I have been using the following method to troubleshoot puck discs (sensors). There are two different styles: ‘old,’ with a slot where the connector sits and blind holes on the top disc, and ‘new,’ with a pocket where the connector sits and through holes on the top disc.
Starting from assembly, I have generally been using 5/16″ screws on the new style and 1/4″ screws on the old style. If I am screwing into the old style puck, I place my finger over the other end of the screw hole on the top and screw the discs together until I can start to feel (with my finger) the screw start to breech the surface of the top disc. This should put the tension on the sensor in a good range to test. Assembling the new style pucks is a bit easier: you can visually confirm that the screws are tight enough because of the through holes. I screw in until the screws are about flush with the top disc, and then back off a couple turns.
I then try to ensure that each screw is screwed in an equal amount: you can look through the side of the puck, between the discs, and you should notice that as you rock the disc around the sensor from all directions, the two pieces of rubber should stay in contact. If you notice that the pieces of rubber hinge on each other, you probably need to tighten the screw in that direction.
Then I test each puck: connect the sensor to an ohm meter. First ensure that the sensor connector is reliable by holding the puck down and applying force with your hand. Wiggle and rotate the connector to see if it sticks at infinite resistance in any position or other odd behavior. Note that the sensor may naturally fluctuate to open circuit momentarily (this may be a behavior of the meters, not the sensor), but if the connector is faulty it should be quite obvious. The criteria for a good puck (under the current design) that Geb was using was that the resistance should be greater than 6KOhm resting (both with and without a full bottle) and less than 4KOhm under the gentlest press that dispenses lotion. You can increase the sensitivity (if the resistance is too high) by increasing screw tension. If you must increase screw tension so much that the screws bulge out the top half of the disc, you should decrease screw length. However, if you use the screw lengths for the type of pucks as I’ve indicated above you should be okay. I’ll note that I haven’t taken data and determined whether increasing ‘sensitivity’ by increasing screw tension changes the sensor response in a linear or exponential manner, or simply offsets it. This question should not be important to us now (as long as the puck meets OK criteria), but may become important in the future as different triggering mechanisms are implemented.