Continuous trimix blending with an ordinary nitrox stick. (English)

Step 2 with the continuous blending system... trimix.
Ultimately the plan is to use 2 nitrox sticks in series and 2 oxygen sensors.
One after the first stick where you inject helium and one after the second stick where you inject the oxygen.
But more on this later.

For now, it can be done with just one O2 sensor. but then you have to control 2 parameters with just one O2 sensor.
In the previous post you see in the picture a second injector... so you can inject helium here at the same time you inject the oxygen.
First you adjust the helium flow, then the O2 flow.
For example, to mix tx21/35 you first adjust the helium flow so you measure 13.7% O2 going in the compressor.
(35% inert gas injected in the stick gives 35% less oxygen in the air; => 21% * (1 - 0.35) = 13.65%)
Next you adjust the O2 flow so the sensor reads 21%...
As long as you use good flow regulators, the helium and oxygen flow remains constant.
However, after some tests I found that the compressor flow itself is nog constant and drops a bit as the end pressure rises.
Therefor you'll inject too much O2 and helium as the flow drops.
You can adjust the O2 flow to compensate, but you have no idea about the helium content anymore.
Some test show the end result for the helium for a complete fill is about a 3-4% increase in helium percentage.
Not much of a problem, the more helium the better :-)
It's the O2 percentage that's really important.
However, it's not an efficient use of expensive helium.

A small work around;
If you fill an empty bottle, you can for example shut down the O2 flow lets say every 50 bar to check the helium flow and adjust if necessary.
So as to limit the helium increase with the decreasing compressor flow.
It helps if you have a pre-pressure valve at the output of the compressor; this valve will only open at a certain pressure so the compressor always "sees" a constant end pressure, and so the compressor flow will remain more constant, and doesn't drift as much.

Such a valve is also much better for you filtration because the filter stack works better at a high pressure, but that's another story.
My Bauer Junior has such a valve by default by the way, and I suppose many others as well since it's pretty important to have this to get clean breathing air.
You really notice that the compressor flow is pretty constant until 150bar, and starts to drop above this pressure...
coincidentally the set pressure of the check valve...

In practice this way of filling is pure luxury.
You can top-off anything you like as long as the O2 pressure is not too high; and the speeds is higher than with an "affordable booster".
You don't waste anything anymore by draining tanks.
Drawback is you have to monitor the system...
Next step is to build a second stick so I can have 2 O2 sensors to be able to measure the helium content at all times.


Mooie site


Gepost door: peter | 19-11-06

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