Service manual

Appendix page 46
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VARIANCE
The variance analyzer mode is used to check whether the counts seen by the controller are actually ffrom
the proper distribution. If the distribution approaches normal, the resulting number will approach 0.
Any significant deviation from the normal distribution will result in a larger number.
The two most common problems resulting in variance failure are light leaks in the detectors, and
periodic noise in the electronics. Periodic noise will result in a number of about 1, a light leak will
usually result in a number larger than 2. The number displayed during a variance test is the absolute
value of the average of a number of these tests, with one test being performed every nn seconds. The
data is valid after three iterations of nn seconds. The pass/fail criteria varies from unit to unit and is
included in the variance section of the manual on most units.
where:
C = counts per sample time
_
C = mean counts
I = number of iterations
N = number of samples taken
R = sample variance modified to equal 0, rather than 1, for gaussian distribution
_
R = mean variance, this term is refered to as variance in TSA’s manuals
S2 = sample variance
8. PARTS LISTS