Instrutech Hornet User Manual page 41

Instruction Manual IGM401 Hornet
The Pressure, Ion Current (Collector Current) and Emission Current are indications that the ionization gauge
transducer 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
-1
parameters leads to the calculation of pressure given by P = I • (I • S) where I is the Ion Current, I is the
C E C E
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 calculating the pressure as indicated
by the IGM401.
Other important information such as Filament Voltage and Filament Current are also given in the R&D display
screen. These values vary widely dependent 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 current. The interpretation of these displayed
values is a qualitative measure. The values will change over time dependent upon the pressure, filament coating
condition 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 IGM401 may operate at 100 μA emission current with only 3 to 4 watts of power required (e.g., 1.5 V @
2.0 A). By increasing the emission current to 4 mA, the power requirement may increase to 4 or 5 watts (e.g.,
2.0 V @ 2 to 2.5 A). Again, the values mentioned here are representative; the actual values you may see for the
IGM401 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, perhaps, give you an indication of when to
schedule replacement of either the device or the filament assembly.
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 permissible pressure for the emission current setting you have
chosen, you will be able to monitor the values displayed in the R&D 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 IGM401 control circuitry will sense the condition and force an OFF
condition.
The R & D (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 transducer 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 R & D (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.
InstruTech, Inc.
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