208
5 Vacuum System
Gauge controller
Gauge controller
The optional HP 59864B Gauge Controller allows you to use the triode gauge tube
to monitor the pressure in the MSD vacuum manifold. This can aid in everyday
operation and in troubleshooting.
The HP 59864B Gauge Controller includes the controller and a cable for connect-
ing the controller to the triode gauge. A power cord is supplied with a plug appro-
priate for the country from which the order was placed. The gauge controller can
operate on all voltages between 100 and 240 V ac (nominal) and at ac frequencies
of 50 to 60 hertz. The fuse in the gauge controller is appropriate for all allowed
voltages.
The gauge controller regulates emission current to the filament of the triode gauge
tube. It also measures the ion current in the collector. From these data, the gauge
controller calculates and displays the pressure present in the vacuum manifold.
The vacuum manifold pressure (in Torr) is displayed on the front panel of the con-
troller.
The gauge controller is calibrated for nitrogen (N
2
). The carrier gas is usually
helium, which has does not ionize as readily as nitrogen. Therefore, the indi-
cated pressure for helium is approximately 6 times lower than the absolute pres-
sure. For example, a reading of 2.0 × 10
-5
Torr versus an absolute pressure of 1.2 ×
10
-4
Torr. The distinction between indicated and absolute pressure is not impor-
tant for normal operation of the MSD. Of greater concern are changes in pressure
from hour to hour or day to day. These changes can indicate air leaks or other
problems with the vacuum system. All the pressures listed in this manual are indi-
cated pressures for helium carrier gas. The gauge controller setpoints are also
indicated pressures.
See Also To monitor high vacuum pressure, page 44
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