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>Thats because you don't actually know what your mix is when you finish.
>I'll elaborate.
>
>Step 1 - figure your mix, get whatever percentage O2 for your dive.
>Step 2 - (for simplicity sake) drain air out of your tank.  (I KNOW you 
>         don't have too)
>Step 3 - Put x psi of O2 into tank.
>Step 4 - Top off with air until you hit x psi.
>
>Unless steps 3 and 4 are done very slowly, and the tanks are allowed to
>cool to the SAME temperature at each pressure measurement, you'll be off.
>Most dive shops I've seen don't do that.

Regardless of the analyzer used, if proper procedures are not followed you
will wind up with the wrong mix. While you're at it, you may as well bring
up the accuracy and precision of the pressure guages used in mixing.

>So, at a dive shop, your computed psi's won't mean as much since they
>usually don't take into account much in the way of temp adjustments to
>pressure, and they often fill at different rates from different banks, so
>the temp will be different for the air top off and the O2.

That's why I mix my own and follow proper procedures or stand there and
make sure whoever is doing the mixing does it properly.

>Now, you measure the O2.  The gases may not be mixed well.  I've seen a
>dive shop take a tank sit it on the floor, and roll it back and forth a
>few times and announce the mix is homogeneous.  Well, the CGA standards
>call for rolling at a specified (and not remembered) rpm for a period of
>time which I believe is an hour.  A couple of kicks won't do it. (I know
>that there is argument about if this is necessary - don't bust my chops on
>this, thats what the book says is needed for a mixture to become
>homogeneous)

Agreed, but that is why you should analyze your mix prior to use. Of course
if you followed all these guidelines, a three day trip to the Doria would
take six days...to allow for homogenous mixing.

>So, now you calibrate your O2 sensor at 21%.  You do your
>mix.  What do you know?
>Not much.

I think I know that maybe I'm close enough to the required mix to probably
get me out of the water in one sound piece, just like on all those other
dives where I have used those improperly analyzed life critical mixes.

>You know that your fg's are dependenant on the accuracy of your PSI
>measurement, which you know is off.  But you don't know by how much.
>You know that your O2 sensor will be off somewhere from 2-8% w/ a
>calibration of 21%. (which should be 20.9%).  Does the PSI off cancel the
>O2 off?  Are the errors additive?  Did they cancel?  Point is, you don't
>know.  In order to know, you *HAVE* to calibrate the sensor with a gas
>that is within 4% of the target value, or you *DON'T* know.  You're
>guessing.

Yeah, but its not a WAG, its an edumacated guess that has allowed me to do
many dives over the years while only sustaining maximal dain bramage.-yonT

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