Rudimentry sensor mV monitoring

Mike · 17th September 2007, 12:11

Quote: (Originally Posted by silent running)

Hi sabagia, I was refering to what I assumed were mv readings of the cells post cal with what you thought was 100% O2. If the post cal readings in air are .24 or .25, then you didn't have 100% O2 during the high point cal. But I'm not that familiar with the meg cal procedure, so I might be missing something.

Hi Andy
I've seen this on my prism as well, pull the bucket off after a dive and the sensors are reading a little high in air, then drop down over time. Never worried me much, a couple of 100th of an ata of O2 either way doesn't really move my care factor guage.

Mike

Gilles · 17th September 2007, 12:37

Quote: (Originally Posted by gtzavelas)

In long dives my cells develop a lot of condensation and at the end of the dive “under-read” substantially.
In 3+ hour dives I have seen 0.90 instead of 0.99 with O2 (using the calibration kit after the dive) and 0.19 on air (thats around 10% off!!)
I seem to have the same problem in long dives both in cold (11 deg c) or warm (24 deg c) water

Of course as it has been suggested a solution is to do a wet calibration- but that’s a work around not a real solution.

I wonder if anyone has to suggest any ideas ?
Also:
Why do some people have this problem and some not (on the same units)
Could it be a “sensitive” batch of sensors?
Could there be any diver habit that aggravates the condensation?

Hi gtvazelas;

Back up in post 21, I wrote a short essay and therein is an illustration explaining the apparent bias (or voltage offset) that seems to occur when cells get moist.

I still prefer to calibrate the cells moist as this is a calibration of cells in a state that they are expected to operate in (especially for 3 hour long dives). Arguably, not really a "work-around", providing the cells are behaving linearly with PO2.

To this day, I've observed that some cells seem more susceptible than others. The Meg design doesn't keep the cells very dry, and the KISS and Inspo units I have seen all end up with visible water droplets on cell faces post-dive.

I will hopefully get my first AI cells this week and will try them out.

gtzavelas · 17th September 2007, 12:46

so this means that if you dive once per week for example then you would use the calibration of the previous week?
And then in the first part of the dive the cells will over read? Do you adjust your deco?

An other alternative would be to keep the head "wet" for ever. i.e. not to dry it out between dives. could that have any side effects? (life of censors or something else?)

looking forward for your feed back of AI cells.

thanks
George

Gilles · 17th September 2007, 14:03

Quote: (Originally Posted by gtzavelas)

so this means that if you dive once per week for example then you would use the calibration of the previous week?
And then in the first part of the dive the cells will over read? Do you adjust your deco?

An other alternative would be to keep the head "wet" for ever. i.e. not to dry it out between dives. could that have any side effects? (life of censors or something else?)

In the order of your questions asked...

yes up to some 6 weeks and some 20 hrs past, as long as the pre and post dive check at 4m/6m checks at 1.4/1.6 or somewhat greater.

they'll over-estimate the PO2 for a bout a half hour, until the cells get moist based an experiment I did using nitrox diluent.

no adjustment for deco given the conservatism I normally apply.

not sure there's really a point leaving cells to dry overnight. They get moist again anyway shortly after you start breathing the loop again.

Rebreatherlab · 30th November 2007, 13:38

Quote: (Originally Posted by Sutty)

I get condensation droplets on my sensor faces on my classic Kiss, it is a bit of a problem on a several day trip. On the recent rebreatherworld trip I was drying them off at least after every two dives, and resorted to the gentle application of the edge of a clean piece of tissue paper to mop up the bulk of it before air-drying. Cell 2 seems the worst affected.
Once the droplets cover the bulk of a sensor face it under-reads significantly.
I wish the ADV blasted dil at the faces to blow away the droplets - then a dil flush would fix the problem rather than just confirming an under-reading cell.
Anyone got any other solutions?

It sounds like a Pelagian DCCCR would be a solution!

Gilles · 2nd December 2007, 05:17

I'm uncertain that by keeping the sensor faces clear of condensation, you remove the "humid gas" effect on the sensors.

The hydrophobic membrane of the sensor faces is simply a semi-permeable layer with capillaries so small that condensation gets capillary trapped and the membrane thus becomes impermeable to water. It remains permeable however to gas, and water vapour is also a gas that gets through.

