[Louby Lou]

Hi All

Just a quick note to say that Joe Radomski has kindly allowed us to publish his article and photographs demonstrating a procedure to accurately calibrate and verify many CCR controllers.

Joe's article Accurate PO2 Calibration can be found HERE.

Please can you use this thread to post any comments, questions or feedback.

Once again, many thanks Joe.

Lou

[jradomski]

Before anyone asks about check valves and the absence of them it is for a reason..

Anyone calibrating a Meg that has a valve on the water trap is inducing this error.. That means the PO2 is always higher than what is displayed. The "2 point" calibration makes this even worse since AIR was not set under the increased pressure..

All check valves have some required cracking pressure.. for small diameters the best units usually have around .3 psi of cracking pressure, Thats an average error of 2%.. Most check valves are worse than this so the error will e greater.

[jont]

As always you are very thorough Joe, and accurate I have several questions:

1) Does the new HH allow you to calibrate at lets say 80% o2 (travel issues...like truck were the o2 generators put out between 80 and 95% o2)?

2) Do other systems like the insperation (as you know I have 1 working unit and one destroyed unit, the truck incident i told you about) take into consideration that the loop AFTER following the on screen instructions will NOT have a true 100% o2 content?

3) Units like the prism (just an example) say to flush the loop 3 times, but even after 3 flushes you won't have a true 100% o2 content. How does this effect diving ? and when does this become life threatening?

4) Is it possible to calibrate such units (that lack true electronics) with less than 100% o2 SAFELY?


Thanks Joe (I am glad someone knows the answers to this stuff)

[jradomski]

Quote: (Originally Posted by jont)

As always you are very thorough Joe, and accurate I have several questions:

1) Does the new HH allow you to calibrate at lets say 80% o2 (travel issues...like truck were the o2 generators put out between 80 and 95% o2)?

The HH was designed to only calibrate with 100% oxygen.. This being said using the method I describe I can accuractely calibrate at other percentages.. I wouldn't use an 80% source, that tells me something else is wrong.. A PSA system delievers 95.5% o2 when working at optimum, if they push the gas through a bit too fast it will give a lower percentage.. The lowest I would use would be around 90% or so

In theory if the vessel could withstand the pressure I could calibrate with air (assuming the unit calibrates to 1.00/1.013)... but that would mean that the guage pressure would be about 55psi

Quote: (Originally Posted by jont)

2) Do other systems like the insperation (as you know I have 1 working unit and one destroyed unit, the truck incident i told you about) take into consideration that the loop AFTER following the on screen instructions will NOT have a true 100% o2 content?

The inspiration calibrates in a unique manner.. what it does is flush the head with oxygen until it sees no further change on the cells, it then sets the calibration to the pressure setting multiplied by the enetered oxygen percentage.. The cal should be verified and the enetered oxygen percentage could be adjusted +/- 1 percent or so until the correct value is obtained.. that is ambient pressure in bar multiplied by the analyzed oxygen percentage.. The Inspiration displays po2 in bar even though the standard is to express po2 in Atas.. so in reality the displayed po2 (assuming a perfect calibration) is 1.3% lower than the standard.

Quote: (Originally Posted by jont)

3) Units like the prism (just an example) say to flush the loop 3 times, but even after 3 flushes you won't have a true 100% o2 content. How does this effect diving ? and when does this become life threatening?

displayed PO2 is higher than actual

when it becomes dangerous depends on how far the lop was actually off and how close to the edge you push the tables

Quote: (Originally Posted by jont)

4) Is it possible to calibrate such units (that lack true electronics) with less than 100% o2 SAFELY?
Thanks Joe (I am glad someone knows the answers to this stuff)

any unit can be reasonable calibrated as long the calibration value you use is somehow zerod to the scane you want to use, and the entrained loop (or chamber) must be at a pressure that matches what you use in your po2 calibrations.. that is pressure times oxygen percentage..

Thats how my chart works at altitude or another gas percentage.. I know what the result is supposed to be (that is 1.00) so I adjust the pressure so that the gas in use multplied by some pressure equals 1.00ATA,

If the controller sets the PO2 in bar the same offset should work but it will now display 1.013 instead of 1.00

[ROB DAVIE]

Joe,

As always, your work is thorough, and pains-taking! Thank you for sharing it with us!

