Trimix...

[IceDiverInCA]

So the only real benefit is shorter (?) deco stops due to quicker off-gassing and no narcosis?

Reduced level of narcosis.
Ease of breathing He at depth because of lower gas density, which reduces the risk of CO2 build up.
Helium mixes allows you to reduce the amount of O2 to go deeper.

Deco stops aren't necessarily any shorter.

 

[Sideband]

10% N2 at 1 atmosphere = 0.1atm partial pressure
At 210 feet (70m approx) = oops 8 atm = 0.8 atm partial pressure.

That's the partial pressure of the Nitrogen, not the O2. PPN2. Right idea, wrong gas.

Joe

 

[gcbryan]

Just to add that the difference between heliox and trimix isn't just a few pennies. If a trimix fill were $60 you can imagine what a heliox fill would be.

 

[wedivebc]

I'm reading that helium comes out of solution much quicker than nitrogen. Assumption -less time in the water then or is the profile conservative, like if you were on trimix?

Yes but it goes in faster too so saturation of slower tissues occur sooner.

 

[padiscubapro]

I'm reading that helium comes out of solution much quicker than nitrogen. Assumption -less time in the water then or is the profile conservative, like if you were on trimix?

The effect of helium on the overall dive time depends on the length of the dive and the dpth, on short dives itsbest to keep the helium to a minimum on long dives (at depth) its best to have as much he as possible..

You are probably thinking... what the heck?? it comes down to the spped that he diffuses into/out of the body.. if your bottom time is long enough to SATURATE a specific controlling compartment, once you saturate it you cant get any more in.. he saturates much faster than n2, but if you saturate with n2 you will take that much more time to get it out of your tissues.. for MOST OC divers it really comes down to minimizing the Helium, for those on CCRs, it becomes more of a decision since we can do bottom times that MOST OC divers only dream about (I'm not taking about cases in caves where lots of bottles can be staged by other divers)... Our limit is basically how much deco do we want to do in the end..

 

[Arduous]

Padiscubapro has given you the best answer. Also remember that He coming out of solution faster than N2, does not equate to shorter deco’s. It more importantly means that you have to be ever more precise in your deco depth & planning, or you’ll bubble over like a soda ! He coming out of solution too fast will form bubbles just like N2 and will hurt, bend or kill you just as quick and in the same manor.

 

[mrobinson]

Thanks everyone! I do plan on getting training eventually but I still need to master a few basics first.

 

[DeepSeaDan]

Did lots of commercial heliox diving - both bounce & sat. Never got bent. All the tables were in-house, created by staff specialists.

The only "Trimix" we used was air.

Best,
D.S.D.

 

[ScubaSixString]

I've been wondering...
plain old compressed air - max depth of 210 feet due to O2 toxicity and let's say standard deco limits
EAN - shallower max depth due to pO2, but enhanced no-deco limits
Heliox (assuming 80%He and 20% O2) - max depth of 210 feet due to pO2 but much enhanced deco limits
Trimix...what is the point of trimix? Every % of He replaced with N2 will give you N2 absorption. I'm guessing if you were to dive at 210 feet and had 10% N2 you would be diving at your max depth of 210 with about 0.7 pO2.

So...what's the point of adding N2 to the mix?

On DEEEEEEEEEEPPPPP dives adding n2 back into the mix staves off HPNS. but thats not going to be a factor at 210

 

[Curt Bowen]

I've been wondering...
plain old compressed air - max depth of 210 feet due to O2 toxicity and let's say standard deco limits
EAN - shallower max depth due to pO2, but enhanced no-deco limits
Heliox (assuming 80%He and 20% O2) - max depth of 210 feet due to pO2 but much enhanced deco limits
Trimix...what is the point of trimix? Every % of He replaced with N2 will give you N2 absorption. I'm guessing if you were to dive at 210 feet and had 10% N2 you would be diving at your max depth of 210 with about 0.7 pO2.

So...what's the point of adding N2 to the mix?

Helium is very expensive and only comes in LP bank cylinders 2400 psi. Adding air to the helium mix allows you to pump the cylinders to higher pressures increasing the needed volume of gas needed for technical dives. Or you can go buy a haskel gas booster to get higher helium pressures.

The deeper the dive, the more helium used to cut narcosis and O2 tox problems.

Example: I use different trimix mixes for planned dives

15/25 for 200 feet 15O2/25HE
14/33 for 300 feet
10/52 for 400 feet
8/64 for sub 400

All these mixes are Heliair (helium topped with air) Why, because its easy and accurate to mix and I have diving it for 15 years. So I know it works.

N2 also cuts decompression times. A 20 minute exposure at 400 feet will give you about 4 hours of excellerated (using nitrox and oxygen) decompression times. To do this dive on heliox, the deco would probably be around 5 hours?

Helium has very little thermal values, so you will get cold quicker than on air.

So, to wrap it up:

N2 adds volume, cuts deco times, keeps you a little warmer, and its cheeper.

The real answer for deep diving is to buy a rebreather.