colinicky

Ok after my thread here :-
http://www.rebreatherworld.com/shear...ontroller.html
I realise that I still do not have a damn clue about cells , mv, linear or any of it !
When I learnt on a Classic Inspo I was just taught to watch the cells move & as long as they all increased decreased together then they were ok unless you were unlucky & had 3 duff cells at the same time with the same variance of error. (In wich case it wasn't your day anyway ! )
Also that once I got to 6m it was a good idea to manually inject O2 to get the readings upto 1.6 to check they would go that far .
Is this still the case & thats all you need to do ? Was I taught wrong ? (keep in mind this was 5 years ago )

Otherwise please help in very very simple, one syllabul words to explain it to a totally confused & thick **** like me

dave t

Ok I will try

in an ideal world the line between say 0.21 and 3.5bar would be straight, in reality it isnt its curved and usually the higher you go up the po2 scale the more the curve moves away from our ideal straight line.
As the cells die that line becomes shorter, the cell has more and more difficulty to produce a correct current to hit 3.5bar. As time goes on this line gets shorter and shorter until it is below our usable range, this is current limiting

We need to calibrate because of a few things
deviance from the straight line
temperature
humidity
Atm pressure

The calibration process adds a "factor" to make all three sensors read the same at the cal point
that point is altered from day to day by one or all of the above

measuring milivolt output in air gives no indication of the health of the cell

doing the maths as Brent described is still only a good guess due to the unmeasurable curve (unless you have a narked@90 cell checker)

A milivolt reading is interpreted by the A-D converter and displayed as a po2, they are both the same thing just displayed differently
Millivolt readings on say a pursuit are raw readings without the calibration factors added (I think)

HTH

Dave

colinicky

Thanks Dave
I think understand most of that. If it is a "practical, touchy feely " type thing then I am fine & can understand things but when it get technical & theory then I am next to useless .

So does my training still hold true or not ? Wondering if my instructor showed me that way as I would understand it better than a "theory" explanation

dave t

well yes and no

I personally dont think there is enough taught about sensors in some mod 1 classes and its one of the most important things to understand, they are the weakest part of any rebreather and a thorough understanding of what they do is very important

diverreb

As a side note, you have to remember that when you do a cell check of any kind, all that is telling you is the condition of that cell at the time of the test.

It doesn't mean that the cell can't go bad any time after that test. Cell could be fine on a pre-dive check out and then go bad during the dive....

Some guys chart the Mili-volt reading of their cells over time to see how much they degrade over that period.

With Hammerhead Electronics it's easy to verify what the cells should be reading at depth with a simple dil flush and a few button pushes. If you are a whiz at math you can do the calculations in your head, but in a stressful situation I'd rather depend on the handset.

As I understand it, the technology being used in Poseidon's Breather actually verifies the cells automatically during the dive. If it works, I'd love to see that incorporated in other units.

The Cell Checker Software plots the deviation from expected values.

Sometimes it's obvious the cells shouldn't be used, sometimes, it's not so obvious.

Aside from moisture & heat, shock & vibration can cause cell failure.

I suspect that is why some cells can test out before leaving the factory and still end up bad out of the box or go bad after a rough ride out to a dive site.

Anyway, it's easy to see why trusting these cells with our lives isn't easy to do. But for now, it's the best technology available on commercial basis.

Richie

__________________
Richie Burr.... Probing the Deep Blue Seas since 1966!

udtabn

i know different maufacturers use different cels that attach differently but i am stymied why my prism that works in the 16-22mv range cannot use OTHER cels that work in that range? any ideas or what cels would or could work ?........with a possible drought of cels comming im very worried that smi will not have them in stock

wedivebc

I think the point most people miss with cells is that calibration does not change the reading of the cell it changes the conversion factor between the cells output and the display reading.
Cells put out mv in the range of 10mv to 70mv or so in operation. The calibration process converts those numbers to numbers we better understand like .21, 1.0 and 1.6 which relate to PO2. The problem is the cells aren't as linear as we would like so the relationship between PO2 and mv is not quite a straight line and must be corrected as much as possible.

__________________
Cheers,

Dave....

www.wedivebc.com

Scuba_Vixen

Let me try an explanation using some numbers that I've rounded off slightly for clarification.


Let's assume air has 20% oxygen to make the math easy.

