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Andrew

If an instrument reads +/- 2% at full scale then this is the accuracy at   
this end point of the scale.  If you read at an intermediate point, one   
should expect the accuracy to be better than this.  In the case of a   
galvanic cell, we are talking about the linearity of the cell's   
performance and the point of calibration.

To make my point, let's assume that we are doing a 21/28 mix.  I would   
calibrate the instrument in air and would expect a high degree of   
accuracy on the analysis i.e. significantly less than 2%.  With my   
analyser, I get repeatability of readings over several days close to the   
display precision of +/- 0.1% for Trimixes.  Due to the calibration being   
close to the target reading, there will be a high degree of accuracy in   
this reading.

The key thing is to calibrate the cell to an O2 content as close to the   
gas you are trying to analyse.

Dave
shimell@se*.co*

 ----------
From:  owner-techdiver[SMTP:owner-techdiver@aquanaut.com]
Sent:  14 November 1997 12:00
To:  Andrew Drapp
Cc:  Tech Diver
Subject:  Re: Fwd: Re: trimix web page


On Fri, 14 Nov 1997, Andrew Drapp wrote:

> With a +/-2% level of accuracy, the measured value will be within 2% of
> the value, not 4%.  Say you were measuring the F02 (OK, actually PP02)   
of
> a mix with 10% O2.  Exactly 10% O2.  The reading would be somewhere
> between 9.8% and 10.2% (+/-2% of the measured value.) How presice do   
you
> need your FO2 measurements?

No, its +/-2% of full scale (100%), not the reading, so for
10%, it could read from 8% to 12%.   And since I frequently
mix in the 8%-15% O2 range, I'd like it to be more precise.

 -Will
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