Performing Gas Analysis for Oxygen Nitrogen Breathing Mixtures
by Joel D. Silverstein and R. W. Bill Hamilton, PhD.
Gas analysis is one of the critical steps that ensures a diver's safety. All divers who use nitrox need to know how properly to analyze the gas that they will be using. Before discussing the procedures of how to analyze gas. A short discussion of how analyzers work is in order.
|Oxygen analyzers are available from many manufacturers, in different sizes and types; some have a digital display and others use an analog (needle) readout (Fig 1-1). Either type can be used successfully for analyzing breathing gas mixtures. Ideally, an analyzer must be able to read and show oxygen values down to a fraction of 0.001 or 0.1% (one-tenth of a percent). For example, the digital read out should be able to read 32.4% instead of 32% or 35.8% instead of 36%.
|The heart of an oxygen analyzer is its detection method. There are two primary types of oxygen analyzers generally used for breathing gases; paramagnetic and electrochemical.
||Figure 1-1 Several oxygen analyzers. From left, Analox, OMS, Teledyne and Catalyst Research - Mini-Ox .
The paramagnetic analyzer takes advantage of the fact that oxygen is attracted to a magnetic field; these are primarily used in research laboratories and are accurate, stable, relatively expensive and somewhat delicate.
The other category of oxygen analyzer available in portable units comprises those that are electrochemical in function. The electrochemical cell breaks oxygen into ions and electrons and measures the current generated; this current is proportional to the partial pressure of oxygen to which the sensor is exposed. Electrochemical oxygen analyzers use two electrodes made of different metals; these are immersed in an electrolyte solution that is contained by a thin, oxygen-permeable membrane. Oxygen diffuses through the membrane to the cathode, where it is reduced, generating a very small current. This current is linearly proportional to the PO2; it is measured by the unit's electronic circuit and the result displayed.
The electrochemical analyzers are of two types - polargraphic and fuel cell. The function of these is the same to the user and the difference is not relevant here. These are also used in laboratories and industrial settings.
Electrochemical analyzers are relatively inexpensive, can be made portable and rugged, and show little interference from other gases. However, they tend to be unstable and may need frequent calibration, especially as the cell begins to age. Cell life depending on manufacturer and use can be anywhere from 6 to 18 months.
The analysis process
The process of analyzing a cylinder of gas mix involves calibrating the analyzer with a "standard gas" or "calibrating gas" then repeating with the "unknown" or "sample" mix. Both calibrating gas and the sample mix should be passed through the analyzer at the same flow rate.
The need for calibration
No matter what the method or mechanism of a gas analyzer, the analysis is no better than the calibration of the analyzer. An electrochemical cell tends to be quite linear. Some types of analyzers are not linear, and these can be a great deal more trouble to use. Being linear, it need be calibrated in only two places to give reliable readings. This is called "zeroing" and "spanning." In effect this sets the slope and the intercept of the calibration curve. Usually an oxygen analyzer is "zeroed" with an inert gas such as argon, nitrogen, or helium; the gas type does not matter with this mechanism. Some analyzers rely on an electrical zero, but this may not be quite as reliable, since this method does not account for drift of the sensor cell.
Ideally it is best to calibrate or "span" an analyzer with a "standard" gas that is close in oxygen level to the sample. For analyzing oxygen levels in the range of 21% to 40% oxygen, it is satisfactory to calibrate with fresh outside air, using the value 20.95% oxygen. Since values above 20.9% are "extrapolations," if an analyzer is spanned but not zeroed properly it may be off by a percentage point or two at 40%. The best method is to "bracket" the unknown with calibration gases. For meters that do not span, using compressed air at a constant flow rate to calibrate the meter is best.
Need for two analyzers
One properly calibrated analyzer, used correctly, is adequate for checking scuba cylinders. Repeated analysis with the same unit properly calibrated helps build the confidence in the tool. However, while one well calibrated analyzer is sufficient for a given analysis, NOAA requires that two properly calibrated analyzers be used for gas analysis.
