tinb29e1-c (2019-10) VGC083A_B.om 83
From the collector current measured, which is directly proportional to the gas den-
sity inside the ion gauge head enclosure, the pressure inside the ion gauge head
can be calculated. The Research display screen is a very useful diagnostic tool to
troubleshoot issues with the sensor or the electronics.
This mode displays the emission current, ion current, filament voltage and filament
current. In the following emission current is 4.00E-3 amperes, Ion current is
4.00E-11 amperes, filament voltage is 4.3 V (dc) and filament current is 4.2 am-
peres.
Research Display Mode of Critical IG Hot-Cathode Ion Gauge Measurement
Parameters
The Pressure, Ion Current (Collector Current) and Emission Current are indications
that the ionization gauge head is operating and pressure (vacuum) is being
measured. In the above example, most of the critical, measured quantities required
for the calculation of measured pressure are displayed. The relationship of these
parameters leads to the calculation of pressure given by P = I
C
× (I
E
× S)
-1
where
I
C
is the Ion Current, I
E
is the Emission Current and S is the Sensitivity of the
specific ion gauge sensor.
The pressure indication may be checked by using the displayed values and calcu-
lating the pressure as indicated by the top LED display line on the VGC083.
Other important information such as Filament Voltage and Filament Current are
also given in the Research display screen. These values vary widely depending
upon filament design type, material used for constructing the filament, filament
condition and operating emission current. The product of the filament voltage and
current is the power dissipation (V×A) required to maintain the set emission cur-
rent. The interpretation of these displayed values is a qualitative measure. The
values will change over time dependent upon the pressure, filament coating condi-
tion and possibly even surface properties of the grid electrode. It is a good practice
to make note of these values at initial installation and from time-to-time during the
operating lifetime of the ion gauge sensor.
A new ion gauge may operate at 100 μA emission current with only 3.5 A of fila-
ment current. By increasing the emission current to 10 mA, the filament current
requirement may increase to 4.5 A. Again, the values mentioned here are repre-
sentative; the actual values you may see for the ion gauge you are using will be
different and will vary over time and from device to device. In general, keeping a
record of the filament heating power (given by the Filament Voltage and Filament
Current readings) will aid you in monitoring the condition of your gauge filaments
over time and give you an indication of when to schedule replacement of either the
device or the filament assembly. Maximum filament current is 6.5 A.
In cases where the ion gauge sensor does not turn ON or does not stay in the ON
condition after you have made sure that the pressure is below the maximum per-
missible pressure for the emission current setting you have chosen, you will be
able to monitor the values displayed in the RESEARCH screen and determine if
the turn ON / stay ON trouble is related to filament condition. There are maximum
operating levels for filament voltage and current; if either of these is exceeded, the
VGC083 control circuitry will sense the condition and force an OFF condition.
The following table (Research - Ion Gauge Diagnostic Display) is intended to assist
you in using the information provided in this display as a means to ascertain if the
ion gauge head is performing as expected under your vacuum chamber operating
conditions. There are various causes of trouble that at times may make it difficult to
use a single measurement parameter as an indicator of what the problem is. In
general, if you have monitored and recorded the values displayed in the Research
Screen over time, you may be able to detect a pattern or trend that, if correlated to
a particular failure mode, may prove to be a valuable indication of probable cause.
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