Lower than expected pO2

**Steve** · 18th January 2006, 02:15

Here is an update to this dilemma. After a great deal of experimentation and fooling around I have located the culprit. In using the cal kit I was making use of a flow restrictor which had loosened (internal hex screw regulates flow) and was flowing at a rate which was causing a build up of pressure and thus a false high.

**Steve** · 18th January 2006, 22:58

Quote: (Originally Posted by UWSojourner)

Are you using ISC or Shearwater electronics?

I have APECS 2.01 software. This would make no difference to the issues I experienced though as I had artifically increased the preasure inside the head; thus creating the error. This problem would happen on any version of software and any CCR for that matter if one were to have higher than ambient pressure inside their unit when calibrating.

USSMEG · 18th January 2006, 23:10

Quote: (Originally Posted by Steve)

I have APECS 2.01 software. This would make no difference to the issues I experienced though as I had artifically increased the preasure inside the head; thus creating the error. This problem would happen on any version of software and any CCR for that matter if one were to have higher than ambient pressure inside their unit when calibrating.

Do you know if you have the one way valve off the side or is it closed off?
Thanks Paul

**Steve** · 18th January 2006, 23:17

My one way valve is closed off. I was able to replicate this error with a mate’s unit that does have the one way drain fitted. If the valve is being used as the vent in the cal and the flow rate it too high pressure will build up as well. The same principals apply. If there was a problem with the valve it would be easily picked up on pre dive checks.

***jradomski*** · 19th January 2006, 00:23

Quote: (Originally Posted by Steve)

My one way valve is closed off. I was able to replicate this error with a mate’s unit that does have the one way drain fitted. If the valve is being used as the vent in the cal and the flow rate it too high pressure will build up as well. The same principals apply. If there was a problem with the valve it would be easily picked up on pre dive checks.

even with a properly working valve, you are screwing up your calibration by having it in place... I did tests on lots of valves and even the best ones put on average .3psi of back pressure.. (although I did find a few cheapies that "only" put about .15psi of back pressure) This is considerable and will screw up the cal by a few percent (on a good design).. I found the optimum flow rate for a really good cal is about .1 to .2 lpm.. Most restrictors are at least 10 to 20 times this value..
If this small pressure differential doesn't make a difference, I would switch electronics since it should and the methods for either calibrating or reading the sensors is grossly inaccurate..
Keep an eye out for an article that I submitted on calibration.. I submitted it over the weekend, Hopefully it will be up shortly. Attached to it is a chart I use for calibration to fool the HH so I can calibrate at any o2 percentage or altitue.. You will be surprised how little back pressure is needed to alter the calibration..

to get very low differential pressure check valves you need a very large surface, there are valves with virtually zero pressure but you are talking about valves that have diameters well over an inch..

**Steve** · 19th January 2006, 00:34

Hi Joe,

I’ll definitely have a read of your article when it pops up.

Just to be clear are we talking about the same thing here? The water drain valve in the head of some Megs (mostly older ones) is used as the out flow point for the in head only (not flushing the loop) method of calibrating the Meg. I’m not talking about OPVs or dump valves (like on a drysuit)

If you are talking about the Water drain valve could you please let me know where you have located the cheap aftermarket versions? I can’t see why someone would bother marketing these.

***jradomski*** · 19th January 2006, 00:51

Quote: (Originally Posted by Steve)

Hi Joe,

I’ll definitely have a read of your article when it pops up.

Just to be clear are we talking about the same thing here? The water drain valve in the head of some Megs (mostly older ones) is used as the out flow point for the in head only (not flushing the loop) method of calibrating the Meg. I’m not talking about OPVs or dump valves (like on a drysuit)

If you are talking about the Water drain valve could you please let me know where you have located the cheap aftermarket versions? I can’t see why someone would bother marketing these.

yes the water drain valve..

There are lots of sources for these type valves, only a few are actually oxygen/ozone compatible.. the ones that had the least backpressure were the crappiest ones that didn't seal well to start with..

**Steve** · 19th January 2006, 01:09

Well there you go. I'm amazed that there are membranes the correct size and have the correct hole placement out there. What other uses are there for the ones that are used in the Meg?

