Tuesday, 01 May 2001 00:00

Trimix & Heliair - What exactly are they?

By Richard Taylor

What are Trimix & Heliair, and what are the differences? Do we need special labelling to identify each.

Trimix is a term used for a gas which usually contains oxygen, helium & nitrogen. 'Tri' meaning three and 'mix' from mixture. The proportions of the three gases is determined by the depth of the dive and what levels of oxygen (to limit toxicity) and nitrogen (to limit narcosis) partial pressures are desired. The greater the amount of helium in your mix means that decompression will generally be required to start at a deeper depth during the dive. It will also mean that the cost of the mix will be greater as Helium is the most expensive component of the mix. Trimix is usually made by adding oxygen, helium & nitrogen (or Oxygen, helium & nitrox) in separate procedures.

Heliair is also Trimix, but made up of Helium and then adding Air, air being 21% oxygen and 79% nitrogen.

As a guide, use recommended limits on oxygen at 1.5 bar (to limit toxicity) and nitrogen at 4.5 bar (to limit narcosis) during the dive. This means that a 80m dive could use 16% oxygen and 50% nitrogen with a remainder of 34% Helium. The common technical diving term for this mix would be a 16/34 mix (oxygen then helium).

This mix though contains a ratio of oxygen to nitrogen of 16:50. This means that different proportions of oxygen and nitrogen would need to be added to make the desired mix. We could do that by pumping these gases from two separate cylinders or by adding oxygen and then helium & then topping with air having calculated the required amounts.

If using Heliair, the ratio of oxygen to nitrogen would always be constant at 21:79 or about 1:3.8. Therefore a 16% oxygen mix would require you to use 60% nitrogen (and so 24% helium). To stay within the limits used for the 80m dive above (a maximum 16% oxygen and 50% nitrogen) the closest Heliair mix that could be blended would be 14% oxygen, 53% nitrogen and 33% helium. This mix could be easily made by filling a cylinder a third full with the required amount of helium & then topping with air to the desired pressure.

Heliair is easier to make than desired Trimix, but will generally mean that your selection of mixes is limited, resulting usually in longer decompression times or a higher narcosis factor. In addition, as you are using more Helium to make the Heliair mix the cost is also greater.

There are no differences in the labelling. Cylinders need to clearly state that Custom or Mixed Gas is inside and a cylinder contents label must be affixed showing actual contents and maximum safe breathing depth.

What is Heliox and how does it differ from Trimix?

Heliox is simply a mixture of Helium & Oxygen, with no Nitrogen, and is used primarily in the commercial diving industry and in most mixed gas rebreathers. As there is no nitrogen in the mix, there is therefore minimal narcosis. Heliox mixtures can be used at depths over 100msw / 330fsw, though beyond this depth we begin to run the risk of High Pressure Nervous Syndrome, or HPNS.

HPNS is believed to be a result of a lack of nutrition to the brain, resulting from a constriction in the surface tension in membranes caused by the biological effect of absorbed Helium. As nitrogen causes the opposite, a rise in constriction in surface tension in membranes, nitrogen is often used as a "buffer" for deep mixed gas dives. Thus the use of Trimix. The symptoms of HPNS include tremors of the hands, whole body tremors, personality changes and convulsions.

In addition to the physiological factors associated with Heliox, as we are using high partial pressures of Helium we therefore have a far greater rate of absorption and elimination of Helium into and out of the tissues. In comparing Trimix for dives less than 100msw we actually can have significantly reduced decompression times whilst for dives over 100msw we can have an increased decompression liability. Of course the amount of Helium we are using will also directly increase the cost.