Nowadays there are quite a few divers using "Nitrox", and the numbers are increasing all the time. Some of them dive on a regular basis and have therefore mastered the technique used to analyse the blend correctly. Others do not dive very often and tend to forget some critical steps that will give the correct reading without damaging the oxygen analyser. So, let's revisit what you should do before you dive with Enriched Air Nitrox.
In order the get a proper reading you have to give your analyser a reference. Normally, you will calibrate it with normal air. Normal air contains 20.9% oxygen, so the first thing you do is to flood the unit inlet with pure air (which means 20.9% oxygen) until you get a constant reading, whatever that may be. If the reading is not 20.9% you need to calibrate the analyser so it is giving this reading. In order to do this you will turn the small knob every analyser has in order to calibrate it to 20.9 on the display. This knob is very sensitive, so do not turn it very quickly. Another way to flood the inlet is to use the content of an air tank, being sure that that tank contains air (for the gas flow, refer to next section). The analyser can be calibrated with pure oxygen as well, but since we do not use pure oxygen for recreational diving the procedure will not be covered here.
Setting the right gas flow
There are several types of analyser on the market. The most common ones have an inlet that can be put in contact with the valve of the tank. It is VERY important that you open the tank before you put the analyser in front of the valve. You should be able to create a gas flow of about 2 litres per minute. That is a very faint hiss that does not become a high pitch sound, or a slight pressure on your finger if you place it in front of the valve (refer to my January article). Sometimes the valve can be a bit stuck, so you might end up creating a too powerful flow, which is dangerous for your ears, can damage the oxygen sensor in the analyser and does not give you an accurate measure because pressure builds up inside the instrument inlet eventually damaging the sensor.
A few analysers have a din-connector that can be screwed directly in the tank's valve. This connector has a flow restrictor that will create the right flow your analyser needs.
Other analysers have a connector that fits the low pressure hose of your BCD. So just connect it and the job is easily done.
Reading the analyser
Once you have set the right flow, you should put the analyser's inlet in contact with the tank valve. The unit's display should start to show you an increasing number that should stop to climb quickly as soon as you get close to the right amount of oxygen in your blend. When the number does not change anymore, then you have the percentage of oxygen present in your mix.
If you do not get what you were expecting, there can be a few reasons:
- the analyser was not calibrated properly, so re-flood the inlet with air, re-calibrate and re-analyse the mix
- the blend in your tank is not what you were expecting. Re-analyse it anyway to confirm it
- the sensor is at the end of its life. Change the sensor or use another analyser
- the batteries are low. You should get a warning on the display if this happens. Change the batteries or use another analyser
- the gas flow from the tanks was too weak (not enough gas to analyse, so a lower reading) or too strong (too much pressure on the oxygen sensor, so a higher reading). Adjust the flow and re-analyse.
Now you should log your analysis on the dive centre log, the shop log or the club log. This MUST be done before the dive. It may also be standard practice in some dive centres that you are asked to write a few details on the tank by means of a label or a permanent marker (your name, maximum depth of the blend and oxygen content). This is done to avoid confusion and alert other divers to the fact that you are using a Nitrox tank, and you have analysed it for your use.
Caring for the oxygen analyser
The oxygen sensor does not like water, especially salt water. So keep it in its box or away from the water. Please, do not leave it on the bench of a diving boat with dripping suits around or spray coming from the sea. The oxygen sensor will have a longer life the less it is exposed to oxygen. So a good practice at the end of the analysis is to re-flood the inlet with pure air.
If the unit does not have an auto power off system, remember to switch it off, otherwise you will run the risk to have a very low battery next time you use it.
Some analysers have a protection cap screwed into or inserted over the inlet of the unit. These caps have a small hole that will let gas go in, but will keep most of the water and dust out. So, obviously, please leave these caps in place to protect the unit and do not remove them.
Happy Nitrox diving!
If you have any questions you would like 'Agony Aunt' Dan to answer, email him directly here or email email@example.com.