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5
Triode gauge tube
Triode gauge tube
The MSD is equipped with a triode gauge tube connected to the vacuum manifold.
With the optional HP 59864B Gauge Controller, the triode gauge can be used to
measure the pressure (high vacuum) in the vacuum manifold. The triode gauge
will not operate at pressures above 8 × 10
without the gauge controller.
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The triode gauge relies on the ionization of gas molecules to establish a pressure-
dependent current flow. In the triode gauge, a regulated electrical current is
passed through a filament called the cathode, causing it to emit electrons. The
electrons accelerate from the filament toward a surrounding grid which is held at a
higher potential (+150 V dc).
The emitted electrons ionize gas molecules in the tube. Positive ions are driven to
a grounded wire mesh collector. At the collector, the positive ions regain missing
electrons. This generates current in the collector. The number of ions formed is a
function of the number of gas molecules present, that is, the gas pressure. There-
fore, pressure can be calculated based on the current applied to the filament
(cathode) and the current measured in the collector.
Since one end of the triode gauge tube is open to the vacuum manifold, the pres-
sure in the triode gauge is essentially the same as the pressure in the vacuum man-
ifold. To prevent electronic noise from the triode gauge tube from interfering with
the detector, a small z-fold baffle is inserted into the stem of the triode gauge tube,
and a shield is installed between the tube port and the ion source.
Unlike some other pressure gauges that work by ionization, the triode gauge does
not require degassing to remove accumulated ions from the surfaces in the gauge.
In some cases, however, new gauge tubes will not display pressures accurately
until they have been turned on for several hours.
206
Vacuum System
-3
Torr. The triode gauge cannot be used
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