Gamma backscatter thickness

The next process variable to be controlled is film thickness (gauge). Either on-line or off-line measurements allow us to monitor film gauge. On-line devices usually employ a radiation source, such as a gamma backscatter system. These devices measure thickness by emitting radiation that reflects back to the sensor from both the near and far surfaces of the film. They can be mounted to measure a fixed location on the bubble or on a carriage to traverse around the bubble (Fig. 3.21). Also, a unit traversing back and forth across the flattened web can be used, but this measures two-layer thickness. Off-line devices typically operate on a capacitance principle, so they don t require the safety practices employed with radiation sources. 

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