From about the beginning of 2017, the team at the SGF noticed some increased instability in our terrestrial range calibration values at the level of 2-3 mm. It was particularly clear on the first calibration of the day being a low value and so we learnt to take at least two calibrations before starting an observing duty.
2 weeks ago we also saw an increase in the RMS of the calibrations and some periodic behaviour in the calibration range values. We consequently took the kHz laser offline and began using our older 10Hz laser, which had very stable calibration values and RMS.
After some investigation and discussion with High-Q, the team found that the flow rate of the cooling water was much lower than it should have been. The water used is a reservoir of distilled water, but after 10 years of use something had built up to block the waterways.
Temperatures in the laser bed were not being kept at a stabilised constant, causing energy fluctuations and range errors from the start diode. The temperature would change when the laser began firing after being off for a period (20+ minutes). It would stabilise after approximately 10 minutes, during which time calibration range differences were seen.
Running the flow backwards and using a descaler (Durgol was recommended) brought the flowrate up to specification levels. Setting the best operating temperature removed the signals in the range values and now the calibration RMS and repeatability are now within acceptable levels.
Needless to say, we will be checking the flowrate more often from now on.