Instrutech The Hornet IGM402 User Manual page 80

Instruction Manual IGM402 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 IGM402.
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 IGM402 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.,
2.0V @ 2 to 2.5 A). Again, the values mentioned here are representative; the actual values you may see for the
IGM402 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 IGM402 control circuitry will sense the condition and force an OFF
condition.
The following table (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.
The values of filament voltage and current presented below are approximate values that one may see in the
Research screen. The representative, average values used in this table may vary depending on the condition of
the filament and the vacuum chamber environment. For example, you may see a reading of 2.0 V at 100 μA
emission current with a corresponding filament drive current of 2.1 A. This doesn't mean that because the
voltage is higher than the value listed (1.7 V) above that there is a problem with the IG. The readings taken
should be viewed with the notion that there may be an indication of reaching end-of-useful filament lifetime if
the power required to sustain emission current continues to increase over time.
InstruTech
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