Initial reading before calibration

Divejunkie · 25th March 2008, 18:29

Can someone help me with this please.

When I switched the meg handsets on after a week off, in air they were both reading 0.18%.

Even though I went outside to ventilate the head they remained at 0.18%. I continued with the calibration in oxygen. When I removed the O2 supply and the head ventilated, the handsets were reading 0.21

My question is to do with how the sensors are calibrated-

I understand the electronics use a two point calibration system i.e. it needs to start the calibration in air (21%) and finish at the max o2 you have set the handset (99%). But what if the cells wont reach .21?

Is the calibration of the 3 O2 cells similar to an O2E11 analyser in that they need to know that they are in 'air' at 20.9 to start with before they can give an accurate O2%?

or- does it not matter what the initial O2 reading is because you flush with a known 100% (albeit the unit is set at 98%) O2 supply and the electronics basically assign 100% as the maximum and works backwards?

I'm concerned that if the cells start at 0.18 (3% down) p02 then the calibration will be incorrect and will result in a potential decompression probelm.

Does this make sense?

***jradomski*** · 25th March 2008, 18:50

Quote: (Originally Posted by Divejunkie)

Can someone help me with this please.

When I switched the meg handsets on after a week off, in air they were both reading 0.18%.

Even though I went outside to ventilate the head they remained at 0.18%. I continued with the calibration in oxygen. When I removed the O2 supply and the head ventilated, the handsets were reading 0.21

My question is to do with how the sensors are calibrated-

I understand the electronics use a two point calibration system i.e. it needs to start the calibration in air (21%) and finish at the max o2 you have set the handset (99%). But what if the cells wont reach .21?

Is the calibration of the 3 O2 cells similar to an O2E11 analyser in that they need to know that they are in 'air' at 20.9 to start with before they can give an accurate O2%?

or- does it not matter what the initial O2 reading is because you flush with a known 100% (albeit the unit is set at 98%) O2 supply and the electronics basically assign 100% as the maximum and works backwards?

I'm concerned that if the cells start at 0.18 (3% down) p02 then the calibration will be incorrect and will result in a potential decompression probelm.

Does this make sense?

first of all I think you need a refresher.. The handsets do not indicate percentage they indicate po2.. If you are at sealevel the percentage and teh po2 display would be nearly equal, but if you are at altitude there can be a big difference..

for example the po2 at ~900m above sealevel for air would be about 0.18

a cell that is only reading 1% low would only need to be at ~600m to give the same reading..

If you are above sea level please follow the proper calibration procedures. You do not want it reading .21 when it is not supposed to..

I chose 900m because All the territory in England that is over 3,000 feet above sea level is in Cumbria. with the highest altatidue being scafell pike at 3210ft/978m

Divejunkie · 25th March 2008, 21:17

Thanks for the geography lesson

.

You are correct it is PO2 not % O2. For some reason I had it in my head that it starts at 21% so it was therefor 0.21. Dont I feel foolish

.

I'm more or less ar sea level so I would expect it to be 0.21.

Gilles · 25th March 2008, 22:32

Quote: (Originally Posted by Divejunkie)

Can someone help me with this please.

My question is to do with how the sensors are calibrated-

All calibration is, is the conversion of a mV reading to a PO2. As cell mV are expected to respond linearly, their should be a proportional change in mV with PO2.

The 2-point cal system of the Meg is unnecessarily complex. IMV, ISC has done a poor job explaining how this works, and it seems so many perform a procedure without understanding what is going on.

Quote:

... But what if the cells wont reach .21?

...?

It doesn't matter what PO2 the uncalibrated cells read in air or O2. What matters is the relative difference in mV between air and O2.

In pure O2, the mV should read 1/.21 = ~4.8x more than the air reading (assuming the altitude remains constant). If the above factor is 4.8x or a little bit more, than your cells are behaving as they should, and you can calibrate as instructed. Make sure you set the correct altitude as Joe says.

This 2-point system is a design flaw of the Meg. They should change to a more simple 1-point high FO2 system, thus reducing the liklihood of people getting it wrong.

I attach a customized checklist for the Meg, that contains a mV table to facilitate the process.

**wedivebc** · 26th March 2008, 00:26

One thing I have noticed is the new software no longer prompts you before confirming air point calibration. I am not sure it was a good idea to change that.

***jradomski*** · 26th March 2008, 06:24

Quote: (Originally Posted by Divejunkie)

Thanks for the geography lesson

.

.

I just wanted to show that I took extra care answering your question, not trying to be a smart ass...

I had to look up Cumbria.. I knew nothing about it....

Divejunkie · 26th March 2008, 15:20

Quote:

I just wanted to show that I took extra care answering your question, not trying to be a smart ass...

I had to look up Cumbria.. I knew nothing about it...

Joe

I know you weren't- email is so cold though isn't it.

Gilles has basically confirmed what I what I wanted to hear in that it doesn't matter what PO2 the uncalibrated cells read in air or O2. What matters is the relative difference in mV between air and O2.

I'm at sea level near as dammit and the unit is set on 0m. When I switch on if its outside the range 0.20-0.23 PO2 in air then I'll recalibrate.

thanks for the responses.

