Online vs. Offline Testing: What Does Each Tell Us?, 2004 Paper Summit, Spring Technical & International Environmental Conference, (including Papermakers; Process Control, Electrical & Information; Product & Product Quality)

To manage and document product quality, paper and board mills employ a variety of tests and measurements both online and offline. What do online and offline measurements tell us about the quality of the product? Perhaps more importantly, what do they not tell us? i.e. when can both online and offline measurements give misleading information? How should mills use this data for control and operating decisions?

Much of the quality information for paper and board comes from online scanning sensors. In many ways, these devices do a marvelous job generating many measurements and important statistics. However, it is important for papermakers to understand their limitations. Factors such as sensor geometry, scanning speed, and processing power affect what such sensors can “see”. In addition, no scanning sensor can perfectly separate machine direction from cross direction effects. Offline testing of samples taken at reel turnup avoids these MD/CD problems but raises other aliasing issues because of the low frequency of sampling. Examples are discussed illustrating all of these problems.

Both online and offline testing have a role to play. Each shows us part of the overall picture. The key is to understand this and to develop guidelines on how the information should be used.