Megalodon Calibration Procedures

schford

APECS and Shearwater differ in so much as Shearwater is one point calibration (Done in 100% O2) and APECS is 2 point done in air and 100% o2 on both handsets I believe....

The kit is an optional extra that was only brought out about a year ago - before that it was done by trying to bring the loop to 100% o2

In terms of calibration though - if I have thought this through correctly, if you mess up the calibration on the (shearwater) Meg then I think that your displayed PPo2 will be higher than it is in the loop - so am not sure how you would get o2 convultions (at least on the Shearwater with 1 point cal - I have to admit am not sure how 2 point cal woudl affect it)...

eg when you calibrate if the flow of gas is restricted (eg by an anyliser) then you would actually be under a slight pressure in the head so when you set it at .98 or whatever it was it could actually be less eg .9 at ambiant - this could affect your deco but should not cause convultions from breathing to high a PPO2 ...

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Mark Chase

Whats to know?

Do you wish to calibrate?

Yes

Open Mouthpiece?

Yes

Open 02?

Yes

The hand sets ask you for the milibar setting on the start up routine and you put in 1020 if you don't have a VR3 handy and yes we always put in 99% 02 because 02 is rarely 100% at sea level and it alow for a bit of contamination in the head. Your on the boat about to dive so your normally about 1.5m above sea level

But how much does any of this relay matter?

Calibrating on 98% instead of 100% will give you a slightly rich mix by a minuscule 2%. It would be amazing to me if the unit was accurate within 2% any way. I would be surprised if an error of 0.05pp02 made much if any difference in the scheme of things.

I am not sure if refresher training will help divers who couldn't understand how to calibrate a unit despite passing Mod1 and Mod3?

ATB

Mark Chase

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sven

Matthew, Joe.

In principle, I agree with what you have said. Unfortunately, the Internet is frequented by "Internet divers", who don't take proper training and believe, that they know it all after reading a few pages on the Net ... I think it would be better to influence certification agencies to improve their training, rather than to publish this information here. If this site was only for certified divers who want to exchange information and improve their knowledge ... different story.

Let the flame wars begin

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UWSojourner

Withdrawn comment.

teomannaskali

The data sheet of the O2 sensors doesnt say anything about 2 point calibration.

It states that if you calibrate it with air 20.9 you should get accurate readings up oh 1 ata. And if you calibrate it with %100 o2 you will get accurate readings up to 2 ata. (Check the datasheets to see how accurate accurate is)

[quote=Genesis]
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.
[quote]

The data sheet claims that the sensors are not affected by non-condensing humidity. If it condenses thet the O2 cant be absorbed by the sensor and result in a low reading.

The cell accurately measures the ppO2 of Oxygen (again see the data sheet for how accurate). What happens is (I think, I dont claim to know it all) when you use dry air you have 20.9 O2 in the mix and 79.1 N2. But when you do it in humid air you are adding water vapour to what you are measuring. Since the total pressure is the same (1 atm at sea level) this means that there now 3 gasses in where there was 2. So the partial pressure of O2 drops because there really is less of 02. So the sensor measures correctly.

So in a rebreather (on the surface) if there is humidity in the loop there is no way you can get a %100 O2 level.

Teoman

rogeringebo

When I dived my KISS I always filled the loop up with 100% O2 and went down on the bailoutreg to about 6m and made sure that the sensors could come up to 1,6 or close to that....then dil flush and then continue the dive..quite a good way to make sure the sensors can get up to that number..(normally when getting old that would be a problem)

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hyrax

First let me say I’m not YET a rebreather diver and I have not YET built my own loop controller for a rebreather. I am however a professional programmer. I think that it might help if everyone understands what you are doing when you calibrate a PPO2 sensor.

First this is what we are solving, the equation for the line the sensor would make if you graphed the PPO2 on the y axis and the mV reading on the x axis.

y= mx+b

y is your PPO2
m is your mV reading
x is the slope of your line
b is your offset

For a 2 point calibration.

Given on air your PPO2 should be about .21. Lets just say our mV reading is 20. For our O2 to make it simple lets use 1.0 for our PPO2 and 50 for our mV reading. First we want to find x that is just our slope between our first and second point. Remember that grade school math slope is rise over run. In our case.

x = (1.0 - .21)/(50 – 20)

and we get

x = 0.0263333

Now we have to solve for b. So let’s use our numbers we used for the air calibration.

.21 = 20 * 0.0263333 + b

b = .21 – (20 * 0.0263333)

b = -0.316666

So we get:

y = 0.0263333 * x – 0.316666

So 40 mV looks like this.

Y = 0.0263333 * 40 – 0.316666 or PPO2 = 0.736666

If I’ve made any mistakes feel free to tell me what an idiot I am, I have thick skin I can take it.

UWSojourner

Shouldn't it be that way?

If water vapor makes up part of the atmospheric pie we breath, then don't you want the analyser to read lower even if 100% O2 is the source gas? For example, if after calibrating to dry 100%O2 you take a reading in high humidity with O2 the source gas I would expect a reading of about 95% I think.

If I'm thinking about this correctly I think you want the dry calibration AND you want the reading to drop with introduction of humidity. If so, the low end of the two point calibration should also occur with dry gas from an take filled with AIR.

Genesis

Exactly.

My point is that for two-point to be accurate you need the cal to take place with gases that have the same RH! Otherwise you're setting up an offset error - the slope is now wrong because you've introduced an offset into the calibration that is fictional (the cell must produce 0mv in 0% O2, as its an electrochemical cell that relies on the O2 for its activity)

If you read 20.9 in "atmospheric" air but its really 20.6 (due to the water vapor) then you've depressed the slope. If you cal with dry O2 from a tank for the other side you've now offset the "zero point" high, which causes the unit to read below actual. You've pushed both ends of the slope in opposite directions, which makes the errors additive. In effect, the two-point cal becomes less accurate than a single-point because the errors in gas use are additive to the error in the result.

Now that alone shouldn't be enough to tox you - but if you now have positive pressure in the sensor areas, you now are forcing the slope even lower; the errors are again additive. With a 0.5psi positive pressure in the head you'd have introduced about a 3.5% error in the reading. Add to that another couple of percent for the RH offset (ambient air cal on a humid day in Florida) and you could easily be off by 5% or so - close to a full tenth off at a 1.6 PO2.

This STILL shouldn't be enough to "get you". But - if you combine this with a non-linear response to hyperbaric PO2s due to cell fault or age......

If you studiously validate the hyperbaric PO2 readings before relying on them then you're PROBABLY ok, as you'll catch any gross errors and smaller ones aren't material enough to matter. But if you don't........

I put two-point cal into my controller's firmware and found that I didn't like the offset values that resulted at the zero end. The algabraic equation should be X = c(PO2); any "offset" is by defintion an error.

Reducing it to single-point resulted in an error of less than 2% at 1.6 PO2, and within the 1% tolerance of the sensor at a 1.0 PO2, all calibrating using dry (tank) air. Using atmospheric air results in an error of less than 3% full scale, which is less than half of one significant digit (that is, it won't result in a 1.6 actual PO2 being 1.5 nor 1.7). Two-point calibration frequently DID lead to errors large enough to affect the display, especially when tank O2 was used for the high point and humid atmospheric air for the low. This makes sense, because the errors are additive.

All these cals are currently being done with the sensors "loose" as the head's not yet done, so there's no possible positive-pressure issue to deal with (yet)

If you do two-point calibration and wind up with an equation that is X = c(PO2) + b, for any value of "b" that is material you need to fail the cal, because that equation is known false. The slope must intercept 0,0 for the calibration to be valid.

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DonalWalsh

Gotcha- that makes more sense.