Wow! Meg design - dont know if I like this

sensor330

What part of the sensor fails? If the sensor body frees itself from the base and the membrane stays put it would still be sealed. I could be wrong though. Out out curiosity I tried to poke a hole in the membrane of a old sensor and it's actually pretty tough to do it.

Scott

Drmike

um the membrane lets gas through otherwise how would the sensor work?

But anyway the membrane comes away with the sensor body (see prev drawing) leaving just a hole.

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Monkey

Can you reinforce the bond with epoxy? or some other material you could run arround the out side of the cell for added support?

Drmike

I think they should be doing it before every dive. - I know I would!

A neg (on the sensor face side) test may not highlight a weak cell join but a +ve on the sensor face side hopefully would.

Either way I have to say I dont like the cell position design used on the meg (cell seeing pressure/temp differential as well as risk of bypass) . I dont see the benefit yet there are clear negatives.

To those that have asked Ive been asked if I can make an alternative cell holder one that will hopefully result in dryer cells too (this in part is why I have Johns Meg on my desk in the first place).

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Drmike

I tried repairing/ regluing mine but its hard to get a good bond against the plastic they are made from. The stuff they come bonded with isnt a rigid bond.

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

RonMicjan

excerpt from meg checklist...

<snip>
____ 16. Inspect sensors and sensor wires.
____ 17. Install sensor moisture pad.
____ 18. Install sensor carriage assembly.
____ 19. Positive pressure test sensor carriage assembly (one-way water check valve).
____ 20. Negative pressure test sensor carriage assembly.
____ 21. Inspect lid assembly for waterproof integrity (O-rings, and all fixed components).
<snip>

item 16 covers actually looking at sensors, verifying they are snug in the threads, thier molex plugs, plug covers, the wires leading to them and one should notice if they are visibly cracked, wouldnt one?

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.

I know mike and know he is very experienced, but I think this just reaffirms the need for crossover training that is rig specific. Mike, unless you do the training and learn the nuts and bolts of every item on the checklist, I suggest you dont dive the rig. (not saying this was your intention)

The checklist is a reminder of all the things one has to do when preparing the rig, it is NOT a comprehensive list, otherwise it would be 4 pages long. There are little sub items that fall under each item that are taught as part of the course. The first time anyone puts the meg together as a student it takes about 4 hours to cover the checklist. (no kidding!) but after some experience it comes down to about 18-30 minutes. I do a complete setup in 18 minutes pretty consistantly.

the pressure across the sensors during diving is minimal, just the resistance of the scrubber material, I dont have the numbers, but its way less than the failure spec of the sensor and less than the positive and negative carriage check will put across them. there is no temperature difference from front to back side as the gas is moving between those two points in a matter of one breath and it will heat evenly in a few minutes of diving.

why the question of moisture on cells? In 250 hours of diving I have not had a single problem with that. Divers attitude in water, the moisture channel around the base of cells and the moisture pad all serve to prevent that problem. I dive in very cold to tropical water and no cell face moisture problems.

__________________

Hk101gr

Very good observation Dr.Mike! This appears to be an important issue on the design of the Meg. Please keep posting news about your new cell holder design since there is a lot of interest around this matter because it is a definite failure point. I dont think the issue here is if and how a cell fails and creates a bypass but to find a way to improve on the design of the holder and eliminate any chance of bypass happening at all.

Outlaw

hi, when you say positive or negative test, do you mean blowing or sucking the air from the loop, and then checking out after 3-4-5 minutes to see if there is a leak? Because if you are talking about something else, then i was not taught that one either....

Spyros

__________________
CMAS - IANTD - TDI
Kamikazi Instrustor Trainer
DIRRebreather Team Member
----------------------------
"Once an Outlaw, always an Outlaw"

wedivebc

The pressure check of the sensor housing involves placing your hand over the inhale breathing hose fitting in the head to create a seal and alternately blowing and sucking on the bottom of the sensor housing where if joins the scubber canister. This area will be sealed and any air flow will indicate a problem. The pressure created far exceeds that which could occur in normal breathing and I think that by practicing this important step you will be aware of any leaking cell problems. This step as well as following the checklist should be part of all meg training.

__________________
Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

Drmike

Hi Ron,

No I dont think it would show up that way (visual inspection). There is a plastic outer covering (the yellow label) that the body sits inside. The label is attached to the baseplate the cell body is in behind it so it can look fine yet a gentle pull and it falls apart.
.
My comments were from an engineering design and risk analysis perspective. iI am not planning on diving a Meg. As an engineer I looked at the design and saw what I considered to be a less than optimum design. The fact there is a pre-dive test to check for a bypass doesnt alter the design. I dont think its a good idea to have a differential across the sensors - I wonder what teledynes view is?

I dont think thats really the concern. I understood that its suposed to be important that the cell sensing element sees the same pressure as the board side of the cell ( temp compensation etc) and asked about that. Like I said I dont know if thats really an issue or not and could be talking bollocks. Maybe Alex knows the sensor spec re: differential/ temp and pressure

I am sure if the meg predive tests are followed then the unit is safe to dive.
but there apears top be quite a few meg divers in this thread that were not aware of this potential issue - indeed we also have meg instructors here saying students arent doing these tests - so I think its wrong to be dismissive and say that thE procedures are there so theres no problem. There clearly is a problem if nothing more than following what Matt said the fact we may need to re emphasise to students about the potential for bypass and maybe to tighten up on training.

At the end of the day IMO from an engineering design fault analysis perspective I think if a Meg diver isnt testing the cell bodies havent seperated from their faceplates before every dive - then they are at risk.


Id better shut up now as I can see people may be get unit defensive.

Just sharing an observation I made today as an engineer not as a Meg diver


Cheers
M

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment