Trimix blending without O2

[decompression]

So I was just thinking about blending trimix without O2 and a booster. So, theoretically, to get a 25/18 mix, add helium from K cylinder (2200 psi) to HP100 with 400 psi (air) and top up with banked 32% to 3500 psi. I used imixer app for the numbers so just wondering if there’s a reason it won’t work or is impractical.

 

[boulderjohn]

Why wouldn't it work?

The classic pure math way to create a trimix blend involved a 3-step process:

1. Calculate the amount of helium to add (elementary arithmetic).
2. Calculate the percentage of nitrox to add to create the intended final mix.
3. Calculate the amount of oxygen to add before topping off with air.

People figured out long ago that if the fill station had nitrox 32 banked, all you had to do was put in the right amount of helium and top it off with nitrox 32. That is how we got standard gases. Put in 35% helium and then top it off with 32%, and you had 21/35. Put in 45% helium and top it off with 32%, and you had 18/45.

This will only eliminate an booster if you have very low levels of gas remaining from a previous dive. It also requires high levels of 32%. No matter what, you still have to have more PSI in your supply cylinder than the tanks you are filling. When I did this years ago, we sent many, many thousands of dollars of helium into the atmosphere to get the tanks down to a level where they could get the gas they needed.

 

[decompression]

Thanks for the reply, John, in the past it’s always been add O2 (with booster), add He and top up with air. There’s only a couple of places locally here for trimix (my shop isn’t one of them), so I’m toying with the idea of having He at home and topping up at the shop. So as long as the required He PSI is lower than the K cylinder then it should be fine.

 

[northernone]

Poor man's mix. Still used in remote locations.

www.advanceddivermagazine.com/ADMEZINE/HeliairPoorMansMix.pdf

Logistics side of things: It's simple to go from cga580 to 1/4 to a hp hose on a first stage. I set myself up for ~14$ to transfill helium (plus the helium bottle).

Heliair???
Old thread mentioning the disadvantages.

Cameron

 

[kensuf]

I regularly blend 18/45 and 21/35 using banked EAN32 and 10/70. It's quite reasonable and easy to do that.

 

[boulderjohn]

There’s only a couple of places locally here for trimix (my shop isn’t one of them), so I’m toying with the idea of having He at home and topping up at the shop. So as long as the required He PSI is lower than the K cylinder then it should be fine.

Here is both the problem with your approach and the solution--especially if you are working with other divers.

Getting the required He PSI lower than the K bottle is a real problem. You will find yourself venting a lot of expensive helium into the atmosphere--where it will eventually drift off into outer space--in order to get that pressure down. You will need 3-4 (or more) helium bottles at home so you can cascade. If you are doing that, you might as well include the oxygen--it's fraction the cost of the helium.

Believe me, I know this. As a trimix instructor in an area with no trimix for sale, I went through that process for years. I had to drive six hours for the diving, and I could only carry so much gas with me. That meant I was very limited with what I could do with my students on a long weekend.

The solution was simple--buy a good booster. I am not talking some mini-operation that takes tons of drive gas and forever to fill a tank. Spend the bucks for a good electric powered booster. I have mine in a van, and I am able to take enough gas along to take care of fills for me, my students, and my fellow divers. I plug into a motel outlet or the outside outlet of a building near the place where we get the fills topped off. I charge just enough for the gas to break even with the cost of the gas, the cost of getting the gas, etc. There is no no more venting helium into the atmosphere. I estimate it took about two years of monthly trips to pay for the booster completely via the saved gas alone. If I were to charge my friends a little more, I would be running a profit, but I am trying to make diving more affordable for everyone.

BTW, I don't know how things work in your area, but I rent my supply bottles. I drive up to the shop, load the tanks in my van, use them up, and return them. I pay a daily fee while I have the tanks, but it isn't much. That way I can adjust how many tanks I have for each trip depending upon the need, and I don't have to use up so much storage space in my home.

 

[decompression]

Great, thanks for the info.

 

[rjack321]

@decompression do you already have a compressor?

 

[Wookie]

I almost never ever ever added O2 to my trimix. I made what I think GUE refers to as the standard mixes, by adding He and dumping in Air or Nitrox of some percentage. 35 heliums and 32% nitroxes yields 21/35, or close enough. 50 heliums and air gets you 10/50, 70 heliums and 32% gets you 10/70, etc. We went through a lot of 10/50 and 16/50 on the Spree.....

 

[boulderjohn]

This approach assumes, of course, that you have access to banked nitrox. Believe me, that does not happen in the Rocky Mountain west.