> > > I am looking for a reliable way to detect water,
> > > although the sensor is covered with a layer of dirt,
> > > oil film, etc.  And the water may be pure or contaminated.
> >
> > Nuclear Magnetic Resonance perhaps?
>
> Have you worked with NMRI?

No. But I was introduced to the concept in 1973 (or 1972?) when it was an
idea in its infancy with no major applications. I still have my original
university notbook showing a coil around a bottle of water and using the
earth's magnetic field! If I had played with that idea then ....

> Cool idea, but it doesn't seem practical.  It's possible that for simply
> detecting the presence of some water it would be far simpler than actual
> NMRI, but I think the field strength wouldl be a concern.

The resolution is dependant on field strength and to get the 1mm odd
resolution of medical MRI you need the field strengths concerned. For
grossly low resolutions (ie the water sample is or isn't there) far lower
field strengths would suffice.

Also the medical units need to produce a uniform (or well controlled) field
over a very large physical distance by normal measures. When your sample can
be all but in contact with your magnet and coils the field strength
requirement is lower AND/OR the magnetics required to produce the field is
smaller. I suspect that a physically small rare earth magnet with the sample
essentially in contact is liable to be in the order of right. (Is that
enough "maybe" words? :-) ).

I have an idea for a simple sensor (not for water) based on this principle
which I hope to get around to trying "one of these days".



                    Russell McMahon

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