Below is a more or less final version of my plot of GFPC anode bias versus atmospheric pressure:

In contrast to previous versions of the plot, I’ve now contoured it (by hand/eye) for dew point temperature, and I’ve also color-coded the data. The curves that I’ve drawn on the plot are given by the following equations, which work well for interpolation:
Channel 1: bias [V] = 7 V/kPa *
Patm [kPa] + 0.52 V/°C *
Tdew [°C] + 909.4 V
Channel 4: bias [V] = 7 V/kPa *
Patm [kPa] + 0.76 V/°C *
Tdew [°C] + 929.7 V
Some unexplained scatter is still present on the plot, and so additional variables are likely significant in addition to atmospheric pressure and dew point temperature. For instance, I don’t really have a good handle on time required for equilibration, which obviously would be particularly important when atmospheric conditions are changing rapidly.
When determining the appropriate anode bias, I should note that I used the JEOL “base level” scan rather than the “high voltage” scan and then adjusted the bias in 2 V increments until I got a distribution centered at/near 4 V. I made the final scan using a step of 0.1 V and dwell time of 1 s; generally I repeated the slow scan at least once in order to assure reproducibility. This is a tedious process, but it produces better results than the “high voltage” scan, which tends to have a broad, gently sloping “peak.”

So I guess now I need to do something about this problem. I’m reluctant to add liquid to the bubbler at the exhaust, as I worry about backstreaming in the event of counter window failure.