Wow! Meg design - dont know if I like this

***Decodiver*** · 28th April 2007, 20:17

Quote: (Originally Posted by wedivebc)

I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low.

Yeah and I knew a bird who could suck the chrome off a bumper!!

silent running · 28th April 2007, 22:03

Quote: (Originally Posted by RonMicjan)

excerpt from meg checklist...

19. Positive pressure test sensor carriage assembly (one-way water check valve).
____ 20. Negative pressure test sensor carriage assembly.
____
19 and 20 are positive and negative tests on the sensor carriage only (not the whole loop, that comes later) and this test will uncover any bypass problems, whether they are o ring or cell body related. I have heard of zero problems in this regard, other than 2 misassembly issues and the next design change will address that.

Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way?

If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy

**wedivebc** · 28th April 2007, 23:18

Quote: (Originally Posted by silent running)

Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way?

If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy

Actually the pressure difference is only the pressure drop across the scubber which is minimal. The +/- check was originally designed to check the o-ring seal in the sensor housing and in earlier models the one way valve on the water drain (later removed by ISC). I don't think it was designed to be a sensor integrity check and frankly I am not sure it is a very common problem other than DR MIke's experience I have not heard of it before.

S. Marc · 29th April 2007, 04:53

Quote: (Originally Posted by wedivebc)

I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low.

Was she in Cornwallis by any chance?

Gilles · 29th April 2007, 08:14

Differences aside, drmike has found a valid issue about the Meg. It is a weak link in a strong system chain. The whole integrity is dependant upon that of the cell(s) installed. Unfortunately I don't like it either.

I normally check the head on build, but...

I never undo the hose to re-check on multi-dive days

, and only on occasion when re-sorbing. I guess I need more discipline

. I use the rational that disassembling the one loop hose every dive introduces yet other risks (orings, threads, fatigue) not to mentioned the PITB

.

Fellow meggies, please let us not get into denial. After some time (to allow someone else to do it), I would like to put this issue on the ISC users forum in the hopes they (ISC) will aknowledge this design weakness (not really a fault because it is mitigated somewhat by the checklist procedure).

Cell failures are a sufficiently rare occurence that it just wasn't accounted for. Doesn't seem an impossibility to design a cell holder or something that protects the loop from a structurally faulted cell.

Gilles · 29th April 2007, 08:30

Quote: (Originally Posted by wedivebc)

Actually the pressure difference is only the pressure drop across the scubber which is minimal.

The pressure drop ("dP") is minimal during normal breathing, but is both rate and viscosity (i.e. density) dependant. Start breathing harder and the dP starts increasing more. Go on a bit of a deep air dive (or equivalent air) and the dP gets more as well.

Somehow get a few litres of water into your canister, and suddenly feel heavy WOB, that's all going to be stress on those sensors (those using axial scrubbers). Add potential CO2 short-circuit to your problems when having what could be a recoverable loop flood.

A radial seems to go a long way to mitigating this. I have observed dryer cells with it. Next outing May 2 to check again.

Gibbon · 29th April 2007, 08:53

Quote: (Originally Posted by schford)

I am probably being slow

I keep on seeing signs that say "Slow Children", do you remember one of these near where you grew up?

Quote: (Originally Posted by mverick)

The air has to pass through the scrubber material. Giving a very slight back pressure. The differential is there. But it's minimal...

Quote: (Originally Posted by UWSojourner)

The pressure will be very small. The pressure of the predive test itself will be large relative to the actual dive pressure. If the cells are that delicate I would think the predive test would provide some possibility for damaging the cells.

Quote: (Originally Posted by silent running)

Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way?

If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy

Quote: (Originally Posted by wedivebc)

Actually the pressure difference is only the pressure drop across the scubber which is minimal. The +/- check was originally designed to check the o-ring seal in the sensor housing and in earlier models the one way valve on the water drain (later removed by ISC). I don't think it was designed to be a sensor integrity check and frankly I am not sure it is a very common problem other than DR MIke's experience I have not heard of it before.

I completely agree the pressure difference will be very small most of the time during a dive - however if you place 100mm (4 inches for you colonials) of water in the bottom of the canister and go vertical, the WOB increases enormously, at that point anyone with strong and committed lungs would be in a position to damage cells.

During MOD1 I was told how to recover from a flood / remove water from the can.. so I went ahead with the exercise and put many liters or water in the can.

During this experiment I was interested to note the different sounds and WOB that the water was creating, I got it to the point that in a vertical position it was quite hard to suck gas through.

I then proceeded to get most of the water out which I was very please with and finished the dive, this was the last exercise of the last dive and I thought that the reason I lost one cell was due to it getting salt water on it. – Perhaps the real reason was the difference in pressure was too much for that 1 of the 3 cells.

Perhaps if I was able to suck a golf ball through a hose pipe then I would have damaged more than one cell.