Gilles · 26th March 2008, 19:15

Due to multiple PM's for an amendable e-copy of the above checklist, I attach an xls here.

Please provide feedback if any, and/or click the green square in the bottom LH of your screen if you find this useful.

25th March 2008, 18:29	#1 (permalink)
Divejunkie New Member Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic Join Date: Nov 2005 Location: Cumbria Posts: 29	Initial reading before calibration Can someone help me with this please. When I switched the meg handsets on after a week off, in air they were both reading 0.18%. Even though I went outside to ventilate the head they remained at 0.18%. I continued with the calibration in oxygen. When I removed the O2 supply and the head ventilated, the handsets were reading 0.21 My question is to do with how the sensors are calibrated- I understand the electronics use a two point calibration system i.e. it needs to start the calibration in air (21%) and finish at the max o2 you have set the handset (99%). But what if the cells wont reach .21? Is the calibration of the 3 O2 cells similar to an O2E11 analyser in that they need to know that they are in 'air' at 20.9 to start with before they can give an accurate O2%? or- does it not matter what the initial O2 reading is because you flush with a known 100% (albeit the unit is set at 98%) O2 supply and the electronics basically assign 100% as the maximum and works backwards? I'm concerned that if the cells start at 0.18 (3% down) p02 then the calibration will be incorrect and will result in a potential decompression probelm. Does this make sense?
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25th March 2008, 18:50	#2 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,796	Re: Initial reading before calibration Quote: (Originally Posted by Divejunkie) Can someone help me with this please. When I switched the meg handsets on after a week off, in air they were both reading 0.18%. Even though I went outside to ventilate the head they remained at 0.18%. I continued with the calibration in oxygen. When I removed the O2 supply and the head ventilated, the handsets were reading 0.21 My question is to do with how the sensors are calibrated- I understand the electronics use a two point calibration system i.e. it needs to start the calibration in air (21%) and finish at the max o2 you have set the handset (99%). But what if the cells wont reach .21? Is the calibration of the 3 O2 cells similar to an O2E11 analyser in that they need to know that they are in 'air' at 20.9 to start with before they can give an accurate O2%? or- does it not matter what the initial O2 reading is because you flush with a known 100% (albeit the unit is set at 98%) O2 supply and the electronics basically assign 100% as the maximum and works backwards? I'm concerned that if the cells start at 0.18 (3% down) p02 then the calibration will be incorrect and will result in a potential decompression probelm. Does this make sense? first of all I think you need a refresher.. The handsets do not indicate percentage they indicate po2.. If you are at sealevel the percentage and teh po2 display would be nearly equal, but if you are at altitude there can be a big difference.. for example the po2 at ~900m above sealevel for air would be about 0.18 a cell that is only reading 1% low would only need to be at ~600m to give the same reading.. If you are above sea level please follow the proper calibration procedures. You do not want it reading .21 when it is not supposed to.. I chose 900m because All the territory in England that is over 3,000 feet above sea level is in Cumbria. with the highest altatidue being scafell pike at 3210ft/978m __________________ 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. Last edited by jradomski : 25th March 2008 at 18:54.
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25th March 2008, 21:17	#3 (permalink)
Divejunkie New Member Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic Join Date: Nov 2005 Location: Cumbria Posts: 29	Re: Initial reading before calibration Thanks for the geography lesson. You are correct it is PO2 not % O2. For some reason I had it in my head that it starts at 21% so it was therefor 0.21. Dont I feel foolish. I'm more or less ar sea level so I would expect it to be 0.21.
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26th March 2008, 00:26	#5 (permalink)
wedivebc FIGJAM Current Rebreather/s: Megalodon Other CCR Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Other CCR Join Date: Feb 2005 Location: Vancouver Island BC Canada Posts: 1,337	Re: Initial reading before calibration One thing I have noticed is the new software no longer prompts you before confirming air point calibration. I am not sure it was a good idea to change that. __________________ Cheers, Dave.... Man is the only animal burdened with the knowledge he will eventually die
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26th March 2008, 06:24	#6 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,796	Re: Initial reading before calibration Quote: (Originally Posted by Divejunkie) Thanks for the geography lesson. . I just wanted to show that I took extra care answering your question, not trying to be a smart ass... I had to look up Cumbria.. I knew nothing about it.... __________________ 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.
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26th March 2008, 15:20	#7 (permalink)
Divejunkie New Member Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic Join Date: Nov 2005 Location: Cumbria Posts: 29	Re: Initial reading before calibration Quote: I just wanted to show that I took extra care answering your question, not trying to be a smart ass... I had to look up Cumbria.. I knew nothing about it... Joe I know you weren't- email is so cold though isn't it. Gilles has basically confirmed what I what I wanted to hear in that it doesn't matter what PO2 the uncalibrated cells read in air or O2. What matters is the relative difference in mV between air and O2. I'm at sea level near as dammit and the unit is set on 0m. When I switch on if its outside the range 0.20-0.23 PO2 in air then I'll recalibrate. thanks for the responses.
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