Water Measurement
Electromagnetic energy absorption at specific wavelengths quantifies the concentration of hydrogen bonds within a substrate. A near infrared moisture sensor directs light onto a moving web of paper or a layer of pulp to detect the attenuation of specific bands that water molecules absorb. Photodetectors capture the remaining light intensity, which corresponds to the total liquid mass per unit area.
This non-contact measurement occurs during the drying stage to allow for rapid feedback to steam valve controls. Precise monitoring maintains the integrity of the fibre structure and prevents curling in finished rolls.
Process Accuracy
Variations in the grammage of the sheet affect the signal intensity received by the sensors. High opacity coatings create barriers that block the penetration of light into the base stock. The calibration of these units relies on the comparison of sensor output against oven-dried samples taken from the same production run.
A change in the filler concentration or mineral composition shifts the baseline signal even if the actual water content remains identical. Operators adjust the gain settings when switching between paper grades to compensate for these inherent substrate differences.
Line Integration
Automated controls receive the continuous stream of data from these units to adjust the machine speed and temperature of drying cylinders. The real-time adjustment of cross-direction moisture profiles reduces the risk of snap-offs and prevents uneven re-wetting during the application of surface treatments. Consistent control of the water fraction minimizes the shrinkage of the web during the cooling phase.
A stable moisture level optimizes the machine efficiency and creates a predictable surface for printing inks. High levels of water in the final product reduce the physical strength of the sheet.
