30-11-06

Continuous trimix blending with 2 nitrox sticks (English)

I've been playing for a while now with a second nitrox stick in series with the first one.
The idea is to be able to pre-mix trimix in a cheap way and still be able the check the mix at all times.

Setup is simple; 2 nitrox sticks in series.
In the first one you introduce the Oxygen to blend nitrox, this nitrox percentage is measured after the first stick.
Now after the first measurement I introduce the helium to the nitrox and at the end of the second stick before the gas enters the compressor is another O2 sensor.

That's right, there is no helium analyzer.
You can easily measure the helium content by calculating the amount of nitrox is replaced by the inert helium.
So for example you want to blend 21/35.
you replace 35% of the air with helium, therefor the oxygen percentage will drop also with 35%.
So when introducing 35% helium to the airflow you'll get (1 - 0.35) * 0.21 = 13.7% oxygen left.

Now we add oxygen back with the first stick until the sensor reads 21%.
You'll see that the O2 sensor after the first stick will now read 32.3% which confirms that 21/35 can be made by topping off helium with 32% nitrox.
You can also calculate this if you don't thrust my numbers, or want to make another mix.

It's very simple:
the total amount of O2 to add in bar (end pressure * desired O2%) for example 21% => 0.21 * 200bar = 42bar O2
This number is then divided by (end pressure - starting pressure - the amount of helium to add in bar)
The amount of helium being ( end pressure * helium percentage ) => 200bar * 0.35% = 70bar
So that will be:
42bar O2 / ( 200 - 0 - 70) = 0.323 or 32.3%

Now you have 2 O2 values you need to keep at the set point to check your mix.

In my earlier test with just one stick I had no idea how much helium I was pumping when the compressor flow was drifting and adjustments were needed to the oxygen and helium flow since I only had one O2 meter for both oxygen and helium.

Some interesting points:
Since you introduce gas in the second stick that gets it's gas from the first stick; the flow in the first stick will drop since part of it is replaced by the introduced gas.
Since the flow of the gas introduced in the first stick is constant it's percentage will rise...
for example if you first adjusted the first stick with O2 to 32.3% and added the helium next, you'll see the 32.3% O2 in the first stick will rise to about 38% O2 and you need to adjust...
This is because the flow in the first stick is reduced by 35% (the helium content that's introduced in the second stick, so that's 35% less gas that needs to come from the first stick)

It is however quite simple to anticipate this.
You can calculate this by subtracting the fraction of O2 in the input of the stick (air => so 0.21) from the desired fraction (in this case 0.323 or 32.3%)
This number is then multiplied by:
(1 - (the amount of He to add in bar / (end pressure - starting pressure))
Now add the fraction of the input gas (0.21) back and you have the adjusted value.

for example: to fill an empty tank with 21/35 we know we have to add 32.3% nitrox to the helium.
For a 200bar fill, we have to add 70 bar of helium in there (0.35 * 200bar = 70bar)
To know the initial set point of the nitrox before you add the helium you can calculate it like this:

((0.323 - 0.21) * (1 - (70 / (200 - 0)) + 0.21 = 0.284 or 28.4%

When you adjust the first nitrox stick to 28.4% and introduce the helium later, the nitrox percentage will climb to 32.3% when the end O2% reaches 21%

note:
The above is only valid when oxygen is introduced in the first stick and the helium is introduced second just before the gas enters the compressor.

air -->Oxygen---------->analyser1--->helium---------->analyser2-------->compressor

When you change the order the above calculations are not correct anymore.
For example, when you first introduce helium, you'll have to account for the drop in the flow of the first stick and have to actually mix a higher percentage of helium in the first stick to end up with the desired percentage at the compressor intake.

I have put all this into an excel sheet for various options for blending.
full continuous blending for both setup options of O2 or He first/last
combined partial blending and continuous blending (handy for deco mixes with high O2%)
re-mixing an existing mix
etc....

When I find out how, I will upload the blender sheet somewhere and post a link to it...
For now, here is already a picture of my setup.

trimix_stick

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