Lost Oxygen in blending? (Nitrox stick)

[Fl_Key_Guy]

The nitrox mix going into the compressor is several percent higher O2 than the mix that results in the bottle. What's "burning"?

I built a blending stick (Thanks, Oxyhacker!) and it seems to work well. The theoretical o2 flow in LPM to the stick for the rate of CFM on the compressor is very close to what it takes to get the percentage reading desired going into the intake. HOWEVER, when I test the finished fill (with the same analyzer) it is several percent low (32%in, 27%out). I tried upping the o2 in by 2-3% and only gained 0-1% on the compressed product. What's eating the oxygen or what's going on? (I do not have a leak of air into the tubing between the stick and the compressor)

 

[rmc77]

What you had in the bottle? Are you calculating the desired O2 flow based in gas you have in the cylinder, right?
I'm not familiar with your system, but in case the gas left in the bottle is not taken into account, 32% O2 flowing into a cylinder with 700 psi of air will give you EAN29 instead of EAN32.

Cheers


Rafael

 

[wedivebc]

You have an air leak in your intake system

 

[dannobee]

there is a restriction in your stick. Too much of a restriction will throw off the sensor. It's not that it is getting eaten, but wasn't there in the first place.

 

[DA Aquamaster]

Lower pressure at the sensor in the stick due to a restriction, will result in actual O2 percentages going into the compressor that are higherthan the sensor says they are as the lower pressure causes the sensor to read lower.

 

[Fl_Key_Guy]

1. My first thought was an air leak past the stick, "watering down" the mix. Nope.
2. The bottles being filled were empty, just visual inspected. It's not a blend with air in the bottle.
3. A restriction in the stick would create a little vacuum, causing a lower partial pressure on the sensor which I think will make it read LOWER than true, whereas sniffing a flow from the full bottle would create a HIGHER partial pressure on the sensor, making it read HIGH. Am I incorrect?

 

[PhilEllis]

1. My first thought was an air leak past the stick, "watering down" the mix. Nope.
2. The bottles being filled were empty, just visual inspected. It's not a blend with air in the bottle.
3. A restriction in the stick would create a little vacuum, causing a lower partial pressure on the sensor which I think will make it read LOWER than true, whereas sniffing a flow from the full bottle would create a HIGHER partial pressure on the sensor, making it read HIGH. Am I incorrect?

I think you are correct with your assumptions. I would again look to the possibility of leaks after the stick.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

 

[rmc77]

If they were visually inspected, they had air inside at 14.7 psi... Which makes no difference. So, that's not the reason. Lets think something else…

Scientifically speaking, there are subtle differences between dynamic and static measurements. I mean, there are sensors suitable for dynamic readings (i.e flow of O2 in a mixture) and sensors that can only read static (i.e O2 contents in the bottle). Some sensors are good for both readings, though. Did U check that already?

 

[Cave Bum]

Usually what causes this is incorrectly calibrating the O2 sensor.

The way the sensor works is that the oxygen causes a galvanic reaction which can be read by a voltage meter as resistance. That's really all an analyzer is, it's a voltage meter. The reading is based on the QUANTITY of oxygen molecules not the PERCENTAGE so higher pressure reads higher O2 and lower pressure reads lower O2.

Try it. Use an analyzer that is open to the atmosphere instead of one that hooks to the inflator line. Calibrate with the tank valve just cracked then open the tank valve up so it's blasting into the sensor. You'll see the O2 reading jump way up. Conversly, of course, the suction side of the compressor has lower pressure and reads lower O2 if the analyzer was calibrated at ambient pressure.

Also there is an issue with humidity. You have a big chunk of nitrogen and a little chunk of oxygen. When you try to squeeze in humidity it lowers the percentages of the other components. I typically see a correctly calibrated O2 sensor reading 20.4% on a day with 50% humidity.

So you have to calibrate for both the factors to be completely accurate.

If you measure the offset between ambient air and pure, dry air straight out of the compressor (not out of the air banks) you can find the correct offset for your current humidity.

Then run the compressor without turning on the oxygen supply and calibrate the analyzer to 20.9% minus the offset that you figured for the paragraph above.

Now turn on your O2 supply and adjust to your 32% (or whatever). This will give you accurate results.

Let me know how it works.

 

[Scuba Duck]

There are two likely sources of the "loss" of oxygen.

1) you are not purging out the "air" from the filters and hoses before you start filling tanks, thereby diluting the Nitrox you are putting in your tank. It helps to have an analyzer on the output side of your compressor at your fill station to let you know when your nitrox has arrived. Dump the air until the nitrox shows up and then start filling tanks.

2) you live in an area that has some level of humidity and need to calibrate the oxygen analyzer accordingly. The below is from a previous post:

And strictly speaking, the air going into your air compressor does not contain 20.9% oxygen. Any humidity in the air will dilute the remaining gases. This matters because your air compressor through the course of pressure drying the air, will remove most of the moisture and concentrate the remaining gases. The net result is that if you are using an oxygen analyzer, you should calibrate that oxygen analyzer to some valve less than 20.9, but not the actual percentage of oxygen in the air, to get the correct mix at the outlet of your air compressor.

In short, if you cal to 20.9, and have a high level of humidity (say 5% of total airborne gases), you will not add enough oxygen and the mixture that comes out of your air compressor will be low. If you calibrate your oxygen analyzer to the actual concentration of oxygen in the humid air, immediately after the Nitrox stik you would have the desired oxygen percentage in your mixture, but your air compressor will remove the moisture and concentrate the remaining gases, and you will end up adding too much oxygen and your mixture will come out rich.

Any apparatus (mixer) at the intake of your air compressor will cause some level of restriction and an associated pressure drop. The idea is to minimize this pressure drop. You should calibrate your oxygen analyzer to the desired value when the air compressor is running, to account for this pressure drop.

Below is a link to a spreadsheet that recommends the correct calibration setting based on your humidity levels, and the desired nitrox percentage.

For people who live in places with moderate levels of humidity, your input oxygen analyzer should be calibrated to 20.7 to 20.8. South Florida in the summertime, will need to calibrate the analyzer to a much lower reading. One touch calibration oxygen analyzers are not recommend, as they only calibrate to 20.9%

http://www.rubberduckiedesigns.com/D...preadsheet.xls

There are several companies that make inexpensive temp/pressure/humidity meters, you can find them on eBay.

http://shop.ebay.com/items/__temp-hu...286.m104.l1199
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