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WELD SENTRY 2
APPENDIX A - SPC FORMULAS
SPC MEMORY - When a Weld Schedule is set for Weld Sentry ON, monitoring results for each related
program are automatically saved in memory. The Weld Sentry SPC memory is capable of saving up to
20,000 individual results for all programs combined.
MEMORY MANAGEMENT - memory management is handled automatically by the Sentry. Memory
capacity is allocated to each program in blocks of 125 welds. A total of 160 blocks of data can be allocated
to individual programs, 125 x 160 = 20,000. When a Sentry Program monitors for the first time a block of
memory will be allocated. When 125 welds have been monitored and saved, a second block of memory will
be allocated. Memory is allocated to related Programs equally. If a Weld Schedule has 5 Programs, A-E,
then each will receive a separate block of memory, 125 x 5 = 625. When all 160 blocks have been
allocated, the oldest block of data will be erased and re-allocated to the next program used. The oldest
block is defined by the chronology of the last weld saved to that block. If only one Sentry Program is ON it
is possible to save results for 20,000 welds for a single program. The collection of results saved in memory
for a program is referred to as SPC Program History.
CALCULATIONS - The following definitions and formulas are used for calculated values accessed through
the SPC Menu:
STATISTICS, RUN CHARTS & HISTOGRAM
x = value of individual results
n = number of "welds used for calculations"
a. Average -
b. Standard Deviation -
CONTROL CHARTS & SUMMARIES - Control chart statistics are based on the subgroup
sample size selected. Control Limits are determined by formulas which approximate the
population standard deviation according to the subgroup sample size.
x bar = average value of results for a subgroup
x bar bar = average value of all subgroups in the "welds used for calculations"
R = range of the sub group
R bar = average range for all subgroups in the "welds used for calculations"
A
= factor used to calculate limits for x bar, see Table 2
2
D
= factor used to calculate limits for R, see Table 2
3
D
= factor used to calculate limits for R, see Table 2
4
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