I theorize that the "humid gas" effect is caused by condensation directly on the lead anodes, thus causing the observed voltage bias constant.

The presence of moisture droplets covers a certain fraction of the surface area of the hydrophobic membrane. Providing some of the surface area is available to allow O2 molecules through, there should be relatively insignificant effect from droplets on the membrane itself.

When lead anode gets covered by a droplet, then you should have proportionally less electro-chemical reaction as this is the "reaction front", not the hydro-membrane of the cell. If this theory is correct, blowing on the cell faces does nothing to remove the condensation on the anodes.

Any thoughts and other opinions from those with more expertise.

Rebreatherlab · 2nd December 2007, 12:52

Quote: (Originally Posted by Gilles)

I'm uncertain that by keeping the sensor faces clear of condensation, you remove the "humid gas" effect on the sensors.

The hydrophobic membrane of the sensor faces is simply a semi-permeable layer with capillaries so small that condensation gets capillary trapped and the membrane thus becomes impermeable to water. It remains permeable however to gas, and water vapour is also a gas that gets through.

I theorize that the "humid gas" effect is caused by condensation directly on the lead anodes, thus causing the observed voltage bias constant.

The presence of moisture droplets covers a certain fraction of the surface area of the hydrophobic membrane. Providing some of the surface area is available to allow O2 molecules through, there should be relatively insignificant effect from droplets on the membrane itself.

When lead anode gets covered by a droplet, then you should have proportionally less electro-chemical reaction as this is the "reaction front", not the hydro-membrane of the cell. If this theory is correct, blowing on the cell faces does nothing to remove the condensation on the anodes.

Any thoughts and other opinions from those with more expertise.

You are correct. Blowing dil across the Cell membranes is a safety meassure to prevent moisture or a "bubble" skin trapping gas inside the cell, thus giving you yesterdays news. Also a cell membrane that is partially covered by water gives a slower respond time than a dry cell membrane.

What you actually breathe towards the end of the dive contains higher amount of vapour which therefore yields a lower Po2.

Any one who have calibrated an O2 analyzer in ambient air in the tropics have seen that analyzing and air tank after gives 0,22 - 0,23 or so.

Eventhough the loop contains more humidity than ambient air in the tropics a cell reading of 1,3 - 1,4 at 6m with O2 in the loop is also likely due to the difficulty of actually getting pure O2 in the loop. Ad the vapor and the fact that you are actually off gassing a bit of inert gas into the loop.

Outlaw · 2nd December 2007, 15:10

Quote: (Originally Posted by Rebreatherlab)

....Ad the vapor and the fact that you are actually off gassing a bit of inert gas into the loop.

Andy,

what if you do a good flash before you try to get the 1.6 at 6m? dont you remove the previously exhaled inert gas? i know you will still be off gassing, but lets say you do a flash every 15 -20 minutes or so? what do you think?

Thanks.

P.S your Bov is working fine mate. lots of people are asking stuff about it.

Cheers.

Rebreatherlab · 2nd December 2007, 15:28

Quote: (Originally Posted by Outlaw)

Andy,

what if you do a good flash before you try to get the 1.6 at 6m? dont you remove the previously exhaled inert gas? i know you will still be off gassing, but lets say you do a flash every 15 -20 minutes or so? what do you think?

Thanks.

P.S your Bov is working fine mate. lots of people are asking stuff about it.

Cheers.

Hi Spyros,

Try to compress your CLs when you flush with O2. Flushing a smaller volume a few times uses less O2 and you should be able to get the Po2 up to maybe 1,55 or so. Now take a picture and enjoy your high reading as it will drop down quite quickly. depending on your logistics it may be better to just do the hang on 1,2 straight through.

Glad to hear you like the Pelagian BOV!

