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Tim,

I've been looking over the posts this last week on O2 sensors, and several
thoughts come to mind:

 - It is fairly easy to build an oxygen meter which allows you to adjust both
offset and scale.  I've built one myself.  I don't know why oxygen meters are
sold without both adjustments.  The calibration proceedure is more complicated,
but lab technicians manage it every day with such things as pH meters.  Any
technical diver should have no difficulties.

 - Even if your oxygen meter has an offset error, you can still get accurate
measurements with it.  As long as the sensor itself is linear.  Here's how:
          Start with two calibration gasses, for example air and 40%.
          Take pO2,air = Pair = 20.8
          Take pO2,40 = P40 = 40.0
          Take the meter measurement for air.  Call it Mair.
          Take the meter measurement for 40% O2.  Call it M40.
          Take the meter measurement for the gas you wish to check.  Call it
Munknown.
          Calculate the desired result:  Punknown = Pair + (P40 - Pair) *
(Munknown - Mair) / (M40 - Mair)
   This technique will adjust for both offset and scale errors.

 - The above technique may be more trouble than it is worth, even aside from
possible human error.  If your O2 meter is within 0.5%, the O2 error is about
the same as the error in a typical electronic depth guage (1-2%).  It is also
about the same as the accuracy of your N2 loading calculated by a dive computer.
The error is probably at least one order of magnitude smaller than the
variations in individual susceptability to DCS or O2 toxicity.

 - If you get one of the meters David Drake was talking about, you should be
careful about storing and using the sensor.  Apparently the meter is adjusted
for the sensor's offset when the sensor is equilibrated with air.  If you store
the sensor in inert gas for a long time, it will have finished off-gassing O2,
and its offset will have changed.  Take the sensor out of storage some time
before using it.  This may not be a problem since it should probably come up to
temperature anyway.

[David:  Do you know if this is correct?  If so, how long does it take to reach
equilibrium?  I am assuming that the offset error is mostly due to O2 absorption
and release by the surrounding material.]

Paul Ostby

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