Moisture in paper, board and pulp

Moisture meters for paper, board and pulp are specialised instruments for measuring the moisture content of paper products at every stage of the production chain: paper machines, quality control of finished reels, acceptance of recovered paper deliveries, warehouse storage control, and moisture checks in printing and converting. The moisture content of paper significantly affects its mechanical properties (tensile strength, stiffness, flatness), its printing characteristics, its dimensional stability and its behavior during processing. The technologies used are mainly electrical (resistive and capacitive) for quick checks, while the precision reference remains the gravimetric drying method.

Portable moisture meters

Portable contact moisture meters use a plate sensor placed on the stack of paper or on the reel, measuring capacitively the change in dielectric constant as a function of moisture. The typical range is 3-12% moisture by mass, with an accuracy of ±0.5-1%. The response is fast (a few seconds) and use is extremely simple. Advanced digital models incorporate: a database of paper types (with calibration curves for each grammage and composition); statistical functions; GO/NOGO thresholds; and data output for archiving. Pin models also exist for spot measurement on single sheets (more precise but slower).

Laboratory moisture meters

The quality control laboratories of paper mills use thermogravimetric balances or drying ovens to determine paper moisture by the ISO 287 gravimetric method: a sample of known area (generally 100 cm² or 200 cm²) is weighed, dried in an oven at 105 °C for 1-4 hours to constant weight, and reweighed; the percentage difference gives moisture with excellent accuracy (±0.1%). The method is destructive and slow, reserved for formal checks and for validating electronic in-line instruments.

Online sensors

Paper machines are fitted with in-line sensors for continuous moisture measurement: multi-wavelength IR sensors (exploiting the IR absorption of water in specific spectral bands); microwave sensors (measuring the attenuation of high-frequency electