I have examined the trending of Pedestals and MIP peaks to check for systematic changes in pmt readouts for pedestals or MIPs. Both calibrations have remained remained fairly stable over time, indicating very little degrading in either the tiles or the circuitry of the ACD. The stability of the calibrations also gives a reasonable idea of how long a particular calibration can be used before being replaced.
Method
I found MIP peak and Pedestal values once every four weeks from week 28 to week 108 (21 data points). I plotted this data against against time units of 10,000 hours. I then fit a straight line to the data. The slope of this line is the rate of change of the calibration per 1.14 years, so by dividing by 1.14 we find the change per year
MIP peaks
The MIP peak values exhibit no systematic drift. The plots below show a histogram of the slope of the trend line (in units of MIP's per year).