Agilent Technologies 1200 Series Reference Manual page 75

6
Theory of Operation
The amplifier and comparator circuit determines the working range of the
vacuum that has to be built up. When the vacuum degasser is turned on and
the vacuum in the chamber is not within working range (above error limit of
190 Torr), the amplifier and comparator circuit sends a signal to the vacuum
pump driver and the timers of the vacuum pump (timer 1) and the solenoid
valve (timer 2).
The vacuum pump is turned on immediately while the solenoid valve closing is
delayed by 15 seconds. This time delay allows the vacuum pump to start
without load before it is connected to the vacuum chamber. The status
indicator turns yellow when the pump is activated. The status lamp is turned
off as soon as the vacuum is below the error limit.
When the vacuum in the chamber reaches its operation mode 1
(approximately 115 Torr) the amplifier and comparator circuit turns off the
solenoid valve and the vacuum pump is turned off by a timer with a delay of
15 seconds.
As soon as the pressure sensor detects that the limit of the operation mode 1
has been exceeded (e.g. when dissolved gas from the solvent diffused into the
vacuum chamber) the vacuum pump is started again as described before.
The pressure signal is available at the auxiliary output. It allows to monitor
the vacuum system. The upper limit of operation mode 1 is 600 mV. Values
below 600 mV on the pressure output indicate sufficient vacuum in the
chamber. If the 600 mV are exceeded the vacuum pump will be started to keep
the vacuum with in its working limit.
The amplifier and comparator circuit also activates the timer 3 when the
vacuum in the vacuum chambers is below operation mode 1. The timer 3 is
reset when operation mode 1 is reached within a maximum of 8 minutes. If
operation mode 1 is not reached and the time (8 minutes) of timer 3 elapsed,
the timer mode (operation mode 2) is activated. In this mode the vacuum
pump is automatically turned on every 2 minutes for a time frame of 30
seconds. Timers 1 and 2 are activated as described earlier.
The error monitor continuously checks the error limit of the degasser
(190 Torr). If the error limit is exceeded (for example, leak in chamber), the
error timer is activated and the yellow status indicator lamp is turned on. The
vacuum pump is turned on continuously. If the vacuum pump cannot reach
either of its two operation modes within 8 minutes (error timer limit) the
error driver is activated. The error driver will deactivate the vacuum pump
and solenoid valve. The status lamp turns red and the error output on the
remote connector will be activated.
1200 Series VD Reference Manual 75