I've been using a pair of the Mopeka sensors for a few months so far and have been quite satisfied with the performance. The key to consistent readings seems to be making sure there's a solid contact with the cylinder bottom by putting a dab of dielectric grease or petroleum gel (Vaseline) on the sensor pad before attaching it. Both of my sensors came with a small tube of grease for that purpose. The sensors come with a set of small aluminum spacers to hold the cylinder high enough off the ground to clear the thin sensor unit, but I found their optional "halo" spacer to be much easier to use.