Cheers,

Andy

bobby · 2nd December 2007, 15:44

I have had this issue with my Meg since I got it and had the issue with my inspiration. I notice the change about 15 minutes into the dive. The cells are linear if they are all in working order and this gives me a base line. When I begin the dive I keep my PO2 a little above my desired set point then let it drop gradually to a little below the set point through the dive. The longer the dive the lower my desired set point is. A one hour wreck dive will be in the 1.1 range and a 3 hour cave dive will be around .9. When I get to my deco stops a quick flush will tell me where the O2 sensors are in relation to what I know I have. I have found that I will get lower readings pending the length of the dive. 1 hour wreck dive will show 1.43 to 1.48 with an O2 flush at 20 feet where a 3 hour cave dive will show 1.35 to 1.45. Presently I have one sensor that is not affected as much as the others and will get voted out during deco on the shorter dives and will sometimes get voted out during the dive on longer dives. I always run my units manually and have looked at sensor readings as information that needs to be analyzed & thought through to decide what is truly happening in the loop. Adjustments are made from there for what I gather for information. It is not scientific & there is a fudge factor with the entire process but it is how I have learned and been diving CCR as well as SCR.One question this brings up in my mind is if there are folks that are getting accurate readings most all the time from their sensors through out their dives? If so what checks are peformed to ensure that that information is in fact accurate? I know by doing an O2 flush at 20 feet and looking at both PO2 reading and MV reading I can have a good idea of what my cells are saying and what is actually in my loop. As long as it is linear then I am comfortable with it. If others think this system can lead to a possible dissaster I would be interested in hearing thoughts on it. Bobby

