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The gain of the PSPC describes the relationship between the energy of the detected X-ray photon and the pulse height produced in the electronics (pulse-shaping, peak-detector, ADC, etc.) connected to the anode (Fig. 3.1). For a perfect PSPC, the relationship is determined only by the electrical potential between the cathode and the anode and by the counter gas composition and density. In reality, there are spatial variations across the face of the detector as well as temporal variations.
The ROSAT PSPC gain has had two significant changes.
First, the gain was lowered by 
30% on 1991 October 11,
when the high voltage was changed from 3060 to 3000V
(one of several measures taken to reduce some detector
hot spot problems
and low pulse height contamination).
Second, the gas flow rate was changed on 1993 June 10,
at which point the mean of the Al K 
line
dropped from channel 103 to 100.