Try attaching a small rare earth magnet to the back of the (UGN 3503U) to
create a field.  Now when the sprocket tooth passes nearby you'll be
measuring a much more significant effect.  You can get fancy by making a
steel "U" so the tooth acts like a pole piece.

That said, the gap between the tooth and the Hall detector still has to be
small (an unscientific wild a** guess is no more than 0.1") and the magnet
will pick up bits of metal as the gears wear over time.  The grease,
however, should be no problem except as a chemical insult to whatever
potting material you encapsulate the detector in.

BTW I seem to recall that there are quite a few Hall detector assemblies
(and proximity detectors) already sealed in convenient stainless steel
studs.  Alas, I don't have any current part numbers to offer you because
we've been "rolling our own" on contact lens lathes for years now.

Cheers,

Win Wiencke

> I have a need to know the position of a shaft, within one degree accuracy,
> that rotates at a speed of one revolution in two minutes. The shaft can
only
> make one turn.
>
> I have been experimenting with a Hall Effect device (UGN 3503U) trying to
> count the teeth on the sprocket that turns the shaft. Although promising,
I
> only get a one millivolt difference between the tooth and the gap and the
> Hall Effect device has to almost touch the sprocket.<snip>

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