It is operational procedure for the diver to analyze the gas before accepting or signing out a nitrox cylinder for use. In addition the diver will want to analyze the gas again just before diving with it. The procedures for analysis are simple to follow, but must be done carefully to maintain the integrity of the resulting analysis. The diver will choose diving tables and oxygen limits based on this analysis. It is standard protocol and essential to check the analyzer's calibration with a known gas (such as air) before performing the analysis.
The best calibration gases are normally obtained from a commercial gas supplier, and should be in the range of or just a little higher than the sample to be analyzed. As an alternative, atmospheric air has a uniform composition everywhere, at a level of 20.95%, or 21% for practical purposes. Industrial "air" obtained in cylinders might vary from this value, so only compressed atmospheric air should be used. The problem with setting an analyzer with air is that mixes in the high end of the enriched air range are then extrapolations, so calibration with air should be done carefully
Proper analysis depends not only on the analyzer itself but on the flow rate of the gas passing in front of the sensor cell. The sample should be read at the same flow rate as is used for the calibration. A flowmeter or flow controller should be used. Different tools are available to regulate the gas flow for analyzing (Fig 1-2). One type uses a special regulator fitted with a flow valve capable of adjusting the flow rate from 0 to 10 liters per minute (the upper end of this regulator is much higher than needed). Ideally, the flow should be between one-half and 2 liters per minute, nominally 1 lpm.
|Figure 1-2 Gas Sampling Flow Meter
One type of special fitting is available that connects to the low pressure inflation hose of a scuba regulator and acts as a flow meter that is either adjustable or preset to 2 lpm. Although it is possible to analyze the gas without a flow meter, readings can be inaccurate and this is not recommended practice.
Figure 1-3. Gas sampling system using an oxygen analyzer with flowmeter set to 2 lpm, using air for calibration showing reading of 20.9%.
An oxygen analysis is only as good as its calibration. Before performing an analysis the analyzer should be calibrated to a known gas. First check or set the zero, using a pure inert gas (nitrogen, argon, or helium) and the method described here, or use the analyzer's electronic zero. Generally the zero does not drift very much.
The flow rate regulator is attached to the source of calibrating gas or compressed atmospheric air. Turn on the oxygen analyzer, set the flow rate at (nominally) 1 liter per minute and let the gas flow through the sensor for approximately one minute or until the reading is stable. Once settled, make sure the reading is set to the value of the calibrating gas, or 20.9%, (Figure 1-3) if air is used; adjust the calibration setting on the analyzer if necessary.
If adjustments are needed too often during re-calibration it may be time to replace the sensor.
||Analyzing the nitrox cylinder
Once the meter has been calibrated, leave it turned on and move the sampling device onto the tank that needs analysis (Figure 1-4). Remember that the flow rate should be the same as that to which the unit was calibrated. Let the meter read the new gas for at least one minute or until stable. (Where the sampling hose between the cylinder and the analyzer is more than 24 inches in length let the flow continue for at least 2 minutes.) The resultant reading is the partial pressure of oxygen in the tank at ambient pressure.
At sea level this is equal to the oxygen fraction, which can be converted to percentage by multiplying by 100. Transfer this number immediately to the cylinder contents label, fill out the rest of the data, and attach it to the cylinder and enter in log book.
Every enriched air cylinder must be properly labeled as to its contents and fill data (Fig 1-5). In some cases, once an cylinder has been analyzed at the fill station it is likely that it will not be analyzed again. Unless analyzed again immediately before use the cylinder contents label is the only way to know what gas is in the tank before diving. The data include fill date, tank pressure, oxygen percentage, maximum operating depth, the name or identification of the person who filled it out, and the users initials verifying that it was analyzed. The contents label or tag should be firmly attached to the cylinder or valve. Plastic re-useable contents tags can be written on in pencil and erased for the next use. Non-reusable labels should be written on with a permanent marker, never with a grease pencil (which may come off), and should only be removed by a gas blending technician before the next fill.
Figure 1-5: NOAA nitrox cylinder
label filled to 3000 psi and 31.8%
Material in this article appears in: NAUI Nitrox A Guide to Diving with Oxxygen Enriched Air, (1997, 1998) and in the NOAA Diving Manual 4th Edition (1999). All material is copyright, RW Hamilton, PhD and Joel D. Silverstein. www.NitroxDiver.com
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