> > > > 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 ....

I suspect that while this is *possible* it is impractical outside a nicely
shielded lab.  The earth's magnetic field strength would likely cause too
low a signal strength for picking anything up outside a good shielding room.

Incidentally, I took my cell phone inside a 15MHz shielding room, and tried
it out.  Clear signal.  Clearer, maybe, than outside the shielding room.

> > 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.

I know, I forgot the difference in resolution when I posted the original
message, but I corrected myself in the second.

> 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? :-) ).

As I said, a .04T magnet, with a small transmitter and reciever coil would
likely do the job.

> 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".

--Brendan

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