Varian 959D, Руководство по эксплуатации

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2-17
959D Leak Detector
2.7 Residual Background Check
2.7.1 Measuring the Background
When testing is complete and helium is no longer entering the leak detector through a leak,
the vacuum system rapidly removes most of the remaining helium. The residual amount,
called background, is steady and can be cancelled by zeroing the leak rate display.
Measure the background to:
❑
Check for a dirty system that needs cleaning or overhaul
❑
Determine whether the system is detecting helium
❑
Determine whether the reading is a result of electrical noise associated with
the preamplifier.
To measure the residual background, refer to Figure 2-2 on page 2-2 and Figure 2-4 on
page 2-4.
1. Verify that the system is warmed up, tuned and calibrated, and ready for testing.
2. Place the test port plug in the test port and tighten the nut.
3. Set RESIDUAL BACKGROUND to CHECK .
When this switch is in the CHECK position, the leak detector is almost completely
insensitive to all gases, including helium.
4. Move RANGE fully counterclockwise to its most sensitive position.
5. Press START and wait until minimum test port pressure is reached.
6. Adjust the ZERO and COARSE ZERO until the leak rate bar graph indicates zero, with
only one bar lit.
7. Move the RESIDUAL BACKGROUND switch to RUN .
The value indicated on the bar graph is the background. Record this value.
CAUTION
If the background reading is unstable or cannot be zeroed, or
the UNDER light is on after the unit was zeroed in the check
position, leak-check the system. Contact Varian Vacuum
Technologies Customer Service at 1-800-8VARIAN for
assistance.