Quote: (Originally Posted by wedivebc)

I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low.

2 cells for sure, would you let her test your carriage assembly?

Quote: (Originally Posted by Decodiver)

Yeah and I knew a bird who could suck the chrome off a bumper!!

All 3 cells. There is no way that would be safe check.

I thought that the design was based around using a radial scrubber where the flooding becomes less of an issue from a pressure perspective - but it still would not explain how you are supposed to test pressure across something that is not designed to take any pressure.

**wedivebc** · 29th April 2007, 11:08

Gilles · 29th April 2007, 14:00

Quote: (Originally Posted by Gibbon)

100mm (4 inches for you colonials)

I thought that the design was based around using a radial scrubber where the flooding becomes less of an issue from a pressure perspective - but it still would not explain how you are supposed to test pressure across something that is not designed to take any pressure.

Hey there pommy

! some of us colonials progressed from that British Imperial system of units

. Don't get us all confused

with my former neighbors South of the border who are having such a hard time to catch on.

Come on down some week so you can try the radial for yourself and also have

teomannaskali · 29th April 2007, 14:39

If the flow is reversed then you would have the little pressure differnece actually forcing the cell in to the slot. That way if the cell isnt really structurally damaged you would not be pulling it apart?
Edit: Hmm then you would be blowing the cell apart...

How about creating s small cap for the cell, or some mechanism to hold it in place? Even a rubber band could help it popping out if it is broken...