Think of cells as little chemical batteries that supply more voltage (actually millivoltage, or thousandths of a volt) as the oxygen pressure around them rises. Keep in mind the concept of partial pressure. The O2 pressure around the cell can increase because the percentage of O2 has increased while the overall pressure has remained constant, the O2 percentage has remained the same but the overall pressure has increased, or any combination of both.

If our test sensor outputs 10mv in our 20% oxygen content air at the surface, then that equates to 20 mv for either 40% O2 at the same overall pressure, or 20 mv for the same 20% air at 2 ata.

If we stay with surface measurement for a moment, we can chart the out put of our cell from air thru 100% O2:

20% O2 (air) = 10mv
40% O2 = 20 mv
60% O2 = 30 mv
80% O2 = 40 mv
100% O2 =50 mv

Notice that the change in mv is constant for the same change of O2 percentage.

The increase in mv is also constant as the partial pressure rises. In other words, 100% O2 at 2 ata would yield 100 mv

That's what linear means; .... as the O2 percentage or partial pressure goes up, the cell output goes up in a matching proportion.

One of the most common failure modes, and a normal sign of aging, is gradual loss of linearity.

What that means is that as the O2 percentage or partial pressure goes up, the mv output doesn't go up in a stright line. Typically, mv output goes up slower than it O2 partial pressure being measured. The displayed PO2 lags the actual PO2, usually by increasing amounts, the higher the PO2 is.

Using our example; if we had a cell output of 75 mv, that would show as 1.5 PO2 on our display. If our cell was in good condition, then indeed we would have a 1.5 PO2 at the cell. Rather high for the dive, but not a imminent tox risk

If, on the other hand, our cell was failing and getting non-linear, we might have a 1.6 or 1.7, or even higher PO2 at the cell, even though the cell's output only told us we havd 1.5 Very real tox risk

That's why it's important to check a cell's linearity past just 100% O2 calibration on the surface.

The other frequent failure mode is current limiting. This is a severe manifestaion of non-linearity. The cell output tracks the O2 partial pressure relatively ok at lower ranges, but reaches a point where with even large increases in partial pressure, the output ceases to increase more than marginally.

Again, current limiting usually manifests above 1 ata, so the O2 flush at 20' is the best way to test for that, other than a cell checker.

jradomski

a better explanantion of PO2 calibration is this..

during calibration we know what the po2 is supposed to be (thats what we are calibrating to)

so when we hit calibrate the po2 monitors record this value and use it to calculate the slope of a line so that ANY po2 can be crossed to a voltage reading..

the equation of a straight line is

y=mx+b where in our case y = the po2, m=slope of line and b= the 0 intercept..

units that do 2 point calibrations will come up with some number for the 0 intercept, on a unit that does single point cal, the value b is assumed to be zero so the equation simplified to y=mx, so once we have a slope we just multiple this number by the current voltage reading and out comes a po2..


to do 2 point cal youneed to know po2s, with 2 measured voltages (actually the value from an a/d) and then solve the equations simultaneously.. 2 unknows with 2 equations..

in the single poiunt case you only solve one equation since you assume b=0 so, 1 unknown with 1 equation..

all of the po2 monitors I am aware of DO NOT caibrate the mv display so the actual mv reading will be off somewhat but will always be off by the same for that cell location so teh calibrated po2 will be accurate..

if someone wanted an accurate voltage reading a seperate cal would have to be done on the voltmete by putting a known fixed voltage and either doing the same to figure out what the actual internal gain is, or have the gain be adjustable resistors so that once a known input voltage is applied, the gain can be adjusted to get the desired output voltage..


so its clear that the calibration of one will not necessarily effect the other..

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

dave t

Scuba_vixen please forgive me if I misunderstood what you said
Are you saying current limiting and non linear are the same thing ,one just more severe than the other?

As I understand it ALL cells will suffer progressively more current limiting as they age. Their ability to hit say 3bar when then are new will reduce to 2bar when they are six months old (example) and this decline will continue until they cannot output at all.

linearity or non is a function of the deviance from a straight line between two points, a cell can be non linear but still have a high milivolt output just the reading from the controller and the actual po2 (loop) reading will be different to one degree or another. This cannot be checked underwater with any degree of accuracy because you are working with unknown reference points (Depth gauge errors ect)

In testing in a hyperbaric environment I have seen sensors drift off as the "dive" progresses, usually due to moisture on the sensor faces but not always


best

Dave