TecEvolution, More about Heliair mixes

Trimix is defined as any mix of Helium, Nitrogen, and Air. Heliair is a particular type of Trimix, consisting in a mix of Helium and air. If agencies training are designed for any trimix gas blend, heliair offers a couple of advantages in term of cost (avoiding the purchase of a helium analyzer) and convenience (partial pressure blending is a lot easier).

The only advantage of trimix over heliair, is that you can dive an optimum gas blend for a defined dive, by optimising your bottom oxygen partial pressure to minimize decompression requirements (not an issue when diving a closed circuit rebreather) .

Heliair offers many advantages over classic trimix blends, and as it is a specific kind of trimix, it does not require any other training than a trimix course. Here is a list of heliair advantages over trimix:

• Analysing Heliair is a lot more easier, as the reduced content of Oxygen in the Blend is only due to air dilution by Helium. Thanks to this direct relationship between Helium and Oxygen content, you can analyse your blend with an Oxygen Analyzer and deduce the percentage of helium in the blend using mathematical formulas (later on this page). When blending a non Heliair Trimix blend, it becomes impossible to do so because you do not know the cause of the oxygen reading changes as it can be the added helium, or the added Oxygen.
• As analysing a non Heliair Trimix Blend is impossible with an oxygen analyzer, it implies to buy a slow and expensive Helium analyzer, or to drain tanks completely before each Fill. Helium being an expensive gas, and as technical divers keep 1/3 of each tank for emergency purposes, you want to reuse as much as possible of the remaining gas in the cylinders. With Heliair, you can top up your tanks an unlimited number of time.
• As there is never any pure oxygen added in the tank, there is no need for the heliair tanks to be oxygen cleaned and clearly reduces the risk of handling pure Oxygen.
• On expeditions or repetitive days of diving when the gas turnover is quite high, blending Heliair is a lot less time consuming and a lot more simple than blending trimix
• Having the optimal mix isn't always optimal. With Heliair, the bottom ppO2 (partial pressure of Oxygen) is always less than the optimal 1.4, which consequently allows (gas, O2 exposure and decompression information allowing) to extend an exploration dive beyond the planned maximum depth.
• On deep and long dives, the maximum oxygen exposure (wrongly called CNS% or CNS clock) can be a limiting factor for the dive, and reducing the bottom ppO2 can be convenient.

• Formula 1 : Which Heliair mix to use for a planned depth and desired END :

Let's first calculate the fraction of helium (FHe):

                      ( END + 10 )
FHe = 1.0 - -------------------
                 ( Max_Depth + 10 )

Then calculate the fraction of oxygen (FO2):

                  ( END + 10 ) * 0.21
FHe = 1.0 - ----------------------
                   ( Max_Depth + 10 )

So For a dive at a planned depth of 60 metres with a desired 30 END :

                      ( END + 10 )            40
FHe = 1.0 - ------------------- = -------- = 0.57 = 57% He
                 ( Max_Depth + 10 )        70

                  ( END + 10 ) * 0.21       40*.21
FHe = 1.0 - ---------------------- = ------- = 0.12 = 12% O2
                   ( Max_Depth + 10 )         70

The ideal Heliair Mix would be Tx 12/57

• Formula 1 : How much Helium for What mix :

PHe (Helium Pressure) = FHe * Final_Pressure

So For a 200 bars cylinder with 33% desired Helium :

PHe = 0.33 * 200 = 66 bars of Helium

• Formula 2 : What Fraction of Oxygen for that mix ?

FO2 = ( 1 - FHe ) * 0.21

So in our case :

FO2 = ( 1 - 0.33 ) * 0.21 = 0.14

I should analyse 14% of oxygen in the mix to confirm the 33% Helium Content


• Formula 3 : How much Helium left from the previous dive ?

PHe = FHe * Remaining_pressure

So in our case, if there are 60 bars left in the cylinder :

PHe = 0.33 * 100 = 33 bars of Helium Left

How to top up for which mix ?

With these 100 bars leftover in my tank, how do I top up for a 200 bars 17/19 mix ?

To get a 17/19 mix from empty I need : .19 * 200 = 38 bars of Helium

38 bars - 33 bars Remaining = 5 bars

All I need to do is to top up with 5 bars of helium before toping up with air

Important:

• Sometimes, you will need a booster pump or a full helium cylinder to add enough He in the cylinder