testing (measurements #1 and 2), but significant drift was measured after each JPL thermal test  (measurements #3 - 6). Discussion with the original cognizant engineer responsible for measurements #3  and 4 revealed that the ohmmeter was never zeroed, which would have put a constant offset error on all  values measured. If that offset had been approximately -170 Ohms, there would be a uniform downward  trend across nearly all measurements. I became concerned that if this trend continued, the daily thermal  cycles experienced by the Rovers might cause the total resistance of the pots to drift out of spec.  0 1 2 3 4 5 6 7  Measurement Number  Figure 8. Total Resistance measurement history  When notified of this observation the pot manufacturer said that it was typical for the total resistance of a  conductive plastic pot to change after thermal cycling. The mechanism was explained as follows:  conductive plastic is made of carbon particles in an insulating matrix, co-cured at 160" C (and rated for  operation up to 125" C). Thermal cycles change the insulating matrix on a microscopic level, allowing  better conductivity between the carbon particles. Macroscopically these changes appear as a drop in the  total resistance. The manufacturer said that as the number of thermal cycles builds, the rate at which the  resistance drops will decrease and the resistance will eventually stabilize. The scratches and drops of  silver paint used to linearize the pot have an effect on the rate at which the resistance stabilizes. The  manufacturer assured me that the resistance changes would be uniform throughout the length of the  resistance element and that the pot's linearity would not be affected by changes in the total resistance.  Interestingly enough, the pot manufacturer also said that just leaving the pot on a shelf for a time after  thermal cycling will cause the pot to recover some, but not all, of the resistance loss which occurs during  thermal cycling. They called this a "memory" effect.  LESSON 13. To gain insight, plot every measured quantity and look for trends. If a measurement is  worth recording, it is worth plotting.  LESSON 14. Calibrate test equipment before using it on flight hardware. The data is only as good as  the calibrations; i.e., always "zero" an ohmmeter before measuring any resistance.  LESSON 15. Manufacturing considerations and physical mechanisms are the reason a precision pot  has the relatively loose tolerance of * 20% on its total resistance.  Add Vent Holes. Then Glue Them Shut  MIL-PRF-39023-style pot cups trap air so the MER Environmental Requirements Engineer requested a  venting analysis. I did some digging and found that during a design review early in the procurement  process, the original cognizant engineer had requested that a vent hole be added, so the manufacturer  drilled one in the cup between the set screws that hold the lid on. Unfortunately, the original cognizant  engineer had also requested that a Solithane mixture be applied to the set screws to lock them in place.  When this Solithane mixture wicked into the circumferential groove and effectively bonded the lids to the  cups, it also blocked the vent path leading to the drilled vent hole. The only vent path left was the one  through the shaft bearing which is undesirable because vented air often carries contamination that can  become lodged in a bearing.  312