Chapter 5 Parameters Definition
116
Bkg Reset Level +/- (K Delta): This statistical term determines the change to the
background count rate that will cause the background algorithm to discard the
running average-smoothed background value and move immediately to a new higher
or lower value when a step change in rate has occurred. The K Delta is best kept
>3.0 sigma to prevent statistical fluctuations alone from discarding the average
background value. The background must show two consecutive values exceeding the
limit before a change will occur.
Referring to the appropriate Limit (see relevant equations in Background Rate Reset
on page 113), K Delta is used to calculate the upper reset level and the lower reset
level. The default K Delta settings are 4.000 for the Beta/Gamma Channels and
5.000 for the Alpha Channel to compensate for skewed (Poisson) nature of low Alpha
background rate distributions.
Use the CTRL+F10 - Factory Setup, F10 - Methodology Configuration - K
Interpretation setting to pick “one-tailed” or “two-tailed” integration when
computing values to right of Bkg Reset Level (K Delta).
Referring to the appropriate Limit (see relevant equations in Background Rate Reset
on page 113), K Delta is used to calculate the upper reset level and the lower reset
level. The default K Delta settings is “4.000”.
Background Average Period: This sets the time over which the background is
averaged, assuming there is no step change. For most models, the range is 30 s to
3,600 s (1 hour); default value is 300 seconds. The number of background cycles
within the background average period is the Background Average Period divided by
the Background Collect Time. In equations, this is T
b
. To see the affect of the
Background Average Period, see F4 - Monitor Time vs. Bkg Rate on page 246.
Note: If the Background Average Period is less than (F3 - Setup, F2 - Wait
Timeouts) Initialize Background Update n1 (see F2 - Wait Timeouts on page
134 ), times multiplied by Background Collect Time then the monitor will not
be able to leave the Initialize background screen.
Measurement Variance Ratio Limit. Range is 0.1 to 1000. Default is 3.0 for
Gamma and 5.0 while in the “Walk-Through” mode. When Variance Ratio is above
this value at the end of a measurement, it is deemed to signify “Background Change”.
See F3 – Setup, F1 – Common Values – Measurement Variance Ratio Limit
Exceeded setting whether this is applied to detectros only or detctors and sum zones
for action taken. Increasing Measurement Variance Ratio Limit decreases sensitivity
to background change. Also see the F3 – Setup, F3 – Detection Zones, F1 – Alarm
Test Setup – Measurement Variance Ratio Limit Exceeded setting.
F2 – (Zone) Settings
Selecting F2 from the Detection Zones menu brings up the Zone Settings Gamma
Detector/ Sum Zone Pair screen (Figure 55), allowing operational parameters to be
set for each detection zone individually. Any value set in this screen will override the
corresponding value previously set globally using the F1 – Zone Configuration
screen (on page 113) or F5 – Group Settings screen (on page 127) or Shift+F5 –
Global Settings screen (on page 131).
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