Cross-web Measurement

What we all want is total knowledge of coating thickness at every position on the web. What we generally pay for is some compromise.

• Spot measurements. This is the lowest-cost option. The user hopes that the position chosen for the spot measurement is representative of the coating as a whole.
• Variable Spot measurements. This is slightly more sophisticated but much more powerful. The position of the spot measurement can rapidly be adjusted, allowing, for example, a quick "left, middle, right" measurement before setting the spot to its default position, usually the middle. Although this can be done via some automated system, a manual movement is often highly satisfactory and keeps things low-cost. There is an obvious problem with the beta and gamma gauges - unless you are confident that the variation in substrate thickness is smaller than your coating variation
• Multi-spot measurements. If the measurement heads are relatively low-cost then installing 3 of them for left, middle, right, provides obvious benefits. Again, for beta and gamma gauges the problem is the need to install 3 upstream gauges - unless, again, you are confident of your substrate uniformity.
• Scanning gauges. Why do you want a scanning gauge? Is your coating technology so poor that you are worried about constantly shifting coatweights? Maybe you should get a better coating system. Or do you want to detect glitches such as coating misses or lines? Arguably a defect scanner will do a better job as it is generally able (through laser scanning or multiple cameras) to cover essentially 100% of the web all the time. But if you want a scanning head just to keep a good overall view of the coating there are a few questions:
  1. How fast should it go?
  2. How do you visualise and store the data?
  3. How do you adjust the scan width for different coatings?
  4. How do you synchronise the scans for beta and gamma gauges?

This is a pretty simple set of considerations, but it's surprising how often these questions aren't properly thought through - giving a system that is either over-priced or under-performing.