17th September 2007, 12:11	#41 (permalink)
Mike Prism 'prentice Current Rebreather/s: Prism Topaz Other Rebreather/s: Inspiration Classic Evolution Join Date: Feb 2005 Location: Melbourne Posts: 327	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by silent running) Hi sabagia, I was refering to what I assumed were mv readings of the cells post cal with what you thought was 100% O2. If the post cal readings in air are .24 or .25, then you didn't have 100% O2 during the high point cal. But I'm not that familiar with the meg cal procedure, so I might be missing something. Hi Andy I've seen this on my prism as well, pull the bucket off after a dive and the sensors are reading a little high in air, then drop down over time. Never worried me much, a couple of 100th of an ata of O2 either way doesn't really move my care factor guage. Mike __________________ Open ....... Closed Mind ........ Loop
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17th September 2007, 12:37	#42 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by gtzavelas) In long dives my cells develop a lot of condensation and at the end of the dive “under-read” substantially. In 3+ hour dives I have seen 0.90 instead of 0.99 with O2 (using the calibration kit after the dive) and 0.19 on air (thats around 10% off!!) I seem to have the same problem in long dives both in cold (11 deg c) or warm (24 deg c) water Of course as it has been suggested a solution is to do a wet calibration- but that’s a work around not a real solution. I wonder if anyone has to suggest any ideas ? Also: Why do some people have this problem and some not (on the same units) Could it be a “sensitive” batch of sensors? Could there be any diver habit that aggravates the condensation? Hi gtvazelas; Back up in post 21, I wrote a short essay and therein is an illustration explaining the apparent bias (or voltage offset) that seems to occur when cells get moist. I still prefer to calibrate the cells moist as this is a calibration of cells in a state that they are expected to operate in (especially for 3 hour long dives). Arguably, not really a "work-around", providing the cells are behaving linearly with PO2. To this day, I've observed that some cells seem more susceptible than others. The Meg design doesn't keep the cells very dry, and the KISS and Inspo units I have seen all end up with visible water droplets on cell faces post-dive. I will hopefully get my first AI cells this week and will try them out. __________________ Gilles http://www.dirrebreather.com
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17th September 2007, 12:46	#43 (permalink)
gtzavelas Rebreather World Writer Current Rebreather/s: Megalodon RB80 / Clone Other Rebreather/s: RB80 / Clone Join Date: Jan 2006 Location: Athens,Greece Posts: 247	Re: Rudimentry sensor mV monitoring so this means that if you dive once per week for example then you would use the calibration of the previous week? And then in the first part of the dive the cells will over read? Do you adjust your deco? An other alternative would be to keep the head "wet" for ever. i.e. not to dry it out between dives. could that have any side effects? (life of censors or something else?) looking forward for your feed back of AI cells. thanks George
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17th September 2007, 14:03	#44 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by gtzavelas) so this means that if you dive once per week for example then you would use the calibration of the previous week? And then in the first part of the dive the cells will over read? Do you adjust your deco? An other alternative would be to keep the head "wet" for ever. i.e. not to dry it out between dives. could that have any side effects? (life of censors or something else?) In the order of your questions asked... yes up to some 6 weeks and some 20 hrs past, as long as the pre and post dive check at 4m/6m checks at 1.4/1.6 or somewhat greater. they'll over-estimate the PO2 for a bout a half hour, until the cells get moist based an experiment I did using nitrox diluent. no adjustment for deco given the conservatism I normally apply. not sure there's really a point leaving cells to dry overnight. They get moist again anyway shortly after you start breathing the loop again. __________________ Gilles http://www.dirrebreather.com
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30th November 2007, 13:38	#45 (permalink)
Rebreatherlab RebreatherWorld Sponsor Current Rebreather/s: Pelagian Other Rebreather/s: Not Bought Yet Classic Kiss Pelagian Join Date: Sep 2005 Location: Thailand Posts: 200	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by Sutty) I get condensation droplets on my sensor faces on my classic Kiss, it is a bit of a problem on a several day trip. On the recent rebreatherworld trip I was drying them off at least after every two dives, and resorted to the gentle application of the edge of a clean piece of tissue paper to mop up the bulk of it before air-drying. Cell 2 seems the worst affected. Once the droplets cover the bulk of a sensor face it under-reads significantly. I wish the ADV blasted dil at the faces to blow away the droplets - then a dil flush would fix the problem rather than just confirming an under-reading cell. Anyone got any other solutions? It sounds like a Pelagian DCCCR would be a solution! __________________ www.rebreatherlab.com
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2nd December 2007, 05:17	#46 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Rudimentry sensor mV monitoring I'm uncertain that by keeping the sensor faces clear of condensation, you remove the "humid gas" effect on the sensors. The hydrophobic membrane of the sensor faces is simply a semi-permeable layer with capillaries so small that condensation gets capillary trapped and the membrane thus becomes impermeable to water. It remains permeable however to gas, and water vapour is also a gas that gets through. I theorize that the "humid gas" effect is caused by condensation directly on the lead anodes, thus causing the observed voltage bias constant. The presence of moisture droplets covers a certain fraction of the surface area of the hydrophobic membrane. Providing some of the surface area is available to allow O2 molecules through, there should be relatively insignificant effect from droplets on the membrane itself. When lead anode gets covered by a droplet, then you should have proportionally less electro-chemical reaction as this is the "reaction front", not the hydro-membrane of the cell. If this theory is correct, blowing on the cell faces does nothing to remove the condensation on the anodes. Any thoughts and other opinions from those with more expertise. __________________ Gilles http://www.dirrebreather.com Last edited by Gilles : 2nd December 2007 at 05:20.
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2nd December 2007, 12:52	#47 (permalink)
Rebreatherlab RebreatherWorld Sponsor Current Rebreather/s: Pelagian Other Rebreather/s: Not Bought Yet Classic Kiss Pelagian Join Date: Sep 2005 Location: Thailand Posts: 200	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by Gilles) I'm uncertain that by keeping the sensor faces clear of condensation, you remove the "humid gas" effect on the sensors. The hydrophobic membrane of the sensor faces is simply a semi-permeable layer with capillaries so small that condensation gets capillary trapped and the membrane thus becomes impermeable to water. It remains permeable however to gas, and water vapour is also a gas that gets through. I theorize that the "humid gas" effect is caused by condensation directly on the lead anodes, thus causing the observed voltage bias constant. The presence of moisture droplets covers a certain fraction of the surface area of the hydrophobic membrane. Providing some of the surface area is available to allow O2 molecules through, there should be relatively insignificant effect from droplets on the membrane itself. When lead anode gets covered by a droplet, then you should have proportionally less electro-chemical reaction as this is the "reaction front", not the hydro-membrane of the cell. If this theory is correct, blowing on the cell faces does nothing to remove the condensation on the anodes. Any thoughts and other opinions from those with more expertise. You are correct. Blowing dil across the Cell membranes is a safety meassure to prevent moisture or a "bubble" skin trapping gas inside the cell, thus giving you yesterdays news. Also a cell membrane that is partially covered by water gives a slower respond time than a dry cell membrane. What you actually breathe towards the end of the dive contains higher amount of vapour which therefore yields a lower Po2. Any one who have calibrated an O2 analyzer in ambient air in the tropics have seen that analyzing and air tank after gives 0,22 - 0,23 or so. Eventhough the loop contains more humidity than ambient air in the tropics a cell reading of 1,3 - 1,4 at 6m with O2 in the loop is also likely due to the difficulty of actually getting pure O2 in the loop. Ad the vapor and the fact that you are actually off gassing a bit of inert gas into the loop. __________________ www.rebreatherlab.com
(Offline)

