MV reading on cell

eanxdiver · 21st January 2008, 14:01

Quote: (Originally Posted by jiseson)

Optima has MV readouts too...

rEvo too

Gilles · 21st January 2008, 14:20

Quote: (Originally Posted by dave t)

Can you explain how you do that please? and why it can only be done if you have a MV display?

thanks

Dave

Linear response means that for a given input, an output responds proportionally via a constant. This contant depends on the nominal voltage in air. For e.g., your nominal air mV(MVair) is 8.1

lets say your O2 purity is 96%, thus the mV reading of 96% O2 at 1 ATA is

0.96/0.209 x 8.1 = 37.2 mV.

If you had a cell checker and you used the same purity to 2 ATA then

2 x 0.96/0.209 x 8.1 = 74.4 mV

Cells are not perfectly linear however. Often they go above this theoretical value and so far I consider this a sign of a healthy cell. If the same starts to go below the theoretical value at 1 ATA, then I consider this a sign of wear and current limitation and I'll start scrutinizing it more in water.

You can do this without a mV display, but it is more hassle as you have to make contact across two small leads inside your head with a potentiometer.

I attach a mV table for your convenience.

Hope this helps.

H

Stephane · 21st January 2008, 14:58

Quote: (Originally Posted by eanxdiver)

rEvo too

Not really. It displays the mV value when you enter the calibration mode. If it's not within a defined range, calibration does not occure.
So to know the mV value at 6m, you'll have to switch off the rEvodream, switch it on, tap to enter calibration, write down the values.
With the Hammerhead, you have a menu to display the mV values of the 3 cells. And it can be activated underwater if I understood correctly the manual (didn't dive with mine yet).

dave t · 21st January 2008, 15:12

Quote: (Originally Posted by Gilles)

Linear response means that for a given input, an output responds proportionally via a constant. This contant depends on the nominal voltage in air. For e.g., your nominal air mV(MVair) is 8.1

lets say your O2 purity is 96%, thus the mV reading of 96% O2 at 1 ATA is

0.96/0.209 x 8.1 = 37.2 mV.

If you had a cell checker and you used the same purity to 2 ATA then

2 x 0.96/0.209 x 8.1 = 74.4 mV

Cells are not perfectly linear however. Often they go above this theoretical value and so far I consider this a sign of a healthy cell. If the same starts to go below the theoretical value at 1 ATA, then I consider this a sign of wear and current limitation and I'll start scrutinizing it more in water.

You can do this without a mV display, but it is more hassle as you have to make contact across two small leads inside your head with a potentiometer.

I attach a mV table for your convenience.

Hope this helps.

H

Ok thanks for the explanation but all that does is show you are good at maths

A cells output is in milivolts, the computer or controller converts it to a po2 reading, so if you take a reading and youir maths dont add up how do you know where the error is? is it the A-D converter or is the o2 content not what you think it is!

sorry but in realtime I cant see any value in that, and anyway what we should be worrying about is current limiting, a much more hazardous situation in my humble opinion.

best

Dave

eanxdiver · 21st January 2008, 15:54

Quote: (Originally Posted by Stephane)

Not really. It displays the mV value when you enter the calibration mode. If it's not within a defined range, calibration does not occure.
So to know the mV value at 6m, you'll have to switch off the rEvodream, switch it on, tap to enter calibration, write down the values.
With the Hammerhead, you have a menu to display the mV values of the 3 cells. And it can be activated underwater if I understood correctly the manual (didn't dive with mine yet).

Thanks for the clarification. While rEvo mV readings may not be as extensive as the HH, it still serves to help note mV trends of cells during calibration.

Gilles · 23rd January 2008, 01:13

Quote: (Originally Posted by dave t)

Ok thanks for the explanation but all that does is show you are good at maths

A cells output is in milivolts, the computer or controller converts it to a po2 reading, so if you take a reading and youir maths dont add up how do you know where the error is? is it the A-D converter or is the o2 content not what you think it is!

sorry but in realtime I cant see any value in that, and anyway what we should be worrying about is current limiting, a much more hazardous situation in my humble opinion.

best

Dave

The value of mV readings is when you are prepping/checking/calibrating your cells with the unit on the bench. AFAIC, there is no value in real time, and never said there was.

Two point calibrations mitigate cell non-linearity somewhat, but by checking the mV readings (at surface), you can get an indication of impending non-linearity (the main cell failure mode), before going in the water.

