Todays' on-line quality control systems (QCS) scan the sheet and provide information to the paper manufacturer on paper properties. These systems also provide statistical information regarding the variation in the process and the product.

TAPPI TIP 1101-01 provides a definition of terms and method of comparison so that people in the industry will be able gain a further understanding of the role variability statistics play in monitoring the performance of the process. This TIP provides the user with a recommended method of calculating variance from scanning sensor measurements. Examples of scanning sensor measurements include basis weight, moisture, caliper, and other paper property parameters. The sensors traverse the sheet of paper in the cross direction during normal operation of the paper machine. The machine direction rate of paper machine travel is greater than the cross direction rate of sensor travel, the result is a diagonal path across the paper, the system must then differentiate between machine direction (MD), cross direction (CD), and total variation. It is desirable to use this information as an aid to assessing paper quality, determining paper machine performance, and to compare performance between paper machines.

As a prerequisite to the use of this document, the user must have general knowledge regarding scanning sensor measurements and any vendor specific functions. To aid in this understanding, a discussion of the factors that influence sensor measurements is presented. The interested reader is encouraged to consider the references in the bibliography for further information on the subject.