@Akimbo all heliox for ease of mixing though right?
To be accurate, it is more complicated (some of this
@TBone already knows but I'll go over it for others). Historically, I would "guess" that HeO
2 was chosen over Trimix due to unknown (unproven) physiological concerns. The investigation into using Helium-Oxygen to prevent Nitrogen narcosis was first suggested during the salvage of the submarine F-4. She sank in 1915 off Pearl Harbor Hawaii in 306'/93M of water. See:
US Navy Experimental Diving Unit
I imagine that analysis was a relatively minor consideration because their understanding of ingassing and outgassing rates for Helium was unproven and decompression calculations were based on assumptions from animal studies. The calculations were entirely manual (probably aided by slide-rules). Helium was not nearly as expensive for the US government since there was more supply than demand. Significant Helium reserves were discovered in natural gas wells in the US in 1903. The Helium Act in 1925 banned exporting Helium. As a result, it was impractical for other countries to experiment with it.
The vast majority of Helium used for diving today is by commercial diving companies providing services to the offshore oil industry. Of that, the vast majority is used in
saturation diving and related work including subsea pipeline welding operations. The quantities required are so vast that helium reclamation and surface-based recirculating systems for diver's breathing gas was implemented decades ago.
Today, the main reason for using HeO
2 in sat is to reduce WOB (Work Of Breathing). Remember that divers are working at depth much harder and longer than recreational rebreather divers. The cost savings would be relatively minor considering that Helium loss rates are typically in the low single digit range (as a percentage). It is also true that mixing and analysis required for Trimix would be much more complex on sat systems because the gas is recycled many times and removing large volumes of Nitrogen for deeper work is slow and expensive.
From personal experience, I can say the the WOB on HeO
2 around 900'/275M is quite noticeable. Just getting geared up and out of the hatch was much more difficult. To me, WOB on HeO
2 didn't seem any different around 300'/100m than air at the surface.
Mixing for open-circuit surface supplied diving (today) is not a problem since industrial gas suppliers can easily provide very precise and complex gas mixes (when relatively large volumes are ordered). Trimix is used commercially by some companies for the small amount of surface supplied mixed gas work done today. Most deep water oil and gas fields have DSVs (Diving Support Vessel) on contract with sat systems onboard so all but the smallest mixed gas jobs are done by them.
With that background, using Trimix for very shallow sat dives would be possible -- like less than 50M/165'. However, the additional analysis and controls required make it between rare and impractical. It would also make banking gas much more complicated. Removing Nitrogen from breathing mix gas banks for deeper sat dives would also be difficult and expensive when shifting to deeper water. It is much less expensive to run HeO
2 with surface-based closed circuit systems than even the lowest Helium content in Trimix on open circuit.
The first open sea use of Trimix for deep diving that I am aware of was on the
Hannes Keller's 1,000' dive in 1962. Keller used Trimix a year or two earlier in lakes and chamber dives.