If the cell mV responds as expected at the surface, it thus worthwhile to go in the water and check them there (using PO2). If this all checks then your ok to go.

dave t · 23rd January 2008, 07:58

Ok agreed but thats my point you can do that just as easy with a po2 readout. The controller displays the MV reading as a po2 or as a MV reading they are one and the same just different interpretation by the electronics not by the cell!!

Many people seem to think that they can catch a faulty cell if the have a MV readout (as opposed to a po2 display)this is not the case.

Most cells are linear between 0.21 and 1bar anyway even if they loose it at about 1.5 or so (old cell) and I have some concerns about a two point calibration scheme. Isnt there a danger of programing in and offset rather than taking one out due to uknown cal gasses,? I mean when is air air! and as its o2 content can change depending on humidity and atmos pressure ect, ect,

Actually the main cell failure is current limiting not non-linear they can still be pretty linear but current limited to something that is maybe below your setpoint, then its trouble!!

sorry, pet subject

best

Dave

Gilles · 23rd January 2008, 09:56

Quote: (Originally Posted by dave t)

Ok agreed but thats my point you can do that just as easy with a po2 readout. The controller displays the MV reading as a po2 or as a MV reading they are one and the same just different interpretation by the electronics not by the cell!!

At calibration time, this is where you may be in error. The purpose of calibration is to make settings so that mV are correctly translated to PO2.

In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.

Quote:

Many people seem to think that they can catch a faulty cell if the have a MV readout (as opposed to a po2 display)this is not the case.

If your on the bench calibrating, you have no indication of cells going off unless you're checking the mV response. I have cells that started off good, and then started not reaching their theoretical mV's @PO2 = 1.0. This helps me to prevent gearing up, jumping in the water only to find out I've got a dying cell.

Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.

Quote:

Most cells are linear between 0.21 and 1bar anyway even if they loose it at about 1.5 or so (old cell) and I have some concerns about a two point calibration scheme. Isnt there a danger of programing in and offset rather than taking one out due to uknown cal gasses,? I mean when is air air! and as its o2 content can change depending on humidity and atmos pressure ect, ect,

Actually the main cell failure is current limiting not non-linear they can still be pretty linear but current limited to something that is maybe below your setpoint, then its trouble!!

A 2 point scheme mitigates a current limited cell response. Note current limited = non-linear.

Attached is an essay on the topic I uploaded a long time ago. The impact of 1 and 2 point calibration is therein illustrated.

Quote:

sorry, pet subject

best

Dave

No need to apologize. Actually I enjoy providing clarification on points that meet scrutiny.

Dive safe and dive plenty

dave t · 23rd January 2008, 11:36

[quote=Gilles;164283]At calibration time, this is where you may be in error. The purpose of calibration is to make settings so that mV are correctly translated to PO2.>>>>

sorry dont understand that, as I said before the controller interprets the MV output as a po2, all you do in calibration is to correct the reading to a known factor (stored?) dont you? but you cant change the MV output so if the cell puts out say 10MV in air and air is assumed to be 0.21 thats the reading you will see??

<<<<In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.>>>>>

only as long as you are accurate with cal gas and depth measurement!

<<<<If your on the bench calibrating, you have no indication of cells going off unless you're checking the mV response. I have cells that started off good, and then started not reaching their theoretical mV's @PO2 = 1.0. This helps me to prevent gearing up, jumping in the water only to find out I've got a dying cell.>>>>>

agreed but whats the difference in watching a po2 decay or a MV decay?

<<<Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.>>>

if you have a test chamber, yes but still the po2 readout or MV readout from a raw data will decay the same

<<<A 2 point scheme mitigates a current limited cell response. Note current limited = non-linear.>>>>

dont agree, with a 2 point cal you are correcting the error at two points (maybe) introducing an error that wasnt there due to inacurate cal gas/depth sensor/ atmos pressure. There are some that say calibrating in air is better than calibrating in (O2 or both) due to the fact that an air sample is more consistantly reliable than an o2 sample (at least in our operational field) and most cells stay linear beyween 0.21 and 1.00bar anyway. A current limited cell will still calibrate within its range but unless you can calibrate higher than your setpoint how do you know where the current limit is if its outside your one or two cal points?

