Chapter Three: Overview S
stem Desi
n Elements
97
Ion Gauge High Pressure Protection/Disconnect Detection
A characteristic of either hot or cold cathode gauges is that they may provide invalid pressure
readings when the pressure is above 1 x 10
-2
Torr. In addition, hot cathode tubes may be
damaged at high pressures. To address these gauge limitations, the 146 unit has some high
pressure, and pressure rate-of-rise protection built into its hardware, and provides user-editable
protection parameters as well.
Ion Gauge Auto Power Control (Setup Mode Code 12x)
CC and HC
When the pressure reading of an ion gauge exceeds a user-defined pressure limit, the gauge is
powered off. When the pressure drops below the preset limit, the 146 unit restores power to the
ion gauge. If a hot cathode gauge had been degassing when it was powered off, the degas
function will
not
turn back on when the 146 unit restores power to the hot cathode gauge.
HC High Pressure Shutoff (Setup Mode Code 16x)
HC only
When the pressure reading of a hot cathode gauge exceeds a user-defined pressure limit, the
gauge is powered off. The gauge will
not
auto power on again even if the pressure drops below
the shutoff pressure. To power the gauge again, the pressure must be below the shutoff limit,
and
the HC channel must be turned off and then on again. The high pressure shutoff is always
active and it is intended to function as a fail-safe feature. The pressure limit should be set
higher than the auto power pressure threshold.
Ion Gauge Disconnect Threshold (Setup Mode Sensor Calibration screen)
CC and HC
The ion gauge disconnect threshold feature sounds an alarm and changes the gauge’s reading to
E0 when the channel’s pressure drops below the threshold pressure. Set the disconnect
threshold to a value one decade below the system base pressure for a reliable disconnect alarm.
You can also set the threshold pressure to the highest pressure reading the ion gauge can
reliably measure. The factory configuration has this feature disabled (disconnect threshold is
-1.0).
Features Built Into the 146 Hardware
Internal Fast Rate-of-Rise Shutoff (Setup Mode Code 16x)
HC only
If pressure rises rapidly to a level that potentially could harm a hot cathode’s filament, the
gauge is turned off and the E1 error code appears in the display. The exact pressure rate of
rise is dependent upon the type of gauge, but for most Bayard-Alpert tubes it is around 3
Torr/sec. To power the gauge again, the hot cathode channel must be turned off and then on
again. The factory configuration has this feature disabled. This shutoff feature is accessed
through the right hand entry of the Code 16x display. Press the ON button to enable this fast
pressure rate of rise shutoff feature; press the OFF button to disable it.
Internal Sensing of HC disconnect/tube shorts/open filament
HC only
If the 146 unit detects a disconnected hot cathode gauge, or a tube short or open filament in the
hot cathode, the gauge is immediately turned off. To restore power to the gauge, the HC
channel must be turned off and then on again.
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