Many divers benefit from using nitrox when diving for extending bottom times or shorter decompression intervals. There is potentially a safety benefit for using nitrox, however this has not been demonstrated with statistical significance due to the relative number of divers who have experience decompression illness on nitrox compared with air.
Analyzing the oxygen content of gases is critical when using non-air breathing gases. Gas analysis is performed by each diver themselves at a good scuba shop fill station when the cylinders are filled, and again at the dive site on the day of the dive. For this reason, a portable oxygen analyzer should be part of a divers kit. Several types of analyzers and their advantages and disadvantages are discussed below.
All analyzers must be calibrated before they are used to analyze an unknown gas mixture. The measurement must be performed on a known reference gas, e.g. a cylinder of pure oxygen or regular air, and adjusted until the analyzer is in agreement. The analyzer can then be used again immediately on the test gas under identical ambient temperature and pressure conditions, and gas flow rate.
Types of oxygen analyzers
Analyzers are typically available as cheaper handheld units, or more expensive systems that can also measure other gases, e.g. helium content in trimix for very deep dives. Handheld units are sufficient for recreational nitrox diving provided that they are used according to the manufacturers instructions. The oxygen percentage is measured to one decimal place, e.g. 32.1 %, but rounded up to a whole number for use in dive tables, e.g. 33 %.
A handheld oxygen analyzer.
Other features that are available for more expensive systems are attachments for DIN/yoke valves, flow-rate limiters, inline pressure gauges, a hardwired power source and thermal sensors.
Dive shops will typically have a pressure gauge and flow rate limiter for ease of use, allowing the diver to verify the cylinder mixture and pressure in one operation. The flow rate limiter will provide a consistent flow rate from a high pressure source that can be tuned to the optimum rate for the analyzer, and so is consistent when measuring the known reference gas and the gas under test.
A hard power source removes the need to replace batteries. A thermal sensor can be used to measure the cylinder temperature to provide a more accurate reading, but are not commonly used in practice.
How oxygen analyzers work
The heart of the analyzer is a galvanic cell that acts as the detector. The cell has an exposed membrane and a port to connect to the analyzer electronics. When oxygen in the surrounding gas contacts the membrane it reacts chemically producing an electric current and consuming the fuel in the cell. For this reason cells have a limited lifetime and are disposable.
The more oxygen that is present in the gas, the more current is produced. The job of the analyzer is to measure the current and infer the percentage of oxygen that is present in the gas. It is therefore critical that the analyzer is calibrated with each use (or according to manufacturer instructions) so that this calculation remains accurate. Like all sensors, they are designed to work with sufficient accuracy under a range of conditions, like gas flow rate, ambient temperature, and with “normal” gases. Old cells will have the galvanic fuel consumed, and will not be able to produce as high a current, and so can only read up to a maximum oxygen partial pressure that degrades over time. A new cell might be able to read up to 2.0 atmospheres (ata), but fall over its lifetime.
At sea level a mixture of 100 % oxygen will produce a partial pressure close to 1.0 ata, and the degradation will not be noticeable for a few years with normal use and conditions. However, this is a concern for applications where hyperbaric oxygen is present, for example in a rebreather with 100 % oxygen at a depth of 20 feet where the oxygen partial pressure must read 1.6 ata. Hence oxygen sensors in handheld analyzers have a much longer useful lifetime than in rebreathers.
The oxygen sensor is a current source, but this is converted into a voltage in millivolts (mV). This reading is often available on an analyzer, and will also degrade as the sensor ages. The normal range is available from the manufacturer, e.g. 7–14 mV for air at sea level on a Teledyne R-17 sensor.
- Keep new cells sealed in their plastic packaging until they are installed.
- Do not use new sensors immediately after unsealing, wait at least 30 minutes for the sensor to acclimate to the ambient pressure and temperature in case the packaging conditions are different.
- Only use oxygen sensor cells specific to the analyzer specifications.
- Label the install date on the sensor cell in permanent marker when it is removed from its packaging.
- Discard sensor cells 3 years from the date of manufacture, or 1 year from the date of opening.
- Always calibrate the analyzer against a known gas source with the correct flow rate.
- Label analyzed cylinders appropriately immediately after being filled.
- Reanalyze (and calibrate!) cylinders again on the day that they are used to verify the mix.
- Store analyzers in a dry environment close to room temperature.
- Dispose of used batteries and sensors properly, away from water sources.