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Old 27th January 2013, 22:54   #1 (permalink)
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Nitrox stick

Hi all,
I'm in the planning process of building my nitrox stick and have a question on the location of the 02 sensor. I have a trimix analyzer. I was thinking of installing a 1/4" hose barb into the stick (where the o2 sensor would normally go) and plugging in my analyzer thru the tubing. I was told that some have installed a small aquarium air pump between the stick and analyzer to eliminate the negative vacuum pressure so the analyzer gets proper flow and I can get a good reading. Is anyone doing this or tried it? I'm trying not to buy another analyzer, so any suggestion are welcome.
Thanks,
Tony
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Old 27th January 2013, 23:20   #2 (permalink)
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Re: Nitrox stick

Not sure if this answer your question but hopefully it will prove helpful.

Built mine some years ago after reading Oxyhacker. I set up a manometer and tuned the intake length (hose from stick to compressor.) There was a lot variation depending upon length. When I just pump air, I remove the sensor and plug the opening. There is a lot of pulsing at the sensor inlet so you may get enough flow due to pulsing. I have often wondered if this affected my readings but I don't think so. As my stack pressure goes up, I do note a small increase in O2 percentage but this is probably indicative of loading the compressor which would slow it down and therefor increase the O2 percentage. My final tank readings are between the min and max O2 readings that the stick shows which is what I would expect. I will be getting a trimx analyzer later this year but as I will boosting He (don't want to put high percentages of He through may Rix.) I will be blending by calculation and doing He analysis post blending.
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Old 27th January 2013, 23:23   #3 (permalink)
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Re: Nitrox stick

You have to expose the sensor to the gas being sampled for it to work; putting it at the dead end of a length of tubing without flow through it isn't going to do that...I mean, eventually diffusion will get gas to the sensor, but not in a time frame that will be useful to you... The other critical thing is that sensors read partial pressure, not percentage oxygen, so you need to make sure that it's at ambient (there has to be enough flow not to cause a vacuum)

So, you either have to pump the gas through with a sampling pump, or move the sensor.

I'm not sure why you'd have to buy a new meter, though. That makes no sense whatsoever. Just solder an appropriate extension cord in place to your analyzer and move the oxygen sensor to the stick.

On the other hand, if you wanted to continuous trimix blend at some point, the pump solution is a much better alternative...
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Old 27th January 2013, 23:50   #4 (permalink)
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Re: Nitrox stick

Quote: (Originally Posted by camerone) View Original Post
You have to expose the sensor to the gas being sampled for it to work; putting it at the dead end of a length of tubing without flow through it isn't going to do that...I mean, eventually diffusion will get gas to the sensor, but not in a time frame that will be useful to you... The other critical thing is that sensors read partial pressure, not percentage oxygen, so you need to make sure that it's at ambient (there has to be enough flow not to cause a vacuum)

So, you either have to pump the gas through with a sampling pump, or move the sensor.

I'm not sure why you'd have to buy a new meter, though. That makes no sense whatsoever. Just solder an appropriate extension cord in place to your analyzer and move the oxygen sensor to the stick.

On the other hand, if you wanted to continuous trimix blend at some point, the pump solution is a much better alternative...
Thanks for your replies. The aquarium pump would be hooked up between the stick and the analyzer. Draw mix from the stick and provide mix to the analyzer. The analyzer remains stock and let's gas pass thru it, it's not a dead end. I could move the sensor out of the analyzer, but I'd rather not mess with it.
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Old 27th January 2013, 23:53   #5 (permalink)
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Re: Nitrox stick

Quote: (Originally Posted by bowzer) View Original Post
I will be getting a trimx analyzer later this year but as I will boosting He (don't want to put high percentages of He through may Rix.) I will be blending by calculation and doing He analysis post blending.
I think you have the right idea.
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Old 28th January 2013, 03:39   #6 (permalink)
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Tony,
I have done what you are proposing and it works well. I used a small 12 vdc pump to provide constant flow to the sensor.

The next step is to install the sensor into the gas stream, but without purchasing a different analyzer, that's not possible Call me tomorrow and we can talk about it

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Old 28th January 2013, 13:21   #7 (permalink)
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Re: Nitrox stick

I bank Nitrox at my dock. I didnt bother putting a sensor in the suggested Oxyhacker location due to reports of problems.

I have a valve on the line going to the bank. I can open it and either screw in a standard 02 analyzer or just stuff an Analox EII in there.

I use that when Im blending just to check the output mix. The acis test is what comes out of the bank at the fill whip of course :)


I think what you suggested is OK, you just dont need to use a pump or locate it on the stick. Just put a plastic T on the LP gas line somewhere prior to the compressor and use a standard analyzer.

Its not as hard as some people make out. On my system you can drain an 02 bottle and leave the compressor pumping air for 5 mins and it doesnt drop my bank even 1% (depends on the size I guess)

You can also put higher 02 than desired into the compressor and it still doesnt affect the bank. You will quickly see what psi you need to get the mix you want.
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Old 28th January 2013, 15:14   #8 (permalink)
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Re: Nitrox stick

Hello,

I don't know what kind of He analyzer you have, but if it is one which uses sound to measure the He like divesoft, the pump for Premixing TX is not working, Oxygen reading is fine but the He is not.
My guess it is due to the vibration of the gas in the intake of the compressor

I use 2 oxygen sensors to fill the TX, used to be only for reading with 2 el cheapo analyzers, now it is controlled with 2 PID controllers with the same sensors

Stick is made from ordinary PVC plumbing pipe filled with golf balls, O2 en He injection is done with a small pvc pipe drilled with a couple of 2 mm holes blowing in the opposite direction of the gas-flow

regards
Hens
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Old 28th January 2013, 18:49   #9 (permalink)
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Re: Nitrox stick

Quote: (Originally Posted by tony13) View Original Post
Thanks for your replies. The aquarium pump would be hooked up between the stick and the analyzer. Draw mix from the stick and provide mix to the analyzer. The analyzer remains stock and let's gas pass thru it, it's not a dead end. I could move the sensor out of the analyzer, but I'd rather not mess with it.
Gotcha. I was thinking you were just going to hook up a tube. :) The Hargraves miniature pumps are your best bet...you want around 2 lpm, and eBay is your friend for picking them up. Much more compact and reliable than an aquarium pump... consistency is everything in getting a repeatable reading that you can compare across sessions of blending.

They're cheap enough ($55 + $14 for a mount) that you can order direct from the web site, but you can find pretty good deals on them at the usual auction places...
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