[Beanie]

Quote: (Originally Posted by jradomski)

The inspiration calibrates in a unique manner.. what it does is flush the head with oxygen until it sees no further change on the cells, it then sets the calibration to the pressure setting multiplied by the enetered oxygen percentage.. The cal should be verified and the enetered oxygen percentage could be adjusted +/- 1 percent or so until the correct value is obtained.. that is ambient pressure in bar multiplied by the analyzed oxygen percentage.. The Inspiration displays po2 in bar even though the standard is to express po2 in Atas.. so in reality the displayed po2 (assuming a perfect calibration) is 1.3% lower than the standard.

I am I right in thinking that if I have atmospheric pressure of 1010mBAR and a cylinder of 100% I should get a displayed value of 1.01 on the handset?

Then if it's higher I need to adjust my O2% down in the calibration sequence and lower tweak it up? did I get those two the right way round?

[jradomski]

Quote: (Originally Posted by Beanie)

I am I right in thinking that if I have atmospheric pressure of 1010mBAR and a cylinder of 100% I should get a displayed value of 1.01 on the handset?

Then if it's higher I need to adjust my O2% down in the calibration sequence and lower tweak it up? did I get those two the right way round?

Yes thats true.. The inspiration cal% is really a fudged value not what the oxygen has been analyzed at.. I find on most rigs 1% less than the actual value usually gives the right outcome.. When it takes a lower number I find its that the IP on the Oxygen reg is a bit high..

just getting the number to what you expect is important, but make sure if you put 1010 for the atmpospheric pressure its really 1010.. for example in the caribean on a normally sunny day the average pressure is usually around 1020-1030 mbar depending on the location.. while near the poles it averages closer to 1000mbar..

[Scubascooby]

Quote: (Originally Posted by jradomski)

The Inspiration displays po2 in bar even though the standard is to express po2 in Atas.. ..

Just for info, the Atmosphere is an obsolete unit in Britain and Europe, AFAIK it hasn't been used in diving circles for nearly 40 years.

I'm a fan of proper Imperial measure for day-to-day tasks (pint, miles, yards) but for physics and diving, metric is the tool of choice ;-)

Even Bar should be replaced by Pascals but that is taking things a bit too far.

Peter

[jradomski]

Quote: (Originally Posted by Scubascooby)

Just for info, the Atmosphere is an obsolete unit in Britain and Europe, AFAIK it hasn't been used in diving circles for nearly 40 years.

I'm a fan of proper Imperial measure for day-to-day tasks (pint, miles, yards) but for physics and diving, metric is the tool of choice ;-)

Even Bar should be replaced by Pascals but that is taking things a bit too far.

Peter

No atmospheres IS STILL the standard UNIT for PO2.. All the exposure tables are based on ATAs of exposure... the units of measurement for depth doesnt matter, PO2 is still atas..

Most deco software handles this correctly when displaying the calculated po2.. When using metric measurements PO2 is still supposed to be calculated based on atas..

its not just depth times f02 as is commonly taught.. its close enough but it IS wrong.. depth must be converted to atas (/10 for m is not an ata, but /33 for fsw is)then multiplied by f02.. Its not a big difference but it is a difference..

If you don't believe me, just do a standard OC dive using something like vplanner and see the calculated PO2, when you work it out you will see its in Atas not bar.. This is also why it has an option for CCR to select the units for the controller since not all controllers use the correct units..

Here is a line cut from Vplanner

Level 100m 27:07 (50) Trimix 13/60 1.42 ppO2, 28m ead, 34m end


if it was actually in bar it should be 1.43, and the 1.42 is actually rounded up for conservatism in the overall exposure calculations, the calculated po2 is 1.41 atas

and if you really want to be anal, the standard surface pressure is 1.013bar not 1.0 bar as taught.. so if you really want to calculate it properly for sea level it should be ata absoulte = (1.013 + Depth in meters /10 ) / 1.013

the 1.013 can be substituted for any atmospheric pressure (eg altitude)

[ROB DAVIE]

Folks,

Joe is correct here AFAIK. For instance, in aviation we are taught and use 1.013 for standard atmosphere, not 1.000, and this is an international standard.

Also, the formula is: Depth in ata times the fraction of the gas. The conversion in Imperial measurement (Yes, guys, I admit metric works more easily.) is:
Depth in fsw divided by 33, result plus 1 = depth in ata.

Just my ppO2's worth.

(DISCLAIMER: I am not a mathematician nor an engineer, nor do I play either on TV. I am a pilot, and we generally use fancy computers to get our answers these days!! )