Comfort Zone - What range of Gas Analysis?

[Jax]

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. 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?

*Never* more than I wanted to know - I learn so much from you!

---------- Post added February 1st, 2015 at 04:47 AM ----------

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

Yes. I have always been told to calibrate against a known gas closest to what you are measuring -

such as 100% for a deco gas, ~80 > 100

such as air for mix < 50%

 

[The Chairman]

In the immortal words of a fellow ScubaBoarder: Deco theory is like measuring with a micrometer, marking with a chalk and cutting with an axe. Let's say you are indeed 2 points off... instead of 68% N₂ you have 70%. That's less than a 3% variance. Three percent! Is it negligible? It is for me, but then, I never push the NDL envelope and cushion my deco dives with extra hang time.

Secondly, this wound of yours is self inflicted. Temperature, pressure, battery conditioni and humidity can have a non-predictable effect on gas measurements. This has a little to do with the gas you're measuring and a lot to do with the instrument's ability to deal with so many environmental variables. Calibrate, THEN analyse. If the device has been turned off, or any of those variables have changed, re-calibrating is a must.

 

[Jax]

In the immortal words of a fellow ScubaBoarder: Deco theory is like measuring with a micrometer, marking with a chalk and cutting with an axe. Let's say you are indeed 2 points off... instead of 68% N₂ you have 70%. That's less than a 3% variance. Three percent! Is it negligible? It is for me, but then, I never push the NDL envelope and cushion my deco dives with extra hang time.

Secondly, this wound of yours is self inflicted. Temperature, pressure, battery conditioni and humidity can have a non-predictable effect on gas measurements. This has a little to do with the gas you're measuring and a lot to do with the instrument's ability to deal with so many environmental variables. Calibrate, THEN analyse. If the device has been turned off, or any of those variables have changed, re-calibrating is a must.

Ha!

That is why I am an engineer. I delight in exploring the limits of measurements, learning from the experienced, and learning myself my comfort zone.

Besides, crunching numbers is intellectually satisfying. :geek:

 

[Chugwhump]

I wave whatever sensor I end up with in the air around me.
If it hits anywhere between 20.7 to 21.0%, I test the tank with confidence.
Plus or minus a point on "real" O2 content does not concern me.
I dive a 1.4-1.5 MOD based upon my mood, and exceed safety stop and deco times routinely though, so I guess I thow a decent safety/conservative factor in there.
So far, so good.

Chug
Complacent diver.

 

[Jax]

I wave whatever sensor I end up with in the air around me.
If it hits anywhere between 20.7 to 21.0%, I test the tank with confidence.
Plus or minus a point on "real" O2 content does not concern me.
I dive a 1.4-1.5 MOD based upon my mood, and exceed safety stop and deco times routinely though, so I guess I thow a decent safety/conservative factor in there.
So far, so good.

Chug
Complacent diver.

Agree . . . Given the sensor have a +/1 of 2%, and that one should plan for a lower MOD because you just don't know your physiology of the day, and in case of emergencies . . .


My VTI is very accurate, if slow.

I just calibrated to 100%, then left it in the air at normal air pressure. It dropped to 21.0 percent.

Now it measures my tanks as expected.

.5% high, but my instructor always says, do NOT short the Nitrox customer . . . they don't like it.

Me, I'm learning how exact can I make my tanks with my technique and mathematics . . .

 

[JahJahwarrior]

Once had some customers spearfishing deep ask for 36%, which was a 1.6 ppo2 at their planned depth. Tanks came back at 35.4 and could not convince them that the tables are cut for ~1%, that analyzes are only accurate to ~1% and that for them the higher gas would bring more risk. Customer is always right, I worked until I got 36.1 and the customer was happy.

But on my own tanks? If it is within 1% that is within tolerances for the way we measure and fill. Is your fill station gauge very accurate and precise? When was it last calibrated? Partial pressure blending? Did you analyze the O2 before you filled? Tank drained to nothing or did you analyze it to figure out what to add? Let the pressure after adding O2 drop to room temp?

 

[Jax]

Once had some customers spearfishing deep ask for 36%, which was a 1.6 ppo2 at their planned depth. Tanks came back at 35.4 and could not convince them that the tables are cut for ~1%, that analyzes are only accurate to ~1% and that for them the higher gas would bring more risk. Customer is always right, I worked until I got 36.1 and the customer was happy.

But on my own tanks? If it is within 1% that is within tolerances for the way we measure and fill. Is your fill station gauge very accurate and precise? When was it last calibrated? Partial pressure blending? Did you analyze the O2 before you filled? Tank drained to nothing or did you analyze it to figure out what to add? Let the pressure after adding O2 drop to room temp?

Yeah, yeah; using the LDS system and dependent on his time - nor do I want to be a PITA while waiting for cooling.

For me, personally, if my O2 analyzer comes up outside of +/- .5 % of what is expected, I get a "what's wrong?" feeling. I cannot justify it, given the sensor's margin for error and the equipment margin for error, but experience with this particular O₂ analyzer has proven that there is something wrong if it is outside that expected range.

 

[The Chairman]

Ha!

That is why I am an engineer. I delight in exploring the limits of measurements, learning from the experienced, and learning myself my comfort zone.

Besides, crunching numbers is intellectually satisfying. :geek:

Me? I just love to go diving! Or kayaking, or biking, or working on my workshop/truck or...

The limiting factor here is not the measurement. I have a rather expensive Lab Analytics O₂ Analyser that reads to one hundredth of a percent. I have to admit that I only used it until the cell went south. The current crop of analysers are less than half the price of that cell. In addition, there is no need to measure your gas that accutately. I actually round my O₂ content to the nearest even number with anything within one and a half getting rounded to 32%. It's not a cavalier or complacent attitude but the knowledge that other factors play a far bigger role in getting bent or doing the spazzed chicken dance.

 

[Jax]

Then I would guess you have a much wider swing in what your analyzer reports before you question its output?

 

[DiveNav]

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

The other thing you could try is to do a 2 points calibration: basically you calibrate the analyzer with 2 known gasses (like air and O2). In this way the Analyzer can build a more precise reference line (from point A to point B).
Additionally it helps to see the mV output of the cell in addition to the O2% so you can get an idea if the cell is acting weird.