Comfort Zone - What range of Gas Analysis?

[Jax]

I'm working on my gas blender certification, and I brought a couple of [my] cylinders home for testing because they were hot.

They should have come in +/- .2 of 32.2%.

They did not. They came in low. Now, I am fairly confident in my gas analyzer, but I hadn't calibrated it to air under pressure.

So, until I get around to finding a air tank . . . or borrowing the instructors' analyzer . .


So, what range of readings would be in your comfort zone? So far, my calculations and fills have pretty much been spot on. Therefore, I am questioning the calibration of my analyzer. Do you ever question yours? How far out would the swing be before you would be concerned?

I calibrate with 99%O₂ when analyzing 60% or more, and calibrate with air for 21-50%.

 

[tbone1004]

where were they and do you have any other bottles with a known mix that you can cross check against?

 

[Steve_C]

Define low

 

[Jax]

where were they and do you have any other bottles with a known mix that you can cross check against?

Define low

Yes, and it routinely was 2% lower than the other mixes. EDIT: Note of which were air. I don't have an air tank right now.

The last time I calibrated it, I was at about 400 feet elevation. Here at home, I am at 4650 and therefore, I blame the air pressure.

Which is what led me to this question . . . . How far off is too far?

At what point do your spidey senses start tingling and telling you, this isn't right? Just curious.

 

[Akimbo]

You can correct for partial pressure, which galvanic oxygen fuel cells are actually measuring.

 

[Jax]

You can correct for partial pressure, which galvanic oxygen fuel cells are actually measuring.

I'm not sure what you mean by this . . .

Isn't calibrating against a known air tank "correcting for partial pressure"? Am I misunderstanding?

{Side note - I corrected the above to say I do not have a tank of 20.9% against which to calibrate the analyzer properly.}

 

[tbone1004]

it should correct for PP yes, though you are at 0.84 ATA where you are now and was calibrated at 0.98ata. 2% low I'd be a bit skeptical and try to get a 100% and 21% bottle to crosscheck it against. How old is the sensor?

 

[Jax]

I agree I need to calibrate it . . .


But my REAL question is -- at what point do you question the correctness of your measuring device?

As soon as I read 29%, I was "no way can that be right". So after futzing a bit, I checked some other cylinders that I'd analyzed after fill, and it was about 2.3% low.

I'm wondering at what "off" do the rest of you get that "can't be right" feeling.

 

[KevinNM]

If you are doing blending can you check it on a 100% O2 tank?

 

[Akimbo]

I'm not sure what you mean by this . . .

Isn't calibrating against a known air tank "correcting for partial pressure"? Am I misunderstanding?...

It is no different than calculating your PPO2 at 100' for 32% Nitrox, except there is a different conversion factor for feet of air than feet of seawater.

Let’s say you calibrate against the open air in your backyard at sea level. You can be pretty confident that it is darn close to .209 ATA PPO2, which also happens to be 20.9% by weight at sea level — by volume is so close to the same it barely matters. However, the PPO2 of air at 12,000' is less than .209 ATA.

No.	Atomic Weight	Name	Sym.
1	1.0079	Hydrogen	H
2	4.0026	Helium	He
7	14.0067	Nitrogen	N
8	15.9994	Oxygen	O
10	20.1797	Neon	Ne
18	39.948	Argon	Ar
54	131.293	Xenon	Xe
n/a	44.0095	Carbon dioxide	CO2

If you wanted to get really theoretical/anal about it you would also have to correct for barometric pressure, but in practice the sensor isn’t repeatable enough to matter.

Galvanic Oxygen sensors generate an electrical current in direct proportion to the Partial Pressure of Oxygen. I have blown-down unmanned chambers to 500M/1,650' of seawater with pure Helium, all at or near sea level. They started with air or .21 ATA PPO2 and held almost exactly. There are always leaks the first time chambers are pressurized so we expected PPO2 to drop a little from that.

Does this make sense or is it more than you wanted to know?