On a side note I'm impressed you bothered to source them and test them. I just ordered mine without the thing.

***jradomski*** · 19th January 2006, 01:21

Quote: (Originally Posted by Steve)

Well there you go. I'm amazed that there are membranes the correct size and have the correct hole placement out there. What other uses are there for the ones that are used in the Meg?

On a side note I'm impressed you bothered to source them and test them. I just ordered mine without the thing.

The use the the Meg is using it for is a common one.. to isolate 2 chambers and still allow a liquid to pass... or even just a simpel gas check valve to prevent a gas leaking out when a pump has failed....

I should have kept track of the variuos part numbers for my records but I didn;t.. I did the tests some time ago..

Joe · 19th January 2006, 15:59

Quote: (Originally Posted by jradomski)

.. You will be surprised how little back pressure is needed to alter the calibration....

I have been obsessing over this for a while now. One method I tried (on the Meg) was to rubber band a plastic bag with a hole in it over the (bottom) outlet. I'd pinch the hole off while the O2 is flowing and when the bag fills I'd open the hole & flush the O2 through. I'd do this 2 or 3X & then just open the hole & let the gas flow until the mv's stabilize & then calibrate, the logic being that the bag would contain the O2 environment with little or no back pressure.

I intend to make a plug with a digital gauge & a bleed valve so I can pressurize to 1 ata and see if I get a 1.0 readout. I just don't see how a screw in the bottom of a brass fitting can give consistent, accurate lpm & zero back pressure unless the whole system is engineered to take into account the relationship between the inlet and the outlet.

Or am I just being neurotic about this whole thing.....?

18th January 2006, 02:15	#11 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,100	Re: Lower than expected pO2 Here is an update to this dilemma. After a great deal of experimentation and fooling around I have located the culprit. In using the cal kit I was making use of a flow restrictor which had loosened (internal hex screw regulates flow) and was flowing at a rate which was causing a build up of pressure and thus a false high. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.
(Offline)

18th January 2006, 22:58	#12 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,100	Re: Lower than expected pO2 Quote: (Originally Posted by UWSojourner) Are you using ISC or Shearwater electronics? I have APECS 2.01 software. This would make no difference to the issues I experienced though as I had artifically increased the preasure inside the head; thus creating the error. This problem would happen on any version of software and any CCR for that matter if one were to have higher than ambient pressure inside their unit when calibrating. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.
(Offline)

18th January 2006, 23:10	#13 (permalink)
USSMEG USSMEG Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Mar 2005 Location: Mass Posts: 275	Re: Lower than expected pO2 Quote: (Originally Posted by Steve) I have APECS 2.01 software. This would make no difference to the issues I experienced though as I had artifically increased the preasure inside the head; thus creating the error. This problem would happen on any version of software and any CCR for that matter if one were to have higher than ambient pressure inside their unit when calibrating. Do you know if you have the one way valve off the side or is it closed off? Thanks Paul
(Offline)

18th January 2006, 23:17	#14 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,100	Re: Lower than expected pO2 My one way valve is closed off. I was able to replicate this error with a mate’s unit that does have the one way drain fitted. If the valve is being used as the vent in the cal and the flow rate it too high pressure will build up as well. The same principals apply. If there was a problem with the valve it would be easily picked up on pre dive checks. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.
(Offline)

