Megalodon Calibration Procedures

jradomski

Since it is clear to me that many do not understand how calibration works and the consequences of various errors I will try and put together some examples of what some common mistakes will do..

There is another point I think many do not understand.. if you are doing a 2 point calibration like the meg, doing it inside (using ambient air)is not the greatest idea..

Inside a well insulated space with the windows and doors shut the o2 percentage is very rarely 20.9%, its usually lower, with several people around its not uncommon to have actual oxygen percentages indoor of 18-19%,

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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.

tecdivertraining

Joe this would be a great idea and surly beneficial to a lot of CCR divers out their. Wouldn’t it be great if you could charge for all your additional information?

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Ben Field

[quote=jradomski]There is another point I think many do not understand.. if you are doing a 2 point calibration like the meg, doing it inside (using ambient air)is not the greatest idea.. [quote]

(warning potentially idiotic question)
Surely it makes more sense to calibrate off the known O2 and Dil supplies seeing as they are "control" gasses?
Ambient "air" anywhere (in or out of doors) isn't much good for calibration I'd imagine?

Taking your Rebreather apart and holding your thumb or caps over the various inlets and outlets while juggling analysers and handsets sounds like a recepe for diasater IMVHO.

BEN

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bubnotbub

Joe,
it's not a polemic
but this is to look for something of wrong for strength.
I don't think that a scuba diver habitually makes his calibration in a closed room with a lot of people.
There are surely a lot of wrong ideas about calibration but we look
to remain on real things.

Regards

Aldo

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Aldo Ferrucci
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Nad

There's something I noticed: the O2 supplier give me 99.9...O2, with certificate and a couple of time I was there during the analyzing process.
When I analyze it by myself the % is around 97 with three different analyzer.
Then what could you do? Trust the O2 supplier or trust three differents analyzer (with different sensors)?
It could be also part of the thread problem.

Nad

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jradomski

I'm not making any assumptions.. God knows how many times I have seen people "calibrate" their o2 analyzers inside a divestore without using gas from a tank..

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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.

nigelh

We need to be aware of and manage flaws. We must know the problems,
We should be the most critical because we have the most invested, the most to lose.

I'm not a Meg user but tell me about a problem with my YBOD and you are my friend with my best interests at heart. OK not if you just want to do the shovel gag but if you know a stock fault that I don't or something I should watch out for this is on my good information list.

One of the more impressive things I see with rebreather divers is the "Dive/No Dive" attitude to faults. We are very intolerant and stay out of the water if we have a defective system. I don't see the 'give it a go.. it will probably be all right' attitude and I equate that to maturity.

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nigelh

bubnotbub

About this I agree with you
Regards

Aldo

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Aldo Ferrucci
Trimix and Rebreather IT
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Drmike

When the meg is calibrated against air is the FO2 of the air input by the diver or does the controller assume its 21%

Around here air is typically 20.6 or 20.7%

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Genesis

There are a number of potential issues with calibration that are NOT well-explained in any of the material I've seen - including user manuals that I've read.

IMHO that's bordering on criminal, as this is NOT something that should "be left for a class" or anything written by/for an agency. That institutes variability of instruction on top of the rest! This should be in the unit's user manual along with a detailed explanation of how the software works and how it determined if a calibration is "ok".

First, O2 fuel cells are QUITE sensitive to humidity. You want to see this? Its quite easy. Take a tank full of AIR (which is very dry, with RHs in the single digits if the dewpoint is at -50F or better where it should be.) Calibrate your O2 analyzer on atmospheric air to 20.9, then plug in the tank air and analyze it. Notice anything? Heh, how come it doesn't read 20.9 any more? You have two samples, both taken from the same air. The only difference is that one's dry, the other's moist. Uh huh....

This is especially bad with a Rebreather because the loop is almost always at or very near 100% RH. This is an immediate source of error and there's simply no way around it, because your source gas for calibration on a "known source", if using some form of tank gas (e.g. O2) is going to be dry while the ambient environment moist - and humidity makes the cells read LOW.

Second, I don't like two-point calibration. I know why some people DO like it, but the engineer in me doesn't like it. My reason is that the cell is supposed to have a 0mv output in 0% O2. That is, the slope of the response line is set by a single-point calibration but one end-point is fixed.

Two-point calibration does not fix either end point, but rather defines the slope with the two points of calibration and allows the end point to float. Unless the software verifies that the "zero" end point is really zero, there is a danger here that you could easily set a slope that does not conform to reality. It would be relatively easy to end up with a significant error, especially in the case where one or more cells are malfunctioning, or there is a problem with the signal conditioning circuit - yet the "calibrate" can be performed. Looking at the mv levels does not address this, as I've got three cells here which are all "good" and yet which have significant variability in mv output levels; the "range" of acceptable mv levels for functional cells is in the 25% area! That of course would be a HUGE error if that ended up in the displayed (and controlled) value.....

As has already been pointed out calibrating with 100% O2 is potentially problematic in that if there is any positive pressure in the system at the time you will read high, and that error can be fairly significant as well.

There is yet another potential issue in that the sensors I've looked at, with the exception of the little mini sensor used in the SportKiss, all appear to have a specification which only guarantees linearity out to a 1.0 PO2. Of course everyone runs them higher. The little SportKiss sensor has a designed linearity out to 2.0.....

Finally, and I know I've raised this before, I'm not aware of any of the units out there currently which force validation of high-setpoint linearity before they'll attempt to run on "automatic." This is something that bothers me generally (my engineer hat again) in that one cannot in general trust any sort of instrument that hasn't been validated to at least the limits of measurement which is proposed to be in the working range.

Calibrating on a 1.0 PO2 does not prove you can accurately read a 1.2, 1.4 or 1.6. That you were ok two weeks ago on deco at 20' (reading 1.6 or close to it on an O2 flush) doesn't mean that today you can read an accurate 1.6.

Yes, I know that this basically can't be reasonably done "on land". But it CAN be done "in water" by forcing the user to validate a high (and stable) reading before setpoint control over the calibrated range will be permitted. You can validate all the way up to a 1.0 PO2 setpoint on land, of course; validating anything higher requires an O2 flush underwater (e.g. at 20' for 1.6, etc)

As for the argument that we shouldn't talk about this stuff because there's been no official cause of death pronounced in any of the accidents - do you really think we'll ever get a full exposition of what happened, or that we CAN in many cases (E.g. unit is flooded at the time of recovery, damaging or destroying the evidence)? I don't. Heck - we have "official reports" calling a dead guy in a cave, found in a restriction (but not keyholed) who has lots of gas left "death by entrapment in cave". Yeah, I really believe he got caught, then voluntarily decided to drown and removed his reg from his mouth.... Not.

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