Oxygen percent drop from 20.9 to 19.9 after I connect the stick to the compressors

[eisa]

Hi All
Yesterday after I connect the nitrox stick to my compressors I observed the Oxygen percent drop from 20.9 to 19.9 even before injecting the Oxygen to the stick and when I stop the compressors the percent go back to normal.

-The Stick location is between the stick and the compressors.
-the calibration don many time and look Ok.

Any one observed same problem or any one know what the problem is.

Thank you
Eisa

 

[dannobee]

O2 sensors can vary because of pressure/vacuum in the stick. Measure the vacuum with a manometer to see where we are. If it's high, over say, 2-3" H2O (NOT Hg), fix the restrictions.

Also try removing the O2 sensor and check the pressure/vacuum there.

.

 

[cbmech2]

Just guessing here since I am not totally familiar with what your talking about when you refer to a stick. I would think that something inside the stick itself is throwing the readings off, perhaps some sort of contamination?. I would also think that if you are inserting the stick before filtration occurs that would also be a problem. I don't think you have provided enough information to be able to help (e.g. what kind of compressor, what kind of stick, any kind of storage (cascade) system, etc.?). Good luck.

 

[eisa]

Thank you dannobee
I will do so.
And I will tell you the result after I finish.

Thank you again

 

[300bar]

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[deepstops]

Ditto, spot-on advice

O2 sensors can vary because of pressure/vacuum in the stick. Measure the vacuum with a manometer to see where we are. If it's high, over say, 2-3" H2O (NOT Hg), fix the restrictions.

Also try removing the O2 sensor and check the pressure/vacuum there.

.

 

[DA Aquamaster]

O2 sensors work based on a chemical reaction and that chemical reaction is impacted by the PO2, not strictly the FO2.

For example, if I calibrate my sensor in open air, it will read .3% high when I connect it to the metering chamber on the analyzer due to a slight increase in pressure inside the chamber. You can see the same thing on analyser models that are just placed up to the tank valve. If you increase the flow (and the local pressure) the reading will increase even though the mix is the same.

So the calibration options on my analzyer are to calibrate it when connected to the analzyer and with a known gas source (air or 100% O2) flowing through it or, based on operational experience to calibrate it in open air to 20.6% rather than 20.9% to compensate for the observed difference.

You have the same options with a stick. You can just fire up the compressor (which is pumping air - a known 20.9% mix and then calibrate the sensor as the compresor is running, before you introduce the O2. It makes a lot of sense if you think about it.

Or since you have observed a 1% drop due to the lower pressure inside the stick once the compresor is running, you could calibrate it in open air to 21.9% which should give you 20.9 with the compressor running and pumping air.

Of course, given the ease and better accuracy of just calibrating after you start the compresssor, the former procedure makes a lot more sense and either way, you should see 20.9% on the analyser with the compressor running and before you add any O2 as you are indeed pumping 20.9% O2 through the stick.

In short, don't overthink it, just cailibate to what you know is going through it with it running.

If you want to over think something, consider the possible change in flow rate as your compresor reaches higher pressures.

If the compressor starts pumping fewer CFM through the stick, the lower flow rate reduces the amount of pressure drop in the stick due to any venturi effects and that would impact the calibration of the analyser and result in an slight increase in the percentage displayed on the analyser even if the mix is identical. To complicate it further, the final mix itself is not constant as the O2 bleeding into the system is constant as the cfm pumped by the compressor drops, so as the volume pumped drops the mix will get richer unless you also slightly reduce the O2 flow.

So...how much of the increase in the O2 reading on the analyser is due to the calibration error caused by the reduced flow and how much is due to the reduced flow in comparison to the comparatively constant O2 flow? You don't really know, unless you check the vaccuum pressure at the inlet for the sensor with a mamometer when the compressor is putting out gas at low, medium and high pressures to see if any calibration errors will occur.

That is also why its a good idea to ensure you use enough compressor and that you do a good job of matching the size of the stick to the compressor. Lacking a mamometer, you can also keep good records of obervations over time along with the resulting post fill analaysis to know if you need to tweak the mix (as read on the analyser) a bit on the lean side as you are filling if the compressor bogs down a bit at higher pressures to ensure an accurate final mix. If you consistently get final mixes that are a bit rich, backing off a bit on the O2 toward the end of the fill to get a mix a few tenths of a percent under the target value may help improve your accuracy.

With a small compressor, stick blending sometimes becomes something of an art and a bit less of a science, although understanding the science behind what may be happening sure helps.

 

[Scuba Duck]

It sounds like there is a fairly substantial restriction in your Nitrox Stick (I think Eisa bought the Tri-Hunter). Typically this is seen with a cascading orifice, or other mixer design that mixes by reducing the cross sectional area to cause turbulent flow. Be careful, this can ultimately reduce the lifespan of an air compressor. While she would be able to measure the pressure drop with a manometer, you can also do it mathematically based on the reading she has from her oxygen analyzer.

14.7psi*(19.9%/20.9%) = 14.0 psi (pressure at intake) 14.0-14.7 = -.7 psi pressure drop due to the restriction in the continuous gas blender. (.7 psi/14.7 psi)*(34 feet fresh water)=1.6 feet of fresh water (manometer reading).

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/Documents/Nitrox Controller Downloads/Humidity Spreadsheet.xls

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

http://shop.ebay.com/items/__temp-h...idityQQ_ckwZpressure?_trksid=p3286.m104.l1199

 

[rjack321]

O2 sensors can vary because of pressure/vacuum in the stick. Measure the vacuum with a manometer to see where we are. If it's high, over say, 2-3" H2O (NOT Hg), fix the restrictions.

Also try removing the O2 sensor and check the pressure/vacuum there.

Especially in Saudi and really hot places like that, do not run your compressor with a vacuum. Upsize the mixing stick and the inlet hose to keep your compressor running cooler. Right now its sucking through a thin straw and working harder than it needs to pump a given volume of air.

 

[Peter_C]

The compressor we use is a 10 cfm Max-Air and it has a pretty good O2 drop at the stick too. Taking an empty AL80 and filling it with the stick connected, then waiting a while and filling the empty tank again, this time with the stick disconnected, there was less than a 30 second difference. So in our case the stick is not effecting the compressors ability to pump air.

At some point I would like to see what the stick pulls on my magnehelic gauge.