As promised, I've uploaded my excel sheet to calculate the settings for continuous blending with 2 nitrox sticks and 2 O2 analysers.
You can download it HERE.
This sheet is a work in progress for myself, so it's not that structured.
Blue cells can be modified, green/red cells are calculated.
The sheet will warn you if the result gives you O2 concentrations over 36%..
The maximum I am willing to pump.
The top row you enter your current tank contents and the desired tank contents.
The lower rows give you different filling options.
Full continuous blending.
Partial filling with O2 and topping of with pre-entered max. O2 concentration
topp-off with nitrox only
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%
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.
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
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.
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.
OK, fresh start with the blog.
Lets start with re-posting some interesting old messages.
Partial blending is very simple, but it's not very efficient.
Once the pressure in your storage bottles starts to drop you soon can't remix your tanks anymore and have to (partially) drain them.
Therefor you need more oxygen and helium then necessary, and the compressor will also have to run more than would be necessary...
A simple and relatively cheap solution is continuous blending with a nitrox stick
You can find a lot of information about this in the oxygen hackers companion from airspeed press.
A must read for everyone who want to start blending himself.
With such a nitrox stick you can bleed oxygen into a mixing chamber where it blends with the air that's sucked into the compressor; so you'll actually pump nitrox.
The big adavantage is that you can always top-off your nitrox tanks, no matter how much pressure there is still in them or that your oxygen storage bottles are almost empty.
Topping off with nitrox is very handy, especially since almost every GUE standard mix can be made by topping off helium with nitrox 32%
Even for decompression mixes it's handy if you for example can top-off with 40% nitrox instead of air... because you'll need to mix less O2 with partial pressure blending first.
For safety reasons it's not advisable to pump richer mixes than 40% through your oil lubricated compressor.
In the picture below you can see the initiel setup.
On the left side is the oxygen buffer with flow restrictor.
The oxygen runs to the op of the stick where it is injected in the airflow of the compressor intake.
Inside the stick are multiple chambers causing lots of turbulance so the oxygen mixes well with the air.
At the bottom is an O2 sensor where you can measure the oxygen percentage that enters the compressor and your tanks.
Eventually the nitrox goes via the yellow hose to the compressor where it's pumped into the tanks.