Why have O2 on inhalation side?

[robinfante]

Ok all you Meg divers, can someone explain to me the rationale for having manual O2 add on the inhalation side? I can think of powerful arguments con, but don't know of any pro, with the exception that it is easier to deco on pure O2 at 20 (easier at 200' too!)

This is not to start a my-unit-kicks-your-units-*** thread (god they're tiresome), I'm just really curious. I've asked a bunch of folks, including meg divers, and none really had a reason.

ciao
rob infante

 

[caveseeker7]

I don't have a Meg and can't think of a particular intelligent reason why that was done.
It is quite customary to inject O2 before the scrubber to allow time and space for mixing.
Then again, the APD CCRs only manually inject there, solenoid injection is into the sensor chamber.

Anyway, here my best guess:
Leon Scamahorn comes from the MK series rebreathers.
The loop direction on the MK is inhalation from the right hose, exhalation into the left hose.
Like most manufacturers, they decided to place the gases rich = right (O2), lean is left (dil). As customary as having the r=r/l=l is having one gas on one side. Tank, hoses, valves, the O2 related ones on one side, dil ones on the other. Helps to prevent hitting the wrong button.

So you have to make a decision if your gas path runs counter clockwise:
- break the 'separate sides' rule and cross hoses
- break the r=r/l=l rule and place the O2 on the left exhalation side (as SMI does on the PRISM)
- break the 'O2 before scrubber' rule and inject after it

Breaking the r=r/l=l is very unpopular as just about everyone but the DIR chaps carry their bailout/deco gases that way to. By the time most divers reach CCRs they're accustomed to it. Crossing sides sucks as you'd manually inject left and work the valve on the right for the same gas under different circumstances ...

 

[hoopa]

i asked leon about this a while back and forget what he said =-= bit i "think" it was for two reasons-- one if there is a prolem and you need o2 NOW, you can get it-- and also i think he didnt want the staight 02 hitting the cells and giving a false reading... lol hell i dont remember now== never mine im going back to wathing the japanese f1 race on tivo--lol

 

[caveseeker7]

i "think" it was for two reasons-- one if there is a prolem and you need o2 NOW, you can get it

When the hell do you need pure O2? Only at 6 msw or less.
What is the flow rate on the Meg's manual add valve?
It must be really minimal being in the inhalation side to prevent accidental oxtox.
Moreover, you have no way to measure the bag's pO2 before you inhale it.
Personally, I think having O2 added into the inhalation bag is a very risky proposition.
You're asking for an O2 hit in the long run ... . How are you gonna fly the unit manually
in a safe way if O2 is injected between sensors and DSV?

and also i think he didnt want the staight 02 hitting the cells and giving a false reading

That's nonsense. You'd be injecting into the exhaltion bag, gases mix there and in the scrubber, then go past the sensors. Sensor reading should be quite accurate, even with He in the mix. Besides, have look where the solenoid addition injects.

 

[Yann A.]

Hi,

Any one got datas on the time it takes for O2 to mix with a diluent like air, trimix?

Rgds,

Yann.

 

[grouchyturtle]

As long as we're on the subject. I read a thread on Rebreather World that claimed that you can actually rig the Meg so it does ad the O2 on the exhale side.

Does anyone know if there is any truth to this?

 

[grouchyturtle]

Talked to farmerted last week. He's been diving the Meg since the begining, actually trained on unit #3.

He's pretty sure that the r=r/l=l rule is the reason for the O2 on inhalation side.

Seems a lot of guys are starting to reroute the hoses and connect the O2 to the exhalation lung.

Ted is actually planning on ripping his lid apart and rebuilding it so the loop runs in the opposite direction, giving him a Meg with the O2 on the exhalation side and still following the r=r/l=l rule.