<<<<Attached is an essay on the topic I uploaded a long time ago. The impact of 1 and 2 point calibration is therein illustrated.>>>>

thanks interesting stuff

best

Dave

Gilles · 25th January 2008, 00:30

[quote=dave t;164305]

Quote: (Originally Posted by Gilles)

sorry dont understand that, as I said before the controller interprets the MV output as a po2, all you do in calibration is to correct the reading to a known factor (stored?) dont you? but you cant change the MV output so if the cell puts out say 10MV in air and air is assumed to be 0.21 thats the reading you will see??

The controller is a machine. Completely unable to do any interpretation. It converts a mV into a PO2 depending on your calibration. Calibration is not a correction, it is a setting that determines the correctness of your controllers "interpretation" (please tolerate my sarcasm

). The difference between 0.21(air) and 1.0 (O2 @ 1 AtA) is a factor of about 4.8. Your O2 (@1 ATA) mV setting needs to be 4.8x that of your air mV in order for your controller to "interpret" correctly.

Quote:

<<<<In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.>>>>>

only as long as you are accurate with cal gas and depth measurement!

That is true, but it (the correct depth and purity) is largely implied already if your mV at PO2 =1 jives, and you get 1.6 @6m with your cal gas.

Quote:

whats the difference in watching a po2 decay or a MV decay?

The main difference is that in the former, you do not know if you have a calibration issue, or a cell that is changing its nominal mV, or if it's a dying cell.

Quote:

<<<Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.>>>

if you have a test chamber, yes but still the po2 readout or MV readout from a raw data will decay the same

It is best to have a cell checker (test chamber), and yes PO2 and mV readouts will respond equally, it is just that the former is more ambiguous because healthy cells do need periodic re-calibration. When mV readings start to behave excessively non-linear (by this I mean to consistently not reach its theoretical value, something like erectile disfunction), this is a good indication to suspect your cell.

Quote:

dont agree, with a 2 point cal you are correcting the error at two points (maybe) introducing an error that wasnt there due to inacurate cal gas/depth sensor/ atmos pressure. There are some that say calibrating in air is better than calibrating in (O2 or both) due to the fact that an air sample is more consistantly reliable than an o2 sample (at least in our operational field) and most cells stay linear beyween 0.21 and 1.00bar anyway. A current limited cell will still calibrate within its range but unless you can calibrate higher than your setpoint how do you know where the current limit is if its outside your one or two cal points?

For reasons thoroughly articulated in the above-mentioned essay, air-cals are the worst possible types of cals.

Granted your argument about the higher risk of making errors in a 2-point cal.

Oxygen cals are ok because then (at least) you can observe how the cell drops back to air. If the value starts staying above 0.21, this is an indication of cell erectile disfunction

(for reasons detailed in above attachment). A correct 2-point cal however will result in a PO2 readout that is slightly less in error than a O2 single point cal. (again detailed in above essay).

I have observed current limited cells that don't/can't reach 1.5 or so, start not reaching their respective ideal PO2=1 mV by some 2-3 mV. The non-linearity starts to show already at PO2=1.

The above works at least some of the time, and is thus worthwhile if you have the convenience of handset mV's.

21st January 2008, 14:01	#11 (permalink)
eanxdiver Rather be Diving Current Rebreather/s: Classic Kiss rEvo Other Rebreather/s: rEvo Join Date: Feb 2005 Location: USA, SC Posts: 104	Re: MV reading on cell Quote: (Originally Posted by jiseson) Optima has MV readouts too... rEvo too
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21st January 2008, 14:58	#13 (permalink)
Stephane Hey! Ho! Let's go! Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Nantes - France Posts: 725	Re: MV reading on cell Quote: (Originally Posted by eanxdiver) rEvo too Not really. It displays the mV value when you enter the calibration mode. If it's not within a defined range, calibration does not occure. So to know the mV value at 6m, you'll have to switch off the rEvodream, switch it on, tap to enter calibration, write down the values. With the Hammerhead, you have a menu to display the mV values of the 3 cells. And it can be activated underwater if I understood correctly the manual (didn't dive with mine yet). __________________ Stéphane Hammer-rEvo powered!
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21st January 2008, 15:12	#14 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 621	Re: MV reading on cell Quote: (Originally Posted by Gilles) Linear response means that for a given input, an output responds proportionally via a constant. This contant depends on the nominal voltage in air. For e.g., your nominal air mV(MVair) is 8.1 lets say your O2 purity is 96%, thus the mV reading of 96% O2 at 1 ATA is 0.96/0.209 x 8.1 = 37.2 mV. If you had a cell checker and you used the same purity to 2 ATA then 2 x 0.96/0.209 x 8.1 = 74.4 mV Cells are not perfectly linear however. Often they go above this theoretical value and so far I consider this a sign of a healthy cell. If the same starts to go below the theoretical value at 1 ATA, then I consider this a sign of wear and current limitation and I'll start scrutinizing it more in water. You can do this without a mV display, but it is more hassle as you have to make contact across two small leads inside your head with a potentiometer. I attach a mV table for your convenience. Hope this helps. H Ok thanks for the explanation but all that does is show you are good at maths A cells output is in milivolts, the computer or controller converts it to a po2 reading, so if you take a reading and youir maths dont add up how do you know where the error is? is it the A-D converter or is the o2 content not what you think it is! sorry but in realtime I cant see any value in that, and anyway what we should be worrying about is current limiting, a much more hazardous situation in my humble opinion. best Dave
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21st January 2008, 15:54	#15 (permalink)
eanxdiver Rather be Diving Current Rebreather/s: Classic Kiss rEvo Other Rebreather/s: rEvo Join Date: Feb 2005 Location: USA, SC Posts: 104	Re: MV reading on cell Quote: (Originally Posted by Stephane) Not really. It displays the mV value when you enter the calibration mode. If it's not within a defined range, calibration does not occure. So to know the mV value at 6m, you'll have to switch off the rEvodream, switch it on, tap to enter calibration, write down the values. With the Hammerhead, you have a menu to display the mV values of the 3 cells. And it can be activated underwater if I understood correctly the manual (didn't dive with mine yet). Thanks for the clarification. While rEvo mV readings may not be as extensive as the HH, it still serves to help note mV trends of cells during calibration.
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23rd January 2008, 01:13	#16 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 761	Re: MV reading on cell Quote: (Originally Posted by dave t) Ok thanks for the explanation but all that does is show you are good at maths A cells output is in milivolts, the computer or controller converts it to a po2 reading, so if you take a reading and youir maths dont add up how do you know where the error is? is it the A-D converter or is the o2 content not what you think it is! sorry but in realtime I cant see any value in that, and anyway what we should be worrying about is current limiting, a much more hazardous situation in my humble opinion. best Dave The value of mV readings is when you are prepping/checking/calibrating your cells with the unit on the bench. AFAIC, there is no value in real time, and never said there was. Two point calibrations mitigate cell non-linearity somewhat, but by checking the mV readings (at surface), you can get an indication of impending non-linearity (the main cell failure mode), before going in the water. If the cell mV responds as expected at the surface, it thus worthwhile to go in the water and check them there (using PO2). If this all checks then your ok to go. __________________ Gilles http://www.dirrebreather.com
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23rd January 2008, 07:58	#17 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 621	Re: MV reading on cell Ok agreed but thats my point you can do that just as easy with a po2 readout. The controller displays the MV reading as a po2 or as a MV reading they are one and the same just different interpretation by the electronics not by the cell!! Many people seem to think that they can catch a faulty cell if the have a MV readout (as opposed to a po2 display)this is not the case. Most cells are linear between 0.21 and 1bar anyway even if they loose it at about 1.5 or so (old cell) and I have some concerns about a two point calibration scheme. Isnt there a danger of programing in and offset rather than taking one out due to uknown cal gasses,? I mean when is air air! and as its o2 content can change depending on humidity and atmos pressure ect, ect, Actually the main cell failure is current limiting not non-linear they can still be pretty linear but current limited to something that is maybe below your setpoint, then its trouble!! sorry, pet subject best Dave Last edited by dave t : 23rd January 2008 at 08:01.
