Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

Process Control Charts
Two steps on the control strip are selected for measurement. One step should
have a density of 0.6 to 1.0; the other, 2.0 or higher. A specific area of the
step is selected for measurement, andthat same area on each of the two
steps is measured to obtain a high-density value and a lowdensity value.
From the first day on, four values are obtained--the value of the high-density
step and the value of the low density step on the fresh control strip and the
values of the corresponding steps an the control strip exposed the preceding
day--always in the same area on each step. The upper and the lower control
limits for the process density aim can be assigned arbitrarily on the basis of
acceptable tolerances in the process operation (2.0 ± 0.2, for example).
Sometimes it is desirable to calculate more precise control limits, however,
and a statistical method, such as the standard deviation of density values with
three sigma control limits, can be used to determine the limits.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980