19th January 2006, 00:23	#15 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,895	Re: Lower than expected pO2 Quote: (Originally Posted by Steve) My one way valve is closed off. I was able to replicate this error with a mate’s unit that does have the one way drain fitted. If the valve is being used as the vent in the cal and the flow rate it too high pressure will build up as well. The same principals apply. If there was a problem with the valve it would be easily picked up on pre dive checks. even with a properly working valve, you are screwing up your calibration by having it in place... I did tests on lots of valves and even the best ones put on average .3psi of back pressure.. (although I did find a few cheapies that "only" put about .15psi of back pressure) This is considerable and will screw up the cal by a few percent (on a good design).. I found the optimum flow rate for a really good cal is about .1 to .2 lpm.. Most restrictors are at least 10 to 20 times this value.. If this small pressure differential doesn't make a difference, I would switch electronics since it should and the methods for either calibrating or reading the sensors is grossly inaccurate.. Keep an eye out for an article that I submitted on calibration.. I submitted it over the weekend, Hopefully it will be up shortly. Attached to it is a chart I use for calibration to fool the HH so I can calibrate at any o2 percentage or altitue.. You will be surprised how little back pressure is needed to alter the calibration.. to get very low differential pressure check valves you need a very large surface, there are valves with virtually zero pressure but you are talking about valves that have diameters well over an inch.. __________________ 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. Last edited by jradomski : 19th January 2006 at 00:56.
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19th January 2006, 00:34	#16 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,100	Re: Lower than expected pO2 Hi Joe, I’ll definitely have a read of your article when it pops up. Just to be clear are we talking about the same thing here? The water drain valve in the head of some Megs (mostly older ones) is used as the out flow point for the in head only (not flushing the loop) method of calibrating the Meg. I’m not talking about OPVs or dump valves (like on a drysuit) If you are talking about the Water drain valve could you please let me know where you have located the cheap aftermarket versions? I can’t see why someone would bother marketing these. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.
(Offline)

19th January 2006, 00:51	#17 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,895	Re: Lower than expected pO2 Quote: (Originally Posted by Steve) Hi Joe, I’ll definitely have a read of your article when it pops up. Just to be clear are we talking about the same thing here? The water drain valve in the head of some Megs (mostly older ones) is used as the out flow point for the in head only (not flushing the loop) method of calibrating the Meg. I’m not talking about OPVs or dump valves (like on a drysuit) If you are talking about the Water drain valve could you please let me know where you have located the cheap aftermarket versions? I can’t see why someone would bother marketing these. yes the water drain valve.. There are lots of sources for these type valves, only a few are actually oxygen/ozone compatible.. the ones that had the least backpressure were the crappiest ones that didn't seal well to start with.. __________________ 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.
(Offline)

19th January 2006, 01:09	#18 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,100	Re: Lower than expected pO2 Well there you go. I'm amazed that there are membranes the correct size and have the correct hole placement out there. What other uses are there for the ones that are used in the Meg? On a side note I'm impressed you bothered to source them and test them. I just ordered mine without the thing. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.
(Offline)

19th January 2006, 01:21	#19 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,895	Re: Lower than expected pO2 Quote: (Originally Posted by Steve) Well there you go. I'm amazed that there are membranes the correct size and have the correct hole placement out there. What other uses are there for the ones that are used in the Meg? On a side note I'm impressed you bothered to source them and test them. I just ordered mine without the thing. The use the the Meg is using it for is a common one.. to isolate 2 chambers and still allow a liquid to pass... or even just a simpel gas check valve to prevent a gas leaking out when a pump has failed.... I should have kept track of the variuos part numbers for my records but I didn;t.. I did the tests some time ago.. __________________ 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.
(Offline)

19th January 2006, 15:59	#20 (permalink)
Joe Custom Title Allowed! Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Oct 2005 Location: Florida, USA Posts: 248	Re: Lower than expected pO2 Quote: (Originally Posted by jradomski) .. You will be surprised how little back pressure is needed to alter the calibration.... I have been obsessing over this for a while now. One method I tried (on the Meg) was to rubber band a plastic bag with a hole in it over the (bottom) outlet. I'd pinch the hole off while the O2 is flowing and when the bag fills I'd open the hole & flush the O2 through. I'd do this 2 or 3X & then just open the hole & let the gas flow until the mv's stabilize & then calibrate, the logic being that the bag would contain the O2 environment with little or no back pressure. I intend to make a plug with a digital gauge & a bleed valve so I can pressurize to 1 ata and see if I get a 1.0 readout. I just don't see how a screw in the bottom of a brass fitting can give consistent, accurate lpm & zero back pressure unless the whole system is engineered to take into account the relationship between the inlet and the outlet. Or am I just being neurotic about this whole thing.....?
(Offline)

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