990 CLD Autoline™ Leak Detector
4-4
DRAFT 2/2/04
As-Required Maintenance
During normal operation of the leak detector, contaminants build up in the spectrometer
tube, and eventually require it to be disassembled and thoroughly cleaned. While it is
difficult to predict how long a spectrometer tube will operate satisfactorily between
disassembly and cleaning, it is possible to recognize the signs of a tube approaching the
need for maintenance. Careful tracking of the tuning parameters is often used to predict
preventive maintenance intervals.
During operation, organics from the parts being tested and the operating environment build
up within the ion chamber of the spectrometer tube. The contaminants typically take the
form of non-conductive organic deposits, usually dark brown in color, and are primarily
deposited on the ion source, ground slit plate, and within the ion beam bending chamber in
the tube. The level and location of these deposits result in changes to the tuning parameters
as the deposits build up. Since deposits build up on all three areas of the spectrometer tube
simultaneously, the effects described below typically occur together. In addition, these
deposits usually increase the apparent background signal in the machine since they can trap
helium within their layers over time. In extreme cases, the result is not being able to zero
out the background, a condition often referred to as
running out of zero
.
Deposits on the ion source cause the ion beam exiting from the ion source to widen,
de-focus, and vary in exit velocity and a loss of sensitivity in the leak detector. This
shows up as requiring an increased amount of CAL gain to calibrate the leak detector.
Maintenance is required when the CAL pot cannot be increased clockwise far enough to
allow the machine to read the calibrated leak correctly. This condition is often referred to as
running out of electronic gain
. This loss of sensitivity is often accompanied by an increase
in instability, particularly at the most sensitive range of the instrument.
Deposits on the ground slit plate cause shifts in the FOCUS adjustment. While cleaning or
replacement of the ground slit plate sometimes partially remedies the problem, it is best to
disassemble and clean the entire spectrometer tube when this condition is encountered.
Deposits in the ion beam bending chamber cause shifts in the ion voltage required to tune
to helium. Normally, the instrument tunes with the ION dial set to about 4, indicating an ion
voltage of about 250 V. As these deposits build up, the ION voltage must continually be
increased to maintain tuning to helium. Correction of this condition requires disassembly
and cleaning of the entire spectrometer tube.
Continually lowering ion voltages, until a peak cannot be obtained, is an indication of
weakening magnets and that they need replacement. Contact the factory (1-800-8VARIAN,
or 1-800-882-7426) to arrange for a spectrometer tube advance exchange.
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