Analyze Trimix or Heliox
Analyzing your gas at the point of purchase or blending is a good rule of thumb - many facilities will operate a good process and will get you to analyze and sign for your Gas.
Now blending is full of interesting physics (thermal, fluid and gas dynamics) - the behaviours of the gases to be blended have a range of intersting effects - the cruical ones are the slight paramagnetism of oxygen, adebatic heating and differing densities. If you blend a mix really really slowly - it will analzye as the last gas entered (usually air), and this will often be the case for many hours after filling.
The effects of adebatic heating will of course affect the differing gases accordingly. Hence a good blender will know how much 'extra' Oxygen, 'extra' helium and/or 'extra' Air to make the required mix. Many add the Oxygen and Helium and then allow to cool checking the exact final cooling pressures before adding air. You will see good blenders tumble or subject to risk assessment and correct lifting techniques pick up the cylinders and invert them and correct them several times to stir the contents to create a homogeneous mix.
We deliberately mix Nitrox 40%, Nitrox 80%, Trimix 18/50 and Trimix 10/52 in numerous 50L 350 Bar bank cylinders so we can use these well mixed gases to create other blends with a good degree of accuracy and extremely well mixed (=homogeneous).
Now on to the key bit. Why is it very bad (even though this was the practice for many many years before low cost Helium Sensors), to only analyze the Oxygen content in the mix? The answer is an interesting one - your decompression penalty and mix ratio is based on the combination of gases - in Trimix (ignoring the minor extra nobel and other gases) comprises a dominant triad of Oxygen, Helium and Nitrogen - Now Helium is so small an atom and has high kenetic energy that it will find any route possible to escape - hence why Trimix Divers turn off their cylinder valves so tightly (and also the central isolator on a twinset when mixed). The worry is that if this helium escapes you may see little rise in the oxygen % when reanalyzed, however a significant loss of overall pressure and loss of helium may be seen - which will affect decompression requirements (in some instances marginally). Overall the equivalent narcotic depth is increasing (i.e. worsening), so the benefit of Helium is being lost.
I would recommend the use of combined O2/He units now that the price of helium analyzers are now several hundred pounds - they are worth buying and you can more accurately test your gas both at the Gas facility and again prior to your dive (or the morning of the dive). Most training agencies look for +/- one whole % for the oxygen (i.e. Nitrox 36% can be 35.1% to 36.9% as an analyzed gas with no more than 2L/min gas flow through the sensor - or a flow rate as defined and specified by the sensor manufacturer). Helium can be a bit more generous in analysis so an 50% helium mix requirement can be acceptable as 45.1% to 54.9%.
Take can to follow your training and use this accurate % to determine a safe Target Operating Depth for the Gas and it's Maximum Operating Depth - both determined by the Oxygen content and hence Toxicity. Remember to also work out it's Equivalent Narcotic Depth if the mix contains Nitrogen to ensure the mix choice is a sesnible one for the target operating depth (= sea bed or maximum intended depth on the dive plan). Ensure your Gas is correctly marked and it is marked up without interruption. if you get interrupted by others - retest your Gas. Once Labeled then ensure you set decompression computers soon after (if not again right away).
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