Use of Heliox by Commercial Divers

[dougchartier]

In the NatGeo show Delta Divers (a really good show about commercial divers in the Gulf of Mexico), I saw that the divers used heliox for their deep dives. The deepest I've seen them go so far is 300 feet.

I was wondering why these divers use heliox rather than trimix. Or, if you want to turn the question around, why do non-commercial tech divers often use trimix instead of heliox at those depths? Is it just a matter of cost?

 

[Akimbo]

Cost is a factor, as is thermal conductivity for recreational divers. HeO2 tables are much more proven in these depths, especially where Sur-D-O2 (Surface Decompression using Oxygen) is part of the profile. Another factor is reduced respiratory work load and no narcosis regardless of mix.

Reliability in decompression tables for commercial diving is justified well beyond the humanitarian consideration. A little more money in gas is irrelevant compared to screwing up a construction or salvage schedule because somebody takes a hit. They put you in hot water suits if the breathing gas makes you too cold.

 

[ajduplessis]

As mentioned, the greatest advantage is that of elimination of nitrogen narcosis, resulting in a safer working gas. The drawbacks are longer deco times, typically done on a strong Nitrox mix, and possible danger of HPNS at great depths.

 

[Akimbo]

I was thinking about your question and decided that a little more detail on gas supplies might be useful. Smaller scale commercial diving operations buy premixed gas from industrial suppliers in relatively large quantities. The smallest purchase is usually multiple six-packs — six industrial gas bottles in the 250-300 Ft³ range, sometimes called "K" bottles, in a steel frames moved by cranes, fork lifts, and 150'+ steel supply boats.

Entire HP tube trailers you see rolling down the highway are common on larger operations, often using small chamber and bell systems (not always used in saturation mode). Large ship-based saturation systems often have their own mix-makers integrated with 10 HP+ high-purity compressors and helium reclaim systems. These operations usually buy pure helium in tube trailers unless they know they will be operating at a set depth for extended periods. Gas suppliers mix gas more cost effectively so they may also buy tube trailers of HeO2 pre-mix.

In addition, they have at least two 25 HP+ industrial two stage LP (150-200 PSI) air compressors for the decompression chamber(s) and shallower air dives. It is not unusual that the only HP air onboard is in a few bailout bottles for air jumps.

Compare this to recreational/technical gas mixing that starts with HP air in the tanks the diver will wear and adjusts the mixture by adding Helium and/or Oxygen — typically by cascading or pumping from individual industrial bottles. It is expensive to impossible to get industrial suppliers to make custom mixes in small quantities.

Commercial diving operations in the North Sea are much different than you saw in the NatGeo Delta Divers TV program. The last I heard, air diving is limited to 50 Meters/165' and closed bells are required for mixed gas operations. Vessels are generally much more seaworthy due weather conditions so you don't see many small bell systems used for HeO2 bounce dives. As a result, saturation diving dominates where the PPO2 is typically maintained around .30 in the chamber and in the water.