990 CLD Autoline™ Leak Detector
DR
AFT 2/2/04
3-7
Calibrating the Leak Detector
Calibration the detector is normally performed after tuning the leak detector to helium. If
calibration is attempted before verifying tuning, the calibration procedure may not be
successful. The Minimum Detectable Leak (MDL) is a function of pumping speed, vacuum
system architecture, and electrical noise level. MDL, therefore, varies for each application.
The following calibration procedure assumes that a calibrated leak of appropriate size is
used.
Because of the variability of detector installations, it is difficult to provide an exact
calibration of the gain (and possibly the emission current). In some cases, the normal
peaking and calibration procedure below is all that is needed to achieve calibration. In
approximately one-third of installations. however, the helium signal is too strong even for
minimum gain. In those cases, move to the procedure that follows.
To calibrate the detector:
1. Verify that the detector has been tuned to helium as described in
“Tuning the Spectrometer Tube Using a Helium Reservoir Leak” on page 3-6.
2. Place the calibrated leak in the vacuum system as close as possible to the test chamber.
3. Open the valve on the calibrated leak.
4. Start and run the leak detector.
5. Set the front panel RANGE pushbutton so that the exponent in the LEAK RATE display
matches the exponent of the calibrated leak.
6. Close the valve on the calibrated leak and look for a decrease in the displayed leak rate.
7. If required:
❑
Carefully rotate the ZERO adjustment to obtain a zero reading on the LEAK RATE
display.
❑
Move the COARSE ZERO adjustment on the secondary controls panel to obtain a
zero reading on the LEAK RATE display.
8. Re-open the valve on the calibrated leak and observe the LEAK RATE display.
9. Adjust the CALIBRATE potentiometer so that the LEAK RATE display bar graph reading
matches the value of the calibrated leak.
If the display cannot be made to match the leak rate, then change the position of the
SENSITIVITY switch and the RANGE SELECT switch and go to step 5. Refer to
“Sensitivity” on page 3-4.
10. Repeat steps 6 through 8.
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