Agilent Technologies XGS-600, Instruction Manual

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XGS-600 Gauge Controller
36
DRAFT 4/
15/14
HFIG Smart Overpressure Protection
Hot filament ion gauges all exhibit non-linearity at high pressure such that they give
readings lower than the actual pressure. For gauges that are rated up to 10 mTorr or higher,
operating the BA gauge at low emission current can minimize this measurement error.
Unfortunately, many applications require operating the gauge at emission currents that
would result in poor measurement accuracy at high pressure, thereby compromising the
ability to provide effective overpressure protection.
XGS-600 adjusts the overpressure limit based upon the emission current. When the
emission current is 0.7ma or less, the gauge's full high pressure range can be used. If,
however, the emission current is set to higher values, XGS-600 protects the gauge by turning
off the filament at 1 mTorr. Overpressure Protection uses the same fast pressure signal that is
used for fast set points, and is displayed by the
P>MAX error message.
Setting the Auto-ON Pressure Level
When enabling the Auto-ON feature, the XGS-600 defaults the threshold pressure to
5 mTorr. If the convection gauge is installed so that it is measuring the same high vacuum
chamber as the ion gauge, this is a good value to use. However, in many applications users
may want to monitor the high vacuum pump's fore-line pressure instead. In this application,
the foreline may never get down as low as 5 mTorr, especially if a large volume of gas is
being pumped. The XGS-600 allows you to program a threshold pressure as high as 1 Torr.
When Auto-ON is being used, the gauge overprotection feature is still active to protect the
high vacuum gauge.
When the Analog Board is installed, any of the active type gauges, or CDGs, can be used to
turn on/off an ion gauge.
Helium Measurement Capability
Helium has been added to Air and Argon selections in Gas Correction in the Gauge Setup
screen for roughing gauges. This selection allows the controller to read correctly when the
predominant gas is helium. However, it is important to recognize the performance
limitations of heat loss gauges when used with helium. These are:
❑
Poor performance above 200 Torr - The signal to noise ratio at this high pressure
range is very small, therefore stability, resolution, and accuracy are much worse than
a direct pressure gauge such as a CDG, silicon diaphragm gauge, or even a bourdoin
tube. Repeatability and drift is typically several hundred Torr. Performance below
200 Torr is about the same as when measuring in air.
❑
The maximum cable length is about half that when measuring air or argon. The
maximum recommended cable length using standard cables is 150’.
❑
The maximum number of rough gauge channels is reduced. Every gauge channel
setup for helium is equivalent to two channels when set for air or argon. Therefore,
the maximum number of convection cards, if all are set to helium, is three (six
channels).