This is measured by a rebreather computer and converted to a PO2 value. For reasons that should be obvious, careful and accurate monitoring of the PO2 within the breathing loop of a closed circuit rebreather is fundamental to the well being to the diver. Not only is this information vitally important to ensure PO2 levels due not drift into hypoxic or hyperoxic levels, it is also critical for determining decompression obligation. Decompression requirements depend on the partial pressure of inert gas in the breathing loop. Rebreather computers determine the inert gas partial pressure by subtracting the PO2 from the total ambient pressure. Because no two oxygen sensors are exactly alike, they each need to be individually calibrated. The calibration process for the different models of rebreathers will be covered in the manufacturer-specific portion of this course. In addition to individual sensor differences, there are different brands, models, and types of oxygen sensors as well. Not all of them are equally suited for use in rebreathers. Of paramount importance is that the sensor is capable of being pressure balanced (most are). This means that the front and rear of the sensing diaphragms must be pressure-equalized at all points of the dive for accurate measurement of the counterlung PO2 irrespective of depth. Response time is also critical, especially with multi-level profile diving. Sensors with fast response times will be better-suited for use in rebreathers. Regardless of what kind of oxygen sensor is used it also represents a potential weak point. The current generated, and thus the registered PO2, can be affected by both temperature and humidity. Most oxygen sensors are designed to automatically compensate for temperature changes within certain tolerances, and most sensor manufacturers quote an operating range from 0 to 50 degrees centigrade/32 to 122 degrees Fahrenheit. Although generally oxygen sensors are designed to operate in as much as 95% humidity, the humidity in a rebreather loop may exceed this. Droplets of water directly on the sensor can lead to a variety of errors in registered PO2. Like all batteries, oxygen sensors do not last forever. Over time, the sensors gradually lose their ability to generate electricity. For this reason, sensors need to be recalibrated on a regular basis to ensure PO2 conversions are accurate (sensor calibration techniques will be covered in the manufacturer-specific portion of this course). Eventually, the output drops enough that the sensors can no longer be recalibrated to give reliable readings, at which time they must be replaced. The effective life of an oxygen sensor is determined in part by the amount of oxygen it is exposed to. Most sensors will last up to 2-3 years in air (21% oxygen), but the effective life span will be reduced if exposed to higher concentrations of oxygen. Thus, the effective life of oxygen sensors in a rebreather may be extended by flushing an oxygen-rich mixture out of the breathing loop after a dive. To further extend their life, sensors can be removed from a rebreather and placed in a container devoid of oxygen altogether (pure nitrogen, for example). However, if this technique is used, sensors must be allowed a "recovery period" (usually about 15 minutes) before they can be used or recalibrated. As mentioned, humidity also affects the life span of oxygen sensors. Thus, not only is it important to keep sensors dry during operation, it is also important to keep them dry between dives. This is one of the reasons why rebreathers should never be stored with the breathing loop intact (Chapter 6 - Rebreather Maintenance). Because even the best galvanic oxygen sensors are prone to failure, most rebreathers incorporate several sensors to provide a level of redundancy. Two oxygen sensors are not much better than one, because if they do not give the same reading, there is no way to determine which one is correct, and which one is in error. For this reason, most closed circuit rebreathers are equipped with at least three oxygen sensors. With three sensors, there is a "democracy" of sorts, in that "voting logic" may be used to isolate an erroneous sensor reading. This, of course, does not work if two sensors simultaneously fail in the same direction and magnitude, but at least the probability of a false reading is greatly reduced. In most cases, if all three sensor readings indicate the same reading (with certain tolerances of error) an average of the three readings will be taken as the actual PO2. Dan Volker SOUTH FLORIDA DIVE JOURNAL "The Internet magazine for Underwater Photography and mpeg Video" http://www.florida.net/scuba/dive 407-683-3592
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