SIGMA AM-255 - Manual (Page 27)

APPENDIX C. FORMULAS page24
Model AM-255
C.6. 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 varies
from unit to unit and is included in the variance section of the manual on most units.
R = the quality factor
C = each of the individual counts
N = number of counts taken
_
R = mean variance
Where:
I = number of iterations