I have increased the LLCV valve setting to 18% (from 15%) to try and compensate for changing conditions.
Factors that are changing are:
The supply pressure is falling as the liquid level in the storage dewar falls.
Heat leak to the liquid transfer line is increasing with outdoor temperature. This results in more vapor in the line and less liquid to the pump.
Radiation from the beam tube as the beam tube warms.
The attached plot shows 100 days of the control valve setting. The flat line at 15% is due to the failure of the differential pressure sensing- we have had to resort to manual setting of this valve.
For reference the liquid capacity of the pump reservoir is ~77 gallons when full.
I forgot about the heat into the pump due to radiation from the surrounding vacuum chamber in the mid station. Here is a plot of recent VEA temperatures at MID Y.
That is illuminating, thanks. We take feedback controls for granted around here; don't miss it till it's gone. Hope this sophisticated PIL* control algorithm will tide us over until we can regen & fix the level sensor.
*Primate In Loop