Near Infrared Optical Sensing Protocols for Wet End Pulp Slurry Verification
Inline NIR transflectance sensors stabilize wet end pulp slurry consistency within +/- 0.3 percent, cutting fiber giveaway and dryer steam consumption.
Transflectance Optics
Optical Measurement
Combined transmission and reflectance of light through a moving web represents a highly accurate non-contact method for measuring sheet properties in real time. Using transflectance optics, specialized online sensors project infrared or visible light through the paper sheet onto a reflective backing plate. The sensor measures both the light transmitted through the sheet and the light reflected back from the plate.
This approach allows for the simultaneous analysis of sheet moisture and coating thickness.
Sensor Design
Positioning the optical sensors at the dry end of the paper machine allows for continuous monitoring of the product before winding. Because the paper web is moving at high speed, the transflectance optics must be designed to withstand temperature fluctuations and dust. The use of a reference beam corrects for any build-up of dust on the lenses, ensuring accurate readings.
Quality Control
Continuous feed of precise measurement data to the control system enables automated adjusters to correct profile deviations. If the moisture levels drift, the system can instantly adjust the steam flow in the drying cylinders or the spray application at the sizing press. Using transflectance optics ensures that the paper meets the strict moisture tolerances required for high-speed printing presses where uneven moisture causes curling and paper jams.
The non-contact nature of the measurement prevents any scratching of the sheet surface, which is critical for high-gloss coated papers. Regular calibration against laboratory samples maintains the sensor’s accuracy over long production runs.