Measuring O2 in enriched air

[MoistNeoprene]

I just recently completed the PADI enriched air course. My coursework said that it was important to calibrate the O2 measuring device from a compressed air source having the same flow rate as the enriched air source you would later test on. My instructor disagreed, and said that it wouldn't make any difference if you calibrate based on ambient air and then tested on enriched air which is under pressure. After calibrating on ambient air, he then proceeded to test several enriched air containers, and came up at least 2 percent shy of the O2 that had been marked on each of the cylinders by the person who previously filled and tested them. I suggested that perhaps it was a result of him not calibrating on compressed air, and he dismissed me. Since I don't own an O2 measuring device of my own, I can't really test if this is true or not, but I'm just curious -- does it make a difference like PADI says, or is my instructor correct?

 

[rhwestfall]

Numerous testers are impacted by the velocity/flow rate of the sample gas.

 

[Dusty123]

You can jack the ppo2 up if you hammer the analyzer with gas since itll be compressed a bit. With a hand held analox type analyzer i turn the gas on and off and try to let it leak a bit to not introduce error from pressurizing the cell in the analyzer. Fancier models flow through the analyzer with an oriffice before it hits the cell to avoid this.

 

[WarrenZ]

O2 calibration is also affected by the humidity if you calibrated to ambient air in some humid day in Boston then tested the dry air of a tank without adding in your humidity correction to the calibration that could affect your measurements.

Remember nitrox is like shop class: measure with a micrometer, Mark it in chalk, take off your glasses, and cut it with an axe. Small changes add up quickly.

 

[JohnN]

Numerous testers are impacted by the velocity/flow rate of the sample gas.

Measure with a micrometer, mark with chalk, cut with an axe, hammer to fit and paint to cover it up.

There is no science that supports a ppO2 and its associated MOD of 1.2 vs 1.4 vs 1.6 vs 1.8

Yes, you want to make sure some bozo hasn't put 80% O2 in your tank (or you're planning on 32% and get plain-old-air), but beyond that the algorithm your computer uses for NDL is a bigger factor than measuring your tank mix to 0.1%

 

[CptTightPants21]

Yes, the high flow rate can throw off a measurement. 2% seems a tad high in terms of error from flow rate. Usually it is around 1%, maybe a little higher or lower than that. If the tanks were analyzed a while ago you could definitely see a slight change from the sensor calibration or humidity. It's also possible that the person who initially marked the tanks was mistaken--maybe he took the measurement with an incorrectly calibrated sensor or took the analyzer away too quickly and didn't let the number settle or maybe he was the gas filler and the tanks cooled down and his initial O2 calculation was off. When people are all analyzing gas at the same time it is easier to find the incorrect % or faulty technique that might be throwing the number slightly off. In a situation like you experienced with class either the instructor or the initial analyzer could have been incorrect--you would need at least a 2nd analyzer and maybe even a third person to check the percentages and see which of the first two was mistaken.

The rule for a quality dive shop/nitrox fill is +/- .5 from requested nitrox blend.

Also whenever you get a sensor that hasn't been just used (as in used to analyze a tank and handed to you), it is good practice to calibrate the sensor on a tank of air just so you know everything is set and no one accidentally bumped the calibration knob by mistake. Starting from the wrong calibration can easily send your analyzed mix off 3-4% from expected and is a good warning sign to recalibrate and start from scratch.

 

[rjack321]

I just recently completed the PADI enriched air course. My coursework said that it was important to calibrate the O2 measuring device from a compressed air source having the same flow rate as the enriched air source you would later test on. My instructor disagreed, and said that it wouldn't make any difference if you calibrate based on ambient air and then tested on enriched air which is under pressure. After calibrating on ambient air, he then proceeded to test several enriched air containers, and came up at least 2 percent shy of the O2 that had been marked on each of the cylinders by the person who previously filled and tested them. I suggested that perhaps it was a result of him not calibrating on compressed air, and he dismissed me. Since I don't own an O2 measuring device of my own, I can't really test if this is true or not, but I'm just curious -- does it make a difference like PADI says, or is my instructor correct?

Usually they read higher than what they should due to too much pressure from the compressed gas pushing on the membrane in the analyzer.
2% low is more likely just not so great mixing

 

[stepfen]

I don't know if this could be a factor here but it is something I find interesting.

During my advanced nitrox course recently I was told by the instructor that after a rich (50 or more O2%) nitrox mix is made by partial blending one should shake/rotate the tank for a while so that the gases get mixed well before measuring it (!). He said that the richer the mix the more important this becomes - eg an 80% mix needs a lot of shaking for it to settle. I find it kind of difficult to believe that gases can be mixed better by shaking a tank but anyway.

I don't know how and to what % the blends that the OP's instructor measured were made, but maybe this could be a factor? ie the one who made the blends measured them without much shaking (hence the gases were not well mixed) to say a value A. But after some time they got mixed well (spontaneously) and then the OP's instructor measured them to a slightly different value B.

I don't know if this is possible - just saying.

 

[michael-fisch]

I don't know if this could be a factor here but it is something I find interesting.

During my advanced nitrox course recently I was told by the instructor that after a rich (50 or more O2%) nitrox mix is made by partial blending one should shake/rotate the tank for a while so that the gases get mixed well before measuring it (!). He said that the richer the mix the more important this becomes - eg an 80% mix needs a lot of shaking for it to settle. I find it kind of difficult to believe that gases can be mixed better by shaking a tank but anyway.

I don't know how and to what % the blends that the OP's instructor measured were made, but maybe this could be a factor? ie the one who made the blends measured them without much shaking (hence the gases were not well mixed) to say a value A. But after some time they got mixed well (spontaneously) and then the OP's instructor measured them to a slightly different value B.

I don't know if this is possible - just saying.

My experience on getting a homogenius mix after PP Blending and topping off with a 3.5cfm compressor is:
Your mix measurement won't change after 3 days with the tank standing, 24 hrs with the tank lying down, and 30 minutes of being driven around prone in the back of your car.
Rolling a tank back and forth on the floor for a half hour should work too, but I've always been too lazy or not enough pressed for time that I had to measure tonight and couldn't wait untill tomorrow to measure.

Michael

 

[yle]

During my advanced nitrox course recently I was told by the instructor that after a rich (50 or more O2%) nitrox mix is made by partial blending one should shake/rotate the tank for a while so that the gases get mixed well before measuring it (!). He said that the richer the mix the more important this becomes - eg an 80% mix needs a lot of shaking for it to settle. I find it kind of difficult to believe that gases can be mixed better by shaking a tank but anyway

Shaking a tank of gas has no effect on how the molecules mix. Gas molecules move very fast. Any gas mix is effectively mixed, with the cylinder just sitting, rather quickly. Although it would be rather amusing if you could get some video of your instructor shaking his scuba cylinders to mix the gas. Sounds like the kind of prank an instructor would play on his DM candidates.

Unfortunately, people tend to want to understand new things by trying to relate them to other things they already understand. Two liquids in a jar, like salad dressing, can separate due to their different densities (and the fact they are liquids.) And you can see them mix when you shake the jar.

But gases are different. Because they are not liquids.