Selecting Emission Current For Hot Filament Ion Gauges - Varian XGS-600 Instruction Manual

GAS CORRECTION
SYSTEM SETUP
EXIT

Selecting Emission Current for Hot Filament Ion Gauges

When the gauge model number is selected, the XGS-600 automatically selects a default
emission current value that works for most applications. However there are reasons why
you may want to select another value based upon your application.
❑
If you are using a gauge at pressures higher than 1.0E-4 Torr, the gauge performs
better and lasts longer at low values of emission current. In general, for this pressure
range, use 0.1ma.
❑
If you are using the gauge in the UHV range, the default emission setting may not
result in a large enough ion current to measure reliably. In this case, increase the
emission current to the maximum setting of 10 ma. However, there will be more
out-gassing which could adversely affect the system pressure. So, a compromise
between the emission current setting and filament out-gassing must be made. Try to
use the lowest emission current setting that provides good measurement results.
❑
If you have had your gauge calibrated, it would have been done at a specific
emission current. You should enter that value into the emission current setting of the
XGS-600.
❑
The XGS-600 employs a feature that automatically changes the emission current
based on the pressure that is measured. If the emission current is set to values greater
than or equal to 1.1 ma, the controller applies the full value when the measured
pressure is below 1.0E-5 Torr, and 1/10
that level. There is an algorithm to minimize hunting. However, depending upon the
vacuum system, it may still be possible for the gauge to hunt for the proper emission
current setting when at or near 1E-5T. This is caused by the fact that when the
emission current is raised as the pressure goes down, the filament outgases at a
higher rate, causing the pressure to increase a bit. Sometimes it is possible for the
pressure to increase enough that the controller reduces the emission current, which
eliminates the source of gas. This causes the pressure to decrease to where the
controller again raises the current, and the system can get stuck in this cycle. You can
defeat this by setting the emission current to 1.0 ma or less, so the XGS-600
maintains that constant emission current at all pressures.
3-12
XGS-600 Gauge Controller
NOTE
Changing the Sensor Model, defaults Sensitivity and Emission
Current settings.
Use this field to enter a correction for various gas types. For an ion
gauge (HFIG and IMG), enter a nominal value from the Gas
Correction Factor Table in Appendix C. For Convection gauges,
select Air or Argon.
Select to open the System Setup screen (page 3-14).
Select to return to the Main Screen (page 3-3).
th
of that value when the pressure is above