Were you trained on Oxygen sensors with this level of detail?

[Bobby]

I put in a NOT SURE. My first course was 17 yrsa ago. Hard to remember if I had it in the course or learned some of it after (sucks to get old!). Also I'm not sure what point he's trying to make in the last paragraph of the article.
Later,
John

John,
The point that I attempted to make is that we can't rely on the controller to determine if the cells are acceptable when it comes to linear drift or limiting. Regardless of the cells output in air we need to do the math and see what the output is in O2 and O2 @ 1.6 ata. That is the only way to actually know the health of our cells.

 

[decostanza]

Bobby shared this article with me some time ago and it was/is a real eye-opener for me. Well worth the read...

 

[DepthCharge]

John,
The point that I attempted to make is that we can't rely on the controller to determine if the cells are acceptable when it comes to linear drift or limiting. Regardless of the cells output in air we need to do the math and see what the output is in O2 and O2 @ 1.6 ata. That is the only way to actually know the health of our cells.

OK got it now!
John

 

[wedivebc]

John,
The point that I attempted to make is that we can't rely on the controller to determine if the cells are acceptable when it comes to linear drift or limiting. Regardless of the cells output in air we need to do the math and see what the output is in O2 and O2 @ 1.6 ata. That is the only way to actually know the health of our cells.

Have you experienced many cells that were grossly non-linear? I'm not talking about current limiting as that is a separate defect common mostly in older cells but new cells that are offset from the expected value in oxygen? The megalodon prior to APECS 4.0 conducted calibration as a 2 part process to straighten out any offset encountered. They no longer do that in calibration. I'm not entirely sure why.

 

[Capt Jim Wyatt]

Have you experienced many cells that were grossly non-linear? I'm not talking about current limiting as that is a separate defect common mostly in older cells but new cells that are offset from the expected value in oxygen?

Never on a new cell just out of the package.

 

[wedivebc]

One of the factors affecting linearity of the cells has to do with the "purity" of the test sample. As indicated in a previous post where a diver has trouble achieving 1.6 at 20 ft, even getting a pure O2 loop can be a challenge on many rebreathers.
The Meg for instance can be fitted with a head only calibration kit that allows you to flood the cells with pure O2.
The common way of performing O2 calibration on a P2 for example is to flush the unit 3 times with O2 while sucking the counterlungs flat between cycles. This works fairly well and when calibrated in this way comes fairly close to pure O2 loop but due to the incompressible portion of the rebreather, ie canister and hose elbow, will never get a completely O2 saturated cell. Matthew Addison showed me a head only kit for the P2 that I believe he designed.
That still won't help those looking for that 1.6 at 20ft due to the incompressible area of the rebreather that never quite gets flushed enough.
One other aspect many divers ignore is that if you are flushing a loop with O2 looking for a certain cell mv reading you must remember that your body is introducing inert gas into the loop as you exhale gas coming out of saturation in your body. This is why the purity of a loop flush will degrade as you perform a deco stop.

 

[Jack Hammer]

My regular buddy uses this formula to check his cells (he's on a RB, I'm not). I'm surprised how many times I've heard him casually talking to other RB divers at sites about it who say they've never heard of it.

 

[Tienuts]

For the better part of 10 years, I worked for 2 large shops selling rebreather consumables online. At least once per month I'd have a conversation that went something like this:

Customer: You sold me a bad o2 cell.
Me: OK what is it doing? (Or not doing?)
Customer: When I put it in my rebreather and do my O2 flush, it reads .87. The others read .99.
Me: What were the millivolts when you calibrated it?
Customer: I don't understand. It reads .87 on my handset.

Horrifying. I've always said CCR diving is more forgiving than people think, as theres quite a few people out there who have no clue how the most important part of their rebreather works.

 

[paulraymaekers]

John,
The point that I attempted to make is that we can't rely on the controller to determine if the cells are acceptable when it comes to linear drift or limiting. Regardless of the cells output in air we need to do the math and see what the output is in O2 and O2 @ 1.6 ata. That is the only way to actually know the health of our cells.

I think you can, but you have to do it the correct way: use the controller as the calculator
calibrate first with pure O2, then flush with air and you must read 0.21/0.22
if you read higher, this means something is wrong: no correct flush with oxygen, non linear, dramatically current limited.
the 1.6 you check with flushing at depth 6m

 

[Bobby]

Have you experienced many cells that were grossly non-linear? I'm not talking about current limiting as that is a separate defect common mostly in older cells but new cells that are offset from the expected value in oxygen? The megalodon prior to APECS 4.0 conducted calibration as a 2 part process to straighten out any offset encountered. They no longer do that in calibration. I'm not entirely sure why.

It was 10 to 15 years ago that a bad batch of cells came out. Out of the package they were OK however after a dive or two the linear drift was around 10%. Eventually it was figured out that the cause was a bad batch however it was a battle to get the cells replaced.

Fairly regularly I'll get a cell that develops a lot of drift (+10%) within a few months.

Too many cells to count that have become limited.