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23rd January 2008, 11:36	#19 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 621	Re: MV reading on cell [quote=Gilles;164283]At calibration time, this is where you may be in error. The purpose of calibration is to make settings so that mV are correctly translated to PO2.>>>> sorry dont understand that, as I said before the controller interprets the MV output as a po2, all you do in calibration is to correct the reading to a known factor (stored?) dont you? but you cant change the MV output so if the cell puts out say 10MV in air and air is assumed to be 0.21 thats the reading you will see?? <<<<In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.>>>>> only as long as you are accurate with cal gas and depth measurement! <<<<If your on the bench calibrating, you have no indication of cells going off unless you're checking the mV response. I have cells that started off good, and then started not reaching their theoretical mV's @PO2 = 1.0. This helps me to prevent gearing up, jumping in the water only to find out I've got a dying cell.>>>>> agreed but whats the difference in watching a po2 decay or a MV decay? <<<Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.>>> if you have a test chamber, yes but still the po2 readout or MV readout from a raw data will decay the same <<<A 2 point scheme mitigates a current limited cell response. Note current limited = non-linear.>>>> dont agree, with a 2 point cal you are correcting the error at two points (maybe) introducing an error that wasnt there due to inacurate cal gas/depth sensor/ atmos pressure. There are some that say calibrating in air is better than calibrating in (O2 or both) due to the fact that an air sample is more consistantly reliable than an o2 sample (at least in our operational field) and most cells stay linear beyween 0.21 and 1.00bar anyway. A current limited cell will still calibrate within its range but unless you can calibrate higher than your setpoint how do you know where the current limit is if its outside your one or two cal points? <<<<Attached is an essay on the topic I uploaded a long time ago. The impact of 1 and 2 point calibration is therein illustrated.>>>> thanks interesting stuff best Dave
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25th January 2008, 00:30	#20 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 761	Re: MV reading on cell [quote=dave t;164305] Quote: (Originally Posted by Gilles) sorry dont understand that, as I said before the controller interprets the MV output as a po2, all you do in calibration is to correct the reading to a known factor (stored?) dont you? but you cant change the MV output so if the cell puts out say 10MV in air and air is assumed to be 0.21 thats the reading you will see?? The controller is a machine. Completely unable to do any interpretation. It converts a mV into a PO2 depending on your calibration. Calibration is not a correction, it is a setting that determines the correctness of your controllers "interpretation" (please tolerate my sarcasm). The difference between 0.21(air) and 1.0 (O2 @ 1 AtA) is a factor of about 4.8. Your O2 (@1 ATA) mV setting needs to be 4.8x that of your air mV in order for your controller to "interpret" correctly. Quote: <<<<In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.>>>>> only as long as you are accurate with cal gas and depth measurement! That is true, but it (the correct depth and purity) is largely implied already if your mV at PO2 =1 jives, and you get 1.6 @6m with your cal gas. Quote: whats the difference in watching a po2 decay or a MV decay? The main difference is that in the former, you do not know if you have a calibration issue, or a cell that is changing its nominal mV, or if it's a dying cell. Quote: <<<Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.>>> if you have a test chamber, yes but still the po2 readout or MV readout from a raw data will decay the same It is best to have a cell checker (test chamber), and yes PO2 and mV readouts will respond equally, it is just that the former is more ambiguous because healthy cells do need periodic re-calibration. When mV readings start to behave excessively non-linear (by this I mean to consistently not reach its theoretical value, something like erectile disfunction), this is a good indication to suspect your cell. Quote: dont agree, with a 2 point cal you are correcting the error at two points (maybe) introducing an error that wasnt there due to inacurate cal gas/depth sensor/ atmos pressure. There are some that say calibrating in air is better than calibrating in (O2 or both) due to the fact that an air sample is more consistantly reliable than an o2 sample (at least in our operational field) and most cells stay linear beyween 0.21 and 1.00bar anyway. A current limited cell will still calibrate within its range but unless you can calibrate higher than your setpoint how do you know where the current limit is if its outside your one or two cal points? For reasons thoroughly articulated in the above-mentioned essay, air-cals are the worst possible types of cals. Granted your argument about the higher risk of making errors in a 2-point cal. Oxygen cals are ok because then (at least) you can observe how the cell drops back to air. If the value starts staying above 0.21, this is an indication of cell erectile disfunction (for reasons detailed in above attachment). A correct 2-point cal however will result in a PO2 readout that is slightly less in error than a O2 single point cal. (again detailed in above essay). I have observed current limited cells that don't/can't reach 1.5 or so, start not reaching their respective ideal PO2=1 mV by some 2-3 mV. The non-linearity starts to show already at PO2=1. The above works at least some of the time, and is thus worthwhile if you have the convenience of handset mV's. __________________ Gilles http://www.dirrebreather.com Last edited by Gilles : 25th January 2008 at 02:32.
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