20-11-06

Pumping helium through a personal compressor (English)

The easiest way of blending trimix is by partial transfilling from storage bottles.
However, once the supply pressure drops it gets hard to get enough pressure in your dive tanks.
Especially with the expensive helium it gets very ineffective after a while since it’s getting very hard to remix anything and it becomes necessary to just empty your tanks before blending it again… which is a waste of perfectly good gas.

One solution is a booster pump, but they tend to be quite expensive and slow once the storage bottle pressure gets low.
You can also get adventurous and try pumping it with your compressor… which is what I do.
Doing this is not hard.
The hardest part is providing the helium to the compressor.
There are several ways of doing it ranging from bleeding the helium into a garbage bag, or filling via an old demand regulator… and supplying that to the compressor intake.

However I use an industrial pressure regulator.
These things can be bought fairly inexpensive from your local gas supplier.
Mine had an integrated flow restrictor and a l/min readout for argon and CO2.
I removed the flow restrictor since my Bauer Junior needs a lot more flow than the reg. could provide.
Now the flow meter becomes a normal IP pressure gauge with no flow restrictor anymore. (but still has the l/min scale)

From the regulator I fix a 2m piece of 18mm flexible garden hose.
This will transport the helium to the compressor and act as some sort of buffer to dampen the pulsing nature of the compressor intake.

For fixation I shoved a smaller hose made thicker with duck tape inside the garden hose and clamped it down… Not an elegant solution, but it works regulator_640






 

On the other side I shoved a piece of aluminum pipe with an outside diameter of 20mm into the hose and clamped it down.
This pipe fits perfectly into the compressors (Bauer Junior) intake without the need of O-rings etc.

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Now it’s time to get pumping…

What I do is turn the IP adjustment of the regulator all the way down…
Then I open the storage bottle all the way…
The first gauge should read the storage bottle’s pressure.
The second should read “0”
Insert the pipe with garden hose into the compressor intake.
Now adjust the IP slowly to “some” pressure… I use 1 bar overpressure…
The first time it takes a little trial and error to find the sweet spot, but the idea is to start the compressor and immediately adjust the IP to somewhere above “0”… because the high compressor flow will cause a severe pressure drop inside the hose and you don’t want the compressor “sucking” on your regulator creating a vacuum and wearing your compressor out, thus you need to slightly increase the IP of the regulator and therefore  increasing the flow so that there is a slight constant atmospheric overpressure in the hose.
You will see the needle bounce a little bit up and down above the “0” point on the scale, this is the setting you want…

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Once you have figured out the sweet spot, you can set the IP to this setting next time before you start and not have to adjust any more.
In my case it is around 1 bar overpressure, or 8l/min on the second gauge scale…
After I start the compressor the needle drops to just above “0”

To stop I first pull off the hose from the compressor intake, shut it down and close the storage bottle’s valve.

I also use this setup to boost argon… In fact all the pictures on this page are while topping off my small argon travel-buffer.
However, with argon the compressor tends to heat up pretty fast and I limit pumping only by starting with a cold compressor and limit times to 10min max… which is enough to top off the small 10l cylinder I use for transfilling it into my dive argon bottles.
After which I let the compressor cool down completely…
Don’t forget to let the compressor run a couple minutes after pumping argon to flush out all the argon or you will pump it into your diving cylinders next time, and argon seems to be very narcotic.
I found the compressor doesn’t heat up much when pumping helium, but the electric  drive engine gets very hot when pumping high pressures for a long time like +250bar…
Also the compressor flow begins to stall indicating the helium starts to bypass the final stage plunger…
!!!Watch the intake regulator during pumping!!!
You will notice the IP creep up because of the compressor flow reduction…

I generally don’t find a need to pump helium higher than 200 bar… so it doesn’t present a problem.

It is also advisable to use synthetic oil in your compressor…
Normally all recent compressors come with this oil, but old types may still have mineral oil which can’t stand as much heat as synthetic oil and will break down faster.
BEWARE: usually you can’t just drain the mineral oil from your compressor and pour synthetic oil into it…
Please refer to your manual for proper oil change procedures.

Filling this way has made logistics a lot simpler and more efficient.
Now I can pump helium on top of an existing mix and top it off with nitrox from my continuous blender…
But even without a nitrox stick it is still very efficient since O2 is cheap and being able to scavenge every bit of helium helps; also only having 1 storage bottle in stock keeps bottle rental down…

BEWARE:
Don’t be tempted to start pumping pure oxygen through your oil lubricated compressor!!!
Chances are it will go up in flames with a big bang!!!

And always keep an eye on your setup when pumping helium/argon.

Happy boosting.

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11:42 Gepost door Davy in dive gear projects | Permalink | Commentaren (1) | Tags: blending |  Facebook |

16-11-06

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.

15-11-06

Continuous blending (English)

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.

nitrox_setup