> >Differential measurement is NOT an option for this
> >application.
> 
> Hmm.. Why not?
> Push some air down a line that goes to the bottom, and it will max
> out when you start bubbling.
> Pout - Pin and you're there I think.

The system in question (and I apologize for not being more specific
earlier) is a long-term in situ datalogger.  It would be unattended for
a minimum of a few days at a time, up to several weeks.  So the
solution needs to be very low power (which can be accomplished
by turning the sensor on and off as needed), small, and simple.

Differential would require a knowledge of the minimum and maximum
water levels, one "end" of the sensor could remain under water and
the other would be guaranteed to stay out of the water.  We've
considered a tube-and-float approach, or a post-with-tube method,
but they have their drawbacks (float snags on a passing log and
tears off; post must be placed, and tall, which detracts from the
convenience of the system).

It may be that it is impossible to do this without differential
measurement and still meet accuracy requirements.  In that case,
the best be may be an additional pressure only logger which sits
well above the high water line and monitors the ambient air
pressure.  That's good, too, because air pressure affects the 
dissolved oxygen content of the water, which is another thing
we measure.

Mike H.

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