Spectral Signature
Discrete bands of the electromagnetic spectrum where water molecules absorb radiant energy are utilized to measure the moisture content of materials during manufacturing. In infrared spectroscopy, water absorption bands appear as sharp drops in the reflectance of a paper sheet, with prominent peaks located at one point four five, one point nine four, and two point nine five micrometers. Measuring the light intensity at these specific wavelengths allows online sensors to calculate the exact amount of moisture held within the fibrous network.
This non-contact measurement is performed continuously as the paper web travels at high speeds through the paper machine.
Sensor Accuracy
Light sources in online infrared gauges must be tuned to these specific absorption wavelengths while referencing adjacent, non-absorbing wavelengths. This dual-wavelength approach enables the sensor to compensate for changes in sheet thickness, basis weight, and surface scatter. By comparing the absorption peak to the reference baseline, the instrument determines the moisture percentage independent of these substrate variables.
This calculation occurs millisecond-by-millisecond to provide a real-time moisture profile that the mill use to adjust the dryers.
Chemical Interference
Other raw materials in the paper sheet can sometimes interfere with these absorption peaks. For instance, cellulose fibres themselves and certain clay coatings have minor absorption features that sit close to these bands. These overlaps require sophisticated calibration algorithms to ensure that the moisture readings are not distorted by shifts in the coating formula.
