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heyydude · 5th June 2006, 22:01

Mark,

Here's why the HH rejects cells that are LESS than 40 mV when you go to Calibrate them:

Since 1995 or so, we've only used ACTUAL cells in a Pressure Pot for testing and calibration. The reason for that had a lot to do with the experience I've gained through using lots of different cells from lots of different manufacturers - some of whom are no longer around.

I wanted to test whatever system we were building against real Sensors. This way, we could eliminate any noise or other fluctuations from an "artificial" source (I once spent an entire day trying to figure out what was wrong with a set of electronics, only to find a variable potentiometer in our mV generator took a dump). We have both mV generators here, as well as pressure pots aplenty, but we use the pressure pots more.

--> Point 1: I once found a set of cells - brand new - that were current limited to what would have been the equivalent of 1.1 ppO2. Had I gone diving with those cells, they would have killed me.

And remember, in a Voting Logic system, any TWO cells that agree can also kill you.

---> Point 2: That is why one of the first things you see on the HH is the "FO2" command - the unit does a simple math function to tell you what your ppO2 should be at whatever particular depth you are at, based upon your selected Gas Mix. This serves as a "mental-health-check" for those stressful times when you're trying to figure something out very quickly.

When we started to develop the HH, one thing became clear from talking to Inspiration divers - they all had "issues" with the cells. Whether or not this was perception or reality is still a mystery to me, since we've had very GOOD luck with Teledyne products across their range. However, we HAVE found current-limited R-22's - enough, combined with my previous experience with cells to make us take a couple of steps to try to ensure you don't get hurt.

---> Point 3: ANY R-22 that outputs LESS than 40 mV in 100% Oxygen is, in our opinion, suspect. REMEMBER THIS. If you are using ANY kind of CCR and your mV in 100% is 35 mV or less, using an R-22 of any vintage - regardless of it's "in air" mV reading - you are taking a risk diving that set at anything above .75.

So what we did in the HH was install a bit of code that says - "if a cell = <40 mV during Cal, REJECT that cell. The older HH's (Pre Primary 7.06 and Secondary 5.05) used to insert a null value "0.00" in place of any Rejected Cell. That, however, led to lots of guys thinking that the HH was "broken" because no matter what cell they plugged in, it still read "0.00". We changed that to read "FAIL" now, so there is no question.

---> Point 4: In our shop, we use ACTUAL R-22 cells in our Test Pots. We leave these in 100% Oxygen FULL TIME. These cells last about a year, no matter how hard we treat them. As part of our REGULAR QC on each HH, we pressurize them to 3.00 ppO2 to verify linearity of the HH system - any deviation, and we check the mV output of the cell first. R-22's are VERY capable of Linearity to 3.00 and demonstrate so all the time in our shop.

Other CCR electronics allow divers to calibrate at LESS than 100% - Trying to be Polite, and with All Due Respect, WE DO NOT agree with this philosophy. In our Humble Opinion, there are TOO MANY ways a diver can wind up dead using that scenario, and we are not willing to allow our set to do that. Why would anyone trust a $50 "El Cheapo" O2 analyzer (no offence to my great friend, Pat Duffy), which is calibrated in AIR vs, a multi-thousand dollar piece of CCR electronics is beyond me.

And before anyone jumps down my throat about this - remember, the USN, RN and RAN all use Medical or Aviator Grade O2, and flow 4 lpm around their R-10D's for FOUR MINUTES before calibrating. I don't know any civilian that does that - but more of you should. It is a good policy. Gas is CHEAP compared to LIFE.

Joe Radomski and I have had this discussion/argument more times than I can count, but I still come back to the same thing - if you want to actually dive something less than 100% O2, you can always compensate via your Set-Point control and your Conservatism - giving you the actual mV output of each cell should be sufficient to help you calculate using a very precise instrument (the HH) what actual FO2 your O2 bottle has in it.

And, most importantly, 3 years of experience has borne out that the 40 mV limit WORKS.

---> Point 5: Calibrate your sensors back home on some verified 100% - keep track of the mV output of EACH cell. When you are in "Elbonia", or some other place where they can only give you 90%, then re-check your mV and do the math to make sure they are in line with what they should be. IF NOT, DON'T DIVE. Verify your readings with other analyzers and re-test. If ever in doubt - DIVE AT LOWER ppO2's or dive OPEN CIRCUIT.

Listen To Me: You should be doing this with ANY Rebreather you are diving - if for no other reason than to cross-check your Sensors and Analyzer. Since nearly all of us carry our PC's (or Mac's for the Enlightened among us), make up an Excel Spreadsheet to do the math for you, and to log your cells at the same time. Treat this information as if your life depended on it, because it DOES.

Nothing down there is worth dying for, no matter how much damn money you paid to go on that trip.

If there is enough of an upswell of desire, we can build copies of our O2 Sensor Pressure Pots. We've made them from time to time for friends and even one customer (who doesn't use it - not naming Names, but it's spelled "B-A-Z-Z-A"

). I'll send a picture of one to Joe so he can upload it to the site (I still don't know how to do that).

--> Last Point: eCCR's are NOT TOASTERS - these things are Life Support Devices, which means that you need to take them more seriously than you would your iPod. You don't need to know how the iPod plays your favorite tunes - but you DO need to know how your O2 Sensors work, and how your Electronics uses that information to keep you alive. If your iPod shits the bed, you just get mad at Steve Jobs and demand a new one. If your eCCR does the same, your widow gets to cash your policy, and your kids have an excuse to use in the future as to why they're such F**ked up adults...

Enough said...

Safe diving to everyone.

Kevin Juergensen
Juergensen Marine.

