Trimix continuous blender

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aainslie

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Hi,

I've been reading the posts here and have learned mightily from Omer, Pescador, Stone and of course, Oxyhacker while putting together my own mixing station. I had two points I came up with that I wanted to check out on a pubic forum, and perhaps (if they pass pubic scrutiny) suggest for others going forward.

1) I found these little wonders on a web search: http://www.valvestore.com/43assova12va20.html . It would seem to me that if one puts one of these beasts in parallel with the compressor motor's electrical circuit, and the flowline in series with the oxygen going into the mixer, then for a mere $30 one gets an automatic shutoff valve should the compressor stop for any reason. Anyone tried or doing this? Any suggestions? it seems like very cheap insurance on a $5000 compressor!

2) pescador came up with a great idea a while back, but I think his math was a little out. I posted this earlier but on a really old post so I wanted to basically try out the idea on everyone again. His idea was:

a) Start your mixer going with air. calibrate the O2 sensor to 21%
b) Start running He, until the sensor drops to an appropriate level. I came up with a slightly different "appropriate" level to him. I'll get to that.
c) Then slowly increase the O2 until you get the level you want.

For simplicity let's say you want a 17/45 mix (his original suggestion) - then his post says, mix in He until the sensor reads 21*(1-.45) = 11.55%. tehn slowly run up the O2 until it reads 17%.

I don't think this is quite right. Here is the math I put on the earlier thread:

Let's say that you want to get to a partial pressure of he that I'll call PHe and a partial pressure of O2 that I'll call PO2, by adding fraction "a" of O2 and "b" of He. Then allowing for the extra O2 and he being added to the 21% O2 gas stream (aka air), the final partial pressures will be:

PHe=b/(1+a+b)
PO2=(.21+a)/(1+a+b)

Then solving these simultaneous equations you get:

b=(.79/(1-PO2))/(PO2/(PO2-1)+(1-PHe)/PHe) (I'd love it if someone checked my algebra here - it gets a little hairy!). I'll bet one could simplify a bit but with Excel it seems pointless.

Once you've calculated these factors, the actual O2 level you want to adjust to for step 1 of your procedure is to add He until the PO2 reading drops to .21/(1+b), where b is given above, because there is no O2 in the mix yet.

Wneh I use this math, I find that to mix the 17/45 discussed above, one would drop the O2 level to 10.8 instead of 11.5. This is intuitively sensible - you're adding a teenie bit more He to make up for the fact that you're now going to add some more O2 to the mix. Omer gave a similar intuition in his original discussion of why he was worried about this approach - of course, he also had some other worries! - but that was one of the main ones. It seems like only a tiny correction, but that might explain the anomalies Pescador thought were due to blowback.

How many people have tried this very simple approach or a variant of it (Well, the process is simple if the math ain't)? It seems like a great way to do it if one uses the type of compressor that Vance refers to in his guide i.e. one that flows steadily even as the outlet pressure increases. Any thoughts or suggestions?
 
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aainslie

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Phew - a stunning silence!

We just set up our homebrewed nitrox stick yesterday (HUGE thanks to Oxyhacker - what a great book!!) and got almost perfect mixes out the box. It seems we have to aim 1% high, i.e. to mix 32% we ran 33% at the inlet. At some point I intend to put an O2 monitor on the HP sde but for now this does the trick. What astonished me is that the Nuvair compressor we used is super-consistent - once you set up the flowmeter at the beginning you do NOT have to touch it again until full at around 3300 PSI. Amazing!

We used a 2", 3 ft long static mixer tube - maybe overkill but it makes for minimal intake pressure drop and it sure looks sweet. Since this is going into a commercial dive shop it's important that it all looks good. The regulator is medical and absolutely faultless - $60 on Ebay from Osprey, a good crowd to deal with. Right now we have the wrong flowmeter, a 25 lpm unit that we need to upgrade to a 50 lpm - on order. I intend to use an "el cheapo" O2 analyzer from Oxycheq but as a temporary measure I dismantled my Analox unit and screwed it in - works great.

To begin with we are going to use PP to fill with Helium followed by a nitrox top-up to do our trimix fills. But it's my intention to do a Trimix fill in one go within a couple of months, starting with <50% he mixes until we get things consistent. Also I want to have an HP O2 monitor going before I do that.

To those of you considering doing this yourselves it is NOT rocket science, just use Vance's drawings. I like the idea of a $150 static mixer tube, and if you're blending in a public place a medical regulator is probably better than a welding one (!!!! - in my experience saying the word "welding" in dive circles gets the most ridiculous "you'll kill yourself" looks!!), but this is dead easy to do for a lot less money than the $2000 odd they want for a nitrox stik.
 
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