Accurate PO2 Calibration

Beanie

Ah that makes more sense to me as I was thinking I do depth / 10 + 1 - I work in metres which is ATA's

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Beanie
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jont

I need to spend a few days with you or go back to school

Thanks people


JOE: Ps: its not "the bronx" its "DA BRONX"

Thanks Again:

J

jradomski

thats just an approximation.. 1 standard ata is 1.013 bar,

20 metes is NOT 3 atas, its (1.013 + 20/10)/1.013 = 2.97 ata absolute, but 3 is generally close enough..

and if you really want to get crazy.. when the definations of pressure were made, both systems use a different value for the specific gravity of water in the definition..

The actual defination is..

10 m of salt water having a specifig gravity of 1.01972 is 1.00 bar


for comparison
the us NAVY assigns a specific gravity 1.02480 so 1atm = 33.066 fsw, which is normally refined to 33.00 fsw

so depending on which defination you use the conversion fsw to msw is

for 33.066 is 3.2633 fsw/msw
or
for 33.000 is 3.2568 fsw/msw

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

ROB DAVIE

Joe, are you trying to give us ALL a headache??

jradomski

Hey I'm bored, so I figure why not make others think and give me some questions to answer...

Besides its info I know alot of others haven't been taught..

Besides it still fits into the "calibration" perspective in that you REALLY have to know what you are calibrating to..

Most people think, hey water is water, that may be true to most of us, but many the different governing bodies all like to get thier hands into the "standards"

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Underwaterbear

Hi Joe. Nice article and discussion, thanks.

Still bored?

First off - I was getting confused with your interchanging ata for atm. I assume in most places you meant atm (meaning the unit atmospheres) rather than ata (atmospheres absolute (total pressure)).

Quote:

The actual defination is..

10 m of salt water having a specifig gravity of 1.01972 is 1.00 bar

Of course that definition depends on several variables which leads to an interesting discussion about measurement error and whether it makes any difference.

Out of interest I'm sure you're aware seawater at the surface varies in it's salinity generally from 20 - 35 g/L hence it's SG ranges from 1.020 ish to 1.035ish. It also varies with temperature and depth and of course any fresh-salt dynamics such as near rivers, rain or under ice.

Any time we look at our dive computer we rely on an assumption about the SG of seawater at the place we are diving. We like to make it easy for ourselves by rounding out our assumption of SG of seawater so it is nice and simple to do the sums...and thus your point about standards. But the errors in sealevel pressure and the SG of the water probably add up to make it irrelevant.

As far as the calibration discussion goes, and my question isn't the pressure guage you use in your article and ata guage, calibrated to a standard atmosphere?

So are you saying that you would have to induce a positive pressure in the unit until the guage just started to move. At that point you would have matched the unit pressure with that inside the bourdon tube (ie 101325Pa or 14.7ish psi) and you expect a ppO2 =1.00.

So for calibrating at altitude I was thinking it would be easier to look at your VR3 and get your current local ambient pressure at altitude or ring the nearest airfield, then look up your table for expected ppO2 at that pressure to confirm the calibration. To check current limiting you can then use a depth correction table for your depth guage (does the VR3 do this automatically?) and do it underwater. For FO2s less than 100 it just becomes maths.

I think I'm saying I don't see the advantage of the guage, other than in demonstrating that positive pressure is easy to miss if using an inline sampler on your external analyser....I guess maybe what I'm missing is related to how the different units are calibrated. My understanding is based on how the prism does it...and I still might be missing something obvious (writing this in between work - so the brain is not too focussed!)

Cheers

Andrew

Steve

Or you could use a controller that lets you nominate the current altitude and O2 %. Sorry I couldn't resist

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jradomski

The guage isnt an absolute pressure guage its a normal differntial pressure guage.. basically I calculated the pressure offset for each altitude (a few mbar each way is not going to make any significant difference), the back pressures presented in the chart are the pressure offset + the offset to adjust for the diffenent oxygen percentages..

if you have a controller that lets you vary the oxygen percentage, all you have to do is offset the pressure (basically the oxygen adjustment is met internally so the pressure offset could be found by looking at the 100% column), if you can hadle altitude then you just offset the oxygen.. (use the sea level pressure since the pressure would have been internally adjusted)

If you have a controller that does both, just use the physical methods presented to get a good flush and insure that you have zero backpressure.. enettring an altitude setting and having a back pressure will really screw things up.. The slower the flow rate the better.. You can then do a linearity check by increasing the pressure to the total calculated pressure required for say 1.2 bar(ata) or 1.3bar (ata) depending on the controller..

even if you have a controller that lets you enter the oxygen percentage, you can still iincrease the accuracy on most designs because you dont need to use full percents.. if you had 95.5% are you using 95 or 96%, with a pressure offset you could actually use 95.5%

The way the chart is set up if you use the proper offsets the units of bar and ata on the controler dont make a difference becasue the controller will internally set it to the proper value.. in an ata controller, at seal level and 100% its 1.00, but for a bar controller its 1.013 . the required offsets are the same..

My main point about the article is to stop poor calibration methods, like leaving oxygen analyzers and check valves attached.. These create back pressure and can induce significant errors.

For all the measuements they have to be in absolute pressure.. PO2 is a measurement based on absolute pressure in standard atmospheres.. Metric/imperial units don't come into play.. A standard atmosphere is 1.01325 bar.. 10m of water is 1.0 bar and is not an atmosphere..

if you want to be perfectly correct, dpeth is converted to Atas, by first converting to atms and adding the atmospheric pressure.. For calibration it needs to be absolute pressure to work correctly.

As to the variences of depth based on standards.. when somesays they are at 60m, they are not at 60 linear meters, they are at 60meteres of water pressure based on the defined standard. so actual water specific gravity doesnt matter 60m of pressure is the same.. you actual linear depth could be more or less depending on where you are.. dive tables are base on pressure not distance.. the units of meters or feet from that matter are NOT a unit of distance, its a unit of pressure..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.