5th June 2006, 22:01	#10 (permalink)
heyydude King of the Geeks Current Rebreather/s: MK 15.X Optima Other CCR Other Rebreather/s: Other CCR Join Date: May 2005 Location: Addison, Pennsylvania Posts: 358	Re: Mv output on new Bud R22 Cells? Mark, Here's why the HH rejects cells that are LESS than 40 mV when you go to Calibrate them: Since 1995 or so, we've only used ACTUAL cells in a Pressure Pot for testing and calibration. The reason for that had a lot to do with the experience I've gained through using lots of different cells from lots of different manufacturers - some of whom are no longer around. I wanted to test whatever system we were building against real Sensors. This way, we could eliminate any noise or other fluctuations from an "artificial" source (I once spent an entire day trying to figure out what was wrong with a set of electronics, only to find a variable potentiometer in our mV generator took a dump). We have both mV generators here, as well as pressure pots aplenty, but we use the pressure pots more. --> Point 1: I once found a set of cells - brand new - that were current limited to what would have been the equivalent of 1.1 ppO2. Had I gone diving with those cells, they would have killed me. And remember, in a Voting Logic system, any TWO cells that agree can also kill you. ---> Point 2: That is why one of the first things you see on the HH is the "FO2" command - the unit does a simple math function to tell you what your ppO2 should be at whatever particular depth you are at, based upon your selected Gas Mix. This serves as a "mental-health-check" for those stressful times when you're trying to figure something out very quickly. When we started to develop the HH, one thing became clear from talking to Inspiration divers - they all had "issues" with the cells. Whether or not this was perception or reality is still a mystery to me, since we've had very GOOD luck with Teledyne products across their range. However, we HAVE found current-limited R-22's - enough, combined with my previous experience with cells to make us take a couple of steps to try to ensure you don't get hurt. ---> Point 3: ANY R-22 that outputs LESS than 40 mV in 100% Oxygen is, in our opinion, suspect. REMEMBER THIS. If you are using ANY kind of CCR and your mV in 100% is 35 mV or less, using an R-22 of any vintage - regardless of it's "in air" mV reading - you are taking a risk diving that set at anything above .75. So what we did in the HH was install a bit of code that says - "if a cell = <40 mV during Cal, REJECT that cell. The older HH's (Pre Primary 7.06 and Secondary 5.05) used to insert a null value "0.00" in place of any Rejected Cell. That, however, led to lots of guys thinking that the HH was "broken" because no matter what cell they plugged in, it still read "0.00". We changed that to read "FAIL" now, so there is no question. ---> Point 4: In our shop, we use ACTUAL R-22 cells in our Test Pots. We leave these in 100% Oxygen FULL TIME. These cells last about a year, no matter how hard we treat them. As part of our REGULAR QC on each HH, we pressurize them to 3.00 ppO2 to verify linearity of the HH system - any deviation, and we check the mV output of the cell first. R-22's are VERY capable of Linearity to 3.00 and demonstrate so all the time in our shop. Other CCR electronics allow divers to calibrate at LESS than 100% - Trying to be Polite, and with All Due Respect, WE DO NOT agree with this philosophy. In our Humble Opinion, there are TOO MANY ways a diver can wind up dead using that scenario, and we are not willing to allow our set to do that. Why would anyone trust a $50 "El Cheapo" O2 analyzer (no offence to my great friend, Pat Duffy), which is calibrated in AIR vs, a multi-thousand dollar piece of CCR electronics is beyond me. And before anyone jumps down my throat about this - remember, the USN, RN and RAN all use Medical or Aviator Grade O2, and flow 4 lpm around their R-10D's for FOUR MINUTES before calibrating. I don't know any civilian that does that - but more of you should. It is a good policy. Gas is CHEAP compared to LIFE. Joe Radomski and I have had this discussion/argument more times than I can count, but I still come back to the same thing - if you want to actually dive something less than 100% O2, you can always compensate via your Set-Point control and your Conservatism - giving you the actual mV output of each cell should be sufficient to help you calculate using a very precise instrument (the HH) what actual FO2 your O2 bottle has in it. And, most importantly, 3 years of experience has borne out that the 40 mV limit WORKS. ---> Point 5: Calibrate your sensors back home on some verified 100% - keep track of the mV output of EACH cell. When you are in "Elbonia", or some other place where they can only give you 90%, then re-check your mV and do the math to make sure they are in line with what they should be. IF NOT, DON'T DIVE. Verify your readings with other analyzers and re-test. If ever in doubt - DIVE AT LOWER ppO2's or dive OPEN CIRCUIT. Listen To Me: You should be doing this with ANY Rebreather you are diving - if for no other reason than to cross-check your Sensors and Analyzer. Since nearly all of us carry our PC's (or Mac's for the Enlightened among us), make up an Excel Spreadsheet to do the math for you, and to log your cells at the same time. Treat this information as if your life depended on it, because it DOES. Nothing down there is worth dying for, no matter how much damn money you paid to go on that trip. If there is enough of an upswell of desire, we can build copies of our O2 Sensor Pressure Pots. We've made them from time to time for friends and even one customer (who doesn't use it - not naming Names, but it's spelled "B-A-Z-Z-A" ). I'll send a picture of one to Joe so he can upload it to the site (I still don't know how to do that). --> Last Point: eCCR's are NOT TOASTERS - these things are Life Support Devices, which means that you need to take them more seriously than you would your iPod. You don't need to know how the iPod plays your favorite tunes - but you DO need to know how your O2 Sensors work, and how your Electronics uses that information to keep you alive. If your iPod shits the bed, you just get mad at Steve Jobs and demand a new one. If your eCCR does the same, your widow gets to cash your policy, and your kids have an excuse to use in the future as to why they're such F**ked up adults... Enough said... Safe diving to everyone. Kevin Juergensen Juergensen Marine.
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