This can happen when you try to turn on the triode gauge or when the triode gauge is already on. It indicates the pressure in the vacuum manifold is above 8 x 10^-3 Torr. The triode gauge will not turn on if the pressure is above that level. It automatically turns off if the pressure rises above 8 x 10^-3 Torr.
Make sure the line voltage is within the acceptable range for the gauge controller.
WARNING! Only a qualified electrician should check the line voltage.
This may happen routinely if you are doing on-column injections. The solvent peak raises manifold pressure above 8 x 10^-3 Torr. If this is a recurring problem, it may be best to leave the triode gauge off.
If you have recently turned on the MSD, allow the system more time to pump down. If, after 20 minutes, the triode gauge will not turn on, check other causes.
Make sure the cable is securely connected to the triode gauge and the gauge controller.
WARNING! Do not connect or disconnect the cable from the triode gauge while the gauge controller is on or while the MSD in under vacuum.
If other possible causes are ruled out, try replacing the cable.
If its fluid level is low, the diffusion pump will not work effectively. Check the level of the fluid in the diffusion pump. If necessary, replace the diffusion pump fluid.
See the MSD maintenance documentation for instructions for checking and replacing diffusion pump fluid.
Turn off the gauge controller.
Vent the MSD.
Disconnect the cable from the triode gauge tube.
Remove and inspect the tube. If the filament is broken, replace the tube.
See the MSD maintenance documentation for instructions for replacing the triode gauge tube.
Make sure the line voltage is within the acceptable range for the gauge controller.
WARNING! Only a qualified electrician should check the line voltage.