Doc. # 5016
BM-185D/285D/286D
Rev. A
Operation and Service Manual
VARIANCE
The variance analyzer mode is used to check whether the counts seen by the controller are
actually from 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 vary
from unit to unit and are 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 referred to as variance in TSA’s manuals
S2 = sample variance
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