O2 Injection Point

divetheworld

I wouldnt be too harsh on those with different ideas, sometimes there is a lot of underlying information thats worth considering.

Propogation of gases is a dead certain as long as there is kinetic energy in the molecules (above -273.15C). How long it takes depends on the amount of energy, the density of the gases and the volume in the vessel.
A good example is farting in the bed compared to farting in the corner of the room, how long before the missus tells you off?

I'm not known to be the most diplomatic .


Brent.

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Yann A.

Brent,
As I said, I'm open to any suggestions. I guess the shape of the gas path will greatly affect the mixing of the gases too.
I do respect anyone's idea and just try to get some explanations here. Not fuming one or another...

Yann.

jhaaja

I have seen it in real life every time I use the partial pressure method to make trimix. First add helium and then top with EAN. Everytime when I analyze right after the EAN top off the O2 percentage is way off the desired. It takes many hours for the O2 percentage to stabilize.

I don`t tumble but just wait it to mix...

JH

silent running

Hello JH, this is my experience as well. And if it's problematic to get an even mix of tmix when it's under 3000 psi, I imagine that it's not going to mix any easier at 11 ATA. I think that whatever you believe about gas mixing dynamics, injecting O2 in the inhale side makes the already inexact process of reading the PPO2 that much harder.

If you manual add O2 into the inhale lung, you are probably going to wind up inhaling some O2 before the rest gets to the sensors and then your actuall PPO2 will not be the same as what the sensors and integrated deco comp, think it is.

Sure, with some practice you can time your breathing cycle to avoid a PPO2 spike, but why bother, just to keep the Right=Rich thing? Seems like too low of a benefit to justify the additional skill/task loading.

And I'm not sure that this is possible, but if that manual add button gets stuck and starts leaking a little and becomes it's own little KISS valve, you could get a higher PPO2 in the inhale lung, without knowing it. And then it might not show up at the point in the loop where the sensors are placed. Which would mean no HUD/handset warning and possible O2 tox. Does this make sense?-Andy

caveseeker7

For your info, Andy, JH dives an Inspo. The loop runs clockwise and thus the rich=right thing is taken care of with the manual O2 add in the exhalation lung.

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Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

Drmike

No it doesnt make any sense.

The spike would take only one breath to reach the sensors. Too short a duration and a diluted down po2 so i dont beilieve there is any risk of an unexpected tox. as any delay in HUD /Handset warning would only be delayed one breath.

O2 is being squirted into a CL and inhale hose volume of lower ppo2. that and the fact that Humans exchange only around 15% of their lung volume per breath, means the ppo2 spike in the lung will be dampened out quite a lot.

Its no differenet than having a stuck open solenoid - the handsets /hud would warn you then too

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Wreck diving is a sport
Diving in general is a sport

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its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

silent running

Gotcha, thanks for explanation Mike, makes sense. -Andy

caveseeker7

Wouldn't it take two, maybe three breaths on most RBs?
Exhale into CL (that's the one you're skipping, skippy ),
will have to get from there through the scrubber to the
sensors.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

silent running

"Skippy", that's good. I forgot he's only got 1 lung. I was refering to OTS CLs. I think he knew that though... But one other thing I forgot about is the time it might take to sort out a stuck O2 add button. I could see it taking a minute or 2. I wonder how long somebody can take a PPO2 of say, 2.0? But maybe the whole stuck O2 button thing is not so likely. I guess if the spring broke or something got in the button-sand, very small pebble-it could jam open. Then you'd need to disconnect the whip... -Andy

AD_ward9

There is a lot of talk about where to inject, but there is a little fact being ignored: if you move the inject point away from the sensors, that is the same as increasing the response time of the sensor. At the lowest tidal volumes, the response time could be 10 seconds if the inject point is on the exhale breathing bag, and sensors in the normal place on the inhale side of the scrubber. This will increase the PPO2 error.

The amount of the error can be calculated quite easily. Put the extra response time into the Rebreather model we posted, it comes up with 10% PPO2 error. That is the difference between PPO2 set point and actual PPO2. So a 1.0 set point will go up to 1.1 and down to 0.9, even if the injection is perfect. Even the average point can be 10% off under these conditions.

This may seem quite surprising, but is quite logical: if your tidal volume is low, and you put in a huge lag between the injector and the sensor, then you are breathing gas that has been through your lungs once, filling up the exhale bag and scrubber before you inject gas. When you inject gas, you are injecting too far downstream to deal with the gas in the scrubber and most of it in the exhale bag.

It is compounded with assymetric breathing patterns: breathing in fast and out slow.

I will post some graphs and figures from this when I have time over the next few days.

It means there is only one correct place to inject gas: right up close to the O2 sensors, which need to be on the scrubber inhale side (they cannot be in the inhale bag otherwise condensation problems would wreck/cause error in the sensors).

This phenomenon is a problem in the Open Revolution design for the helium sensor, which as a 10 second response time. It means we have to add 10% to the calculated helium loading in deco calculations, whenever we detect a change in the gas mix, for several minutes, unless we find a way to fix the problem mathematically. Same for N2. The change in mix can be from DIL or from a genuine mix change, so will apply through long periods of the dive. This is something we are looking at improving my modelling the sensor response, but the point here is, if you place the O2 inject point far from the sensors, it can increase the PPO2 error enough to make a difference in CNS and in Deco.

Cheers

Alex