Standard gasses

[formernuke]

Spinning off from a comment in the roaring river thread. Maybe because I haven't taken my class yet.

What are the standard diluent mixes? I've heard a few trimix ones mentioned other places. I'm curious as to what all of them are, or maybe it's just because I'm bored.

 

[beldridg]

There are really three (two are closely related) primary factors to consider when selecting a Dil.

The first is the amount of O2. The general rule of thumb is to make sure your Dil has a max ppO2 of 1.0 (or 1.1 for some) at your max planned depth.

The second is to make sure you have enough Helium to avoid narcosis. Most people aim for an END of 100 feet or less.

The third is to have a gas density of 5.2 grams per liter (or less) at max planned depth. It turns out that if you include enough Helium for the second factor, then you generally have a reasonable gas density (you might need a little more Helium to get to 5.2 or less).

Standard OC bottom gasses don't necessarily work for diluents because they are designed around a ppO2 of 1.4-ish at max planned depth (and not 1.0 or 1.1) and they are designed to be blended using Helium and Nitrox32 and gas density isn't as important.

For example, 18/45 is generally used for depths up to 200'/60m. The "problem" with using that "standard gas" for a diluent gas is that 18% O2 has a ppO2 of about 1.3 at 60m.

<Side Note>

I will probably be burned at the stake by instructors and training agencies since I almost always dive with HeliAir mixes because they are easy to mix. However, they can be very lean on O2 and more hypoxic than they should be for a given amount of helium for a given depth -- hence dangerous at shallow depths and the surface.

I also have a general rule of thumb for HeliAir mixes that I can dive to a depth 5x the helium content. For example, I consider 10/50 to be good to 250 feet. It is just a quick way for me to get a quick gauge on which bottle of Dil to pick for a specific dive day that will have a reasonable END and Gas Density.

May the burning commence...

</Side Note>

Regards,

- brett

 

[Imla]

Different agencies use different "standard gases".
GUE for example use a set of standard gases for OC, and generally a rule of thumb is "one gas deeper" for CCR.
In OC GUE uses 1.2-1.3 PO2. So, using the example from the previous poster, 18/45. OC, this would be a 60m gas. So, on CCR it is limited to 45m. To do the 60m dive, the next standard gas would be 15/55.
Standard gases for use on the GUE-CCR are depending on use:
32/00
21/00
21/35
18/45
15/55
12/65
10/70

With the exception of the obvious, these can all be made using preblended 10/70 and 32/00

 

[formernuke]

I've heard of 18/35 as well as wondered if that is because of ease of blending.

 

[grantctobin]

I've heard of 18/35 as well as wondered if that is because of ease of blending.

What ease of blending gets you to 18/35?

 

[formernuke]

No clue that's why I was wondering.

 

[tbone1004]

Spinning off from a comment in the roaring river thread. Maybe because I haven't taken my class yet.

What are the standard diluent mixes? I've heard a few trimix ones mentioned other places. I'm curious as to what all of them are, or maybe it's just because I'm bored.

Most will use standard OC gases, EAN32+He. Many will use one deeper for diluent starting around 70ft. I.e. at 70ft you would kick over to either 30/30 or 21/35, and if you would use 21/35 on OC then 18/45 on CCR, etc etc. I do not believe in air dil unless you are in an area that is really difficult to get nitrox fills and if you are in an area where nitrox is banked and is the primary gas of choice like in cave country that there is exactly 0 reasons you should ever consider it. Deep air is far too tempting on OC and that temptation can bite you far harder and faster on a rebreather when it compounds with residual CO2 in the lungs for a hypercapnia incident. Diving EAN32 based standard gases gets you on a much lower gas density which is never a bad thing with CCR.

 

[beldridg]

What ease of blending gets you to 18/35?

I'm guessing he meant 18/45.

- brett

 

[formernuke]

Most will use standard OC gases, EAN32+He. Many will use one deeper for diluent starting around 70ft. I.e. at 70ft you would kick over to either 30/30 or 21/35, and if you would use 21/35 on OC then 18/45 on CCR, etc etc. I do not believe in air dil unless you are in an area that is really difficult to get nitrox fills and if you are in an area where nitrox is banked and is the primary gas of choice like in cave country that there is exactly 0 reasons you should ever consider it. Deep air is far too tempting on OC and that temptation can bite you far harder and faster on a rebreather when it compounds with residual CO2 in the lungs for a hypercapnia incident. Diving EAN32 based standard gases gets you on a much lower gas density which is never a bad thing with CCR.

So I'm about to start blending nitrox is what your saying.

 

[rx7diver]

I've heard of 18/35 as well as wondered if that is because of ease of blending.

What ease of blending gets you to 18/35?

If a shop has EAN28 banked, then mixing 35% helium and 65% EAN28 will yield 18/35 (actually, FO2 = 18.2, FN2 = 46.8, and FHe = 0.35). Straightforward.

rx7diver