Teoman

28th April 2007, 20:17	#51 (permalink)
*Decodiver* Administrator Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jan 2005 Location: Geneva Posts: 2,197	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by wedivebc) I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low. Yeah and I knew a bird who could suck the chrome off a bumper!! __________________ CCR/OC Trimix Instructor Trainer CCR Training to Mixed Gas in Switzerland, France, UK & Germany on Megalodon/COPIS-Megalodon/KISS/Sport KISS/Ouroboros/rEvo/Inspiration/Evolution/Sentinel www.zerogravitydiving.com Rebreather World Terms & Conditions
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28th April 2007, 22:03	#52 (permalink)
silent running Mature mouth breather Current Rebreather/s: Prism Topaz Other Rebreather/s: Join Date: Jun 2005 Location: U.S.A. Brooklyn, New York Posts: 1,836	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by RonMicjan) excerpt from meg checklist... 19. Positive pressure test sensor carriage assembly (one-way water check valve). ____ 20. Negative pressure test sensor carriage assembly. ____ 19 and 20 are positive and negative tests on the sensor carriage only (not the whole loop, that comes later) and this test will uncover any bypass problems, whether they are o ring or cell body related. I have heard of zero problems in this regard, other than 2 misassembly issues and the next design change will address that. Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way? If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy
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28th April 2007, 23:18	#53 (permalink)
wedivebc Dave Tomblin Current Rebreather/s: Megalodon Other CCR Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Other CCR Join Date: Feb 2005 Location: Vancouver Island BC Canada Posts: 1,447	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by silent running) Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way? If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy Actually the pressure difference is only the pressure drop across the scubber which is minimal. The +/- check was originally designed to check the o-ring seal in the sensor housing and in earlier models the one way valve on the water drain (later removed by ISC). I don't think it was designed to be a sensor integrity check and frankly I am not sure it is a very common problem other than DR MIke's experience I have not heard of it before. __________________ Cheers, Dave.... Man is the only animal burdened with the knowledge he will eventually die
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29th April 2007, 04:53	#54 (permalink)
S. Marc Custom Title Allowed! Current Rebreather/s: MK 15.X Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Dec 2005 Posts: 199	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by wedivebc) I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low. Was she in Cornwallis by any chance?
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29th April 2007, 08:14	#55 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Wow! Meg design - dont know if I like this Differences aside, drmike has found a valid issue about the Meg. It is a weak link in a strong system chain. The whole integrity is dependant upon that of the cell(s) installed. Unfortunately I don't like it either. I normally check the head on build, but... I never undo the hose to re-check on multi-dive days, and only on occasion when re-sorbing. I guess I need more discipline . I use the rational that disassembling the one loop hose every dive introduces yet other risks (orings, threads, fatigue) not to mentioned the PITB. Fellow meggies, please let us not get into denial. After some time (to allow someone else to do it), I would like to put this issue on the ISC users forum in the hopes they (ISC) will aknowledge this design weakness (not really a fault because it is mitigated somewhat by the checklist procedure). Cell failures are a sufficiently rare occurence that it just wasn't accounted for. Doesn't seem an impossibility to design a cell holder or something that protects the loop from a structurally faulted cell. __________________ Gilles http://www.dirrebreather.com
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29th April 2007, 08:30	#56 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by wedivebc) Actually the pressure difference is only the pressure drop across the scubber which is minimal. The pressure drop ("dP") is minimal during normal breathing, but is both rate and viscosity (i.e. density) dependant. Start breathing harder and the dP starts increasing more. Go on a bit of a deep air dive (or equivalent air) and the dP gets more as well. Somehow get a few litres of water into your canister, and suddenly feel heavy WOB, that's all going to be stress on those sensors (those using axial scrubbers). Add potential CO2 short-circuit to your problems when having what could be a recoverable loop flood. A radial seems to go a long way to mitigating this. I have observed dryer cells with it. Next outing May 2 to check again. __________________ Gilles http://www.dirrebreather.com
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29th April 2007, 08:53	#57 (permalink)
Gibbon Southern Monkey Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2005 Location: East Devon Posts: 226	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by schford) I am probably being slow I keep on seeing signs that say "Slow Children", do you remember one of these near where you grew up? Quote: (Originally Posted by mverick) The air has to pass through the scrubber material. Giving a very slight back pressure. The differential is there. But it's minimal... Quote: (Originally Posted by UWSojourner) The pressure will be very small. The pressure of the predive test itself will be large relative to the actual dive pressure. If the cells are that delicate I would think the predive test would provide some possibility for damaging the cells. Quote: (Originally Posted by silent running) Hi Ron, I've never seen a the inside of a Meg head, so I don't know exactly what it looks like, but do you know why the sensors are mounted in such a way as to have this p differential accross them? What is the design advantage of having them mounted this way? If there's 2 steps in the setup to check the carriage seal, then ISC are aware of the potential problem, even if it is not so likely to occur. Just wondering about the thinking behind their placement. Thanks, -Andy Quote: (Originally Posted by wedivebc) Actually the pressure difference is only the pressure drop across the scubber which is minimal. The +/- check was originally designed to check the o-ring seal in the sensor housing and in earlier models the one way valve on the water drain (later removed by ISC). I don't think it was designed to be a sensor integrity check and frankly I am not sure it is a very common problem other than DR MIke's experience I have not heard of it before. I completely agree the pressure difference will be very small most of the time during a dive - however if you place 100mm (4 inches for you colonials) of water in the bottom of the canister and go vertical, the WOB increases enormously, at that point anyone with strong and committed lungs would be in a position to damage cells. During MOD1 I was told how to recover from a flood / remove water from the can.. so I went ahead with the exercise and put many liters or water in the can. During this experiment I was interested to note the different sounds and WOB that the water was creating, I got it to the point that in a vertical position it was quite hard to suck gas through. I then proceeded to get most of the water out which I was very please with and finished the dive, this was the last exercise of the last dive and I thought that the reason I lost one cell was due to it getting salt water on it. – Perhaps the real reason was the difference in pressure was too much for that 1 of the 3 cells. Perhaps if I was able to suck a golf ball through a hose pipe then I would have damaged more than one cell. Quote: (Originally Posted by wedivebc) I knew this girl in Halifax once who could suck start a diesel engine but in most cases the pressure involved is pretty low. 2 cells for sure, would you let her test your carriage assembly? Quote: (Originally Posted by Decodiver) Yeah and I knew a bird who could suck the chrome off a bumper!! All 3 cells. There is no way that would be safe check. I thought that the design was based around using a radial scrubber where the flooding becomes less of an issue from a pressure perspective - but it still would not explain how you are supposed to test pressure across something that is not designed to take any pressure.
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29th April 2007, 11:08	#58 (permalink)
wedivebc Dave Tomblin Current Rebreather/s: Megalodon Other CCR Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Other CCR Join Date: Feb 2005 Location: Vancouver Island BC Canada Posts: 1,447	Re: Wow! Meg design - dont know if I like this .... __________________ Cheers, Dave.... Man is the only animal burdened with the knowledge he will eventually die Last edited by wedivebc : 30th April 2007 at 01:00.
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29th April 2007, 14:00	#59 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Wow! Meg design - dont know if I like this Quote: (Originally Posted by Gibbon) 100mm (4 inches for you colonials) I thought that the design was based around using a radial scrubber where the flooding becomes less of an issue from a pressure perspective - but it still would not explain how you are supposed to test pressure across something that is not designed to take any pressure. Hey there pommy ! some of us colonials progressed from that British Imperial system of units . Don't get us all confused with my former neighbors South of the border who are having such a hard time to catch on. Come on down some week so you can try the radial for yourself and also have __________________ Gilles http://www.dirrebreather.com
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29th April 2007, 14:39	#60 (permalink)
teomannaskali Custom Title Allowed! Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 385	Re: Wow! Meg design - dont know if I like this If the flow is reversed then you would have the little pressure differnece actually forcing the cell in to the slot. That way if the cell isnt really structurally damaged you would not be pulling it apart? Edit: Hmm then you would be blowing the cell apart... How about creating s small cap for the cell, or some mechanism to hold it in place? Even a rubber band could help it popping out if it is broken... Teoman Last edited by teomannaskali : 29th April 2007 at 14:40. Reason: brain started working
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