Trimix blending question

[divezonescuba]

During a recent trimix blending course, I noticed the the Padi/dsat calculator and MultiDeco yielded different answers for blending various mixes both from an empty tank and with an existing analyzed fill.

Curious as to why this might be?

Thanks.

 

[tomfcrist]

One is likely “compensating” for the differing compressibility of He versus air or O2, and the other is not.

 

[rjack321]

One is likely “compensating” for the differing compressibility of He versus air or O2, and the other is not.

What he said ^
Helium is less compressive than N2 and O2. and its a non-linear function. Various calculators slightly different compressibility values and some don't account for it at all.

But in the end its trivial anyway, you can just use a calculator and get it good enough even with helium %ages up to 70%.

 

[TrimixToo]

Having now tried 3-4 different mixing programs, I use the one in Multideco and apply the necessary "Kentucky windage" for things like temperature and how much the He pressure increases, and how quickly. A friend remarked this year that two topoffs I did for him were "perfect," any my rejoinder was that I should have bought a lottery ticket that day.

More seriously, a couple-three percent of He either way won't move the needle nearly as much as a half percent of O2. Get the second one perfect, and eventually the first will fall into line. Sort of. On a good day. More or less, anyway, and more than good enough nearly all the time.

It's a lot like a perfect landing in a small airplane. Everyone walks away - good. It wasn't uncomfortable but there was more "thump" than you needed - better. It was pretty smooth - great! The only indication the wheels touched the ground was the tire squeal - awesome! Rare, but awesome.

 

[Jack Hammer]

A benefit of 'standard gasses' is simplicity of blending and topping off.

I've had good experiences using MultiDecos gas blender and CCR mixer. There is another app I liked. I forget the name, my phone dropped it when I changed to a new one.

 

[ajduplessis]

Check temp

 

[Billy Northrup]

Temp is key.

I toss them in some water to hold temp as constant as possible. Use of regulators.

 

[divezonescuba]

Thanks for the input.

But...

The question is why the different software packages provide different blend calculations? So maybe some include a compressibility factor and some do not?

Some adjustment when actually blending is expected as discussed in the oxyhacker book due to temperature, etc.

 

[rjack321]

Thanks for the input.

But...

The question is why the different software packages provide different blend calculations? So maybe some include a compressibility factor and some do not?

Some adjustment when actually blending is expected as discussed in the oxyhacker book due to temperature, etc.

Because some are written by super precise engineers that agonize over stuff like compressibility at 50bar vs 150bar vs 300bar. Yet for all practical purposes you can just use a flat percent and its close enough.

 

[TrimixToo]

RFC 1925 might as well have been written for diving (see RFC 1925 - The Twelve Networking Truths). The part that applies best to blending is:

(8) It is more complicated than you think.

Variations in fill rates, initial temperatures, the order the gasses are added in, and probably the phase of the moon, the local newt population, and the quantum states of nearby matter affect the compressibility and the predictability of the resulting mix. After a while, wetware can outperform software here as you learn to guess whether and how much He to add above what the mixing program calls for when you are not willing to let everything cool for several hours between gasses and incremental top-off's of individual gasses to hit target pressures. I think that is at least in part because we have no accurate measurements of things like the gas temperatures inside the tanks and the rate of thermal energy transfer across the tank walls.

I don't mix with other blenders, but I personally find it far more convenient to apply "Kentucky windage," let the mix sit overnight or throw it in the car and drive around for 10 minutes or so so it will mix more quickly, test, and top. This gets my O2 within a percent reliably and the He within a percent or two as a rule.