Heliox vs. Tri-mix

I was just wondering why one wouldn't just use Heliox instead of Tri-mix so that you could do away with nitrogen all together? Is it because of the cost of Helium since it would take more helium to do this, or are there other things such as deco procedure or something. I was reading a lot about Navy dive operations and noticed that they used both Heliox and Tri-mix, although it seems they only use the Heliox on reeeeeally deep dives where I am assuming even small amounts of nitrogen could cuse problems.

PS. I am not Tri-mix certified and I am not trying to gather information so that I can go out and try Heliox at 1500fsw, it is just something that I am curious about and I thought this thread would be a good place to ask the question. Thanks

Well there are a couple reasons, as you mentioned Helium is very expensive is the first one. In the commercial diving industry the helium is reclaimed or captured from the helmet and reused as much as possible, reclaim systems are mandatory with most companies now especially since its purpose is deep dives, you can imagine how fast it gets used up doing a quick comparison of cubic feet used at depth...

330 fsw = 10 atmospheres absolute

if you normally use 1 cubic foot a minute at the surface you would be using ten cubic feet a minute at depth...

Most people mixing gas for recreational use at home or in smaller shops utilize the cascade method using the pressure in the bank or supply cylinder to fill the bottle. Unless you have access to a gas transfer pump (also quite expensive, look up Haskel on line) to develop the required pressure you would be restricted to the pressure in your bank for fills.

Tri-mix uses air from your high pressure compressor to top off the cylinder to the rated working pressure.

Check the Navy dive manual (you can download it online) the heliox (mixed gas) decompression tables require very long stops in the water as well...

Jeff Lane

There are a few reasons...

Starting around 400 you want to have some nitrogen in the mix to prevent HPNS.

The body on/off gasses Helium considerably faster than nitrogen so on short dives its best to only use the minimum amount of He necessary to control narcosis to keep deco schedules to a minimum. On long dives the opposite is true, keep the nitrogen to a minimum since saturating with nitrogen will take much longer to offgas..

As Jeff said, cost is the main reason.
Another thing that pops up is HPNS (High Pressure Nervous Syndrome), which is tremors & shaking that occur when fast descents are made while diving high partial pressures of helium. Some nitrogen added to the mix alleviates this effect, though it's not a problem for everyone & doesn't come into play 'till below 400ft.
Folks like to do the fast descents to get as long of a bottom time as possible.

rmediver2002 once calculated...

330 fsw = 10 atmospheres absolute

if you normally use 1 cubic foot a minute at the surface you would be using ten cubic feet a minute at depth...

Click to expand...

Ah close, but no. Assuming 1ATM/33 feet, that's 11 atmospheres absolute or ATA (you forgot to add the 1 ATA at the surface). Or:

330/33 = 10 ATA + 1 ATA surface pressure = 11 ATA.

So if at the surface you breath 1 CF/min at depth you'd be breathing 11/1 = 11 CF/min.

Roak

Thanks for the info guys. So if there is a problem with HPNS bellow 400fsw, what are those helmet divers breathing at 1500fsw???

Commercial divers use heliox, the effects of HPNS seem to be brought on by rapid decent (no one knows the exact mechanism causing it yet). Commercial divers conducting deep work will use saturation diving techniques. After a slow decent to a staging depth or pressure (the compression phase is conducted topside in a chamber were the divers can be observed by the diving supervisor) the divers will remain until the tissues in thier body become equalized or saturated with the inert gas partial pressure for that depth. They are lowered to the work site in a diving bell. (like a mini submersible chamber)

Some experimentation with adding small amounts of nitrogen to the mix seems to help as well.

There has been studies conducted using other inert gases as a dilutant for commercial work as well. Hydrogen is one such example. It has been proven to be stable when using deep mixes (the % of 02 is low enough that the danger of explosion is reduced or eliminated)


Jeff Lane

Thanks Jeff,
And by the way, welcome to the board, I haven't seen you around before. You seem to know a lot about commercial diving, do you have first hand experience or are you just studied up on the subject?

Thanks, I have only been on the board a little while.

I was an Army deep sea diver for 10 years (it is mostly shallow inland diving construction / demolition / salvage ) during that time I would work part time for other commercial companies on the weekends as much as I could as well.

Jeff

rmediver2002 once bubbled...
...
Check the Navy dive manual (you can download it online) the heliox (mixed gas) decompression tables require very long stops in the water as well...

Jeff Lane

Click to expand...

Found what looks to be a decent link for this:

http://www.vnh.org/DivingManual/DMTOC.html

Nearly 50 MB, so kick back and wait for the download