2nd December 2007, 15:10	#48 (permalink)
Outlaw In search of Law breaking Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2006 Location: Cyprus, Nicosia Posts: 627	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by Rebreatherlab) ....Ad the vapor and the fact that you are actually off gassing a bit of inert gas into the loop. Andy, what if you do a good flash before you try to get the 1.6 at 6m? dont you remove the previously exhaled inert gas? i know you will still be off gassing, but lets say you do a flash every 15 -20 minutes or so? what do you think? Thanks. P.S your Bov is working fine mate. lots of people are asking stuff about it. Cheers. __________________ CMAS - IANTD - TDI Kamikazi Instrustor Trainer DIRRebreather Team Member ---------------------------- "Once an Outlaw, always an Outlaw"
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2nd December 2007, 15:28	#49 (permalink)
Rebreatherlab RebreatherWorld Sponsor Current Rebreather/s: Pelagian Other Rebreather/s: Not Bought Yet Classic Kiss Pelagian Join Date: Sep 2005 Location: Thailand Posts: 200	Re: Rudimentry sensor mV monitoring Quote: (Originally Posted by Outlaw) Andy, what if you do a good flash before you try to get the 1.6 at 6m? dont you remove the previously exhaled inert gas? i know you will still be off gassing, but lets say you do a flash every 15 -20 minutes or so? what do you think? Thanks. P.S your Bov is working fine mate. lots of people are asking stuff about it. Cheers. Hi Spyros, Try to compress your CLs when you flush with O2. Flushing a smaller volume a few times uses less O2 and you should be able to get the Po2 up to maybe 1,55 or so. Now take a picture and enjoy your high reading as it will drop down quite quickly. depending on your logistics it may be better to just do the hang on 1,2 straight through. Glad to hear you like the Pelagian BOV! Cheers, Andy __________________ www.rebreatherlab.com
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2nd December 2007, 15:44	#50 (permalink)
bobby New Member Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic MK 15.X Join Date: Jul 2007 Location: Charlotte, NC USA Posts: 34	Re: Rudimentry sensor mV monitoring I have had this issue with my Meg since I got it and had the issue with my inspiration. I notice the change about 15 minutes into the dive. The cells are linear if they are all in working order and this gives me a base line. When I begin the dive I keep my PO2 a little above my desired set point then let it drop gradually to a little below the set point through the dive. The longer the dive the lower my desired set point is. A one hour wreck dive will be in the 1.1 range and a 3 hour cave dive will be around .9. When I get to my deco stops a quick flush will tell me where the O2 sensors are in relation to what I know I have. I have found that I will get lower readings pending the length of the dive. 1 hour wreck dive will show 1.43 to 1.48 with an O2 flush at 20 feet where a 3 hour cave dive will show 1.35 to 1.45. Presently I have one sensor that is not affected as much as the others and will get voted out during deco on the shorter dives and will sometimes get voted out during the dive on longer dives. I always run my units manually and have looked at sensor readings as information that needs to be analyzed & thought through to decide what is truly happening in the loop. Adjustments are made from there for what I gather for information. It is not scientific & there is a fudge factor with the entire process but it is how I have learned and been diving CCR as well as SCR.One question this brings up in my mind is if there are folks that are getting accurate readings most all the time from their sensors through out their dives? If so what checks are peformed to ensure that that information is in fact accurate? I know by doing an O2 flush at 20 feet and looking at both PO2 reading and MV reading I can have a good idea of what my cells are saying and what is actually in my loop. As long as it is linear then I am comfortable with it. If others think this system can lead to a possible dissaster I would be interested in hearing thoughts on it. Bobby __________________ No matter the amount of experience we can all learn and no matter the experience we can all teach. http://www.rebreatherworld.com/photo...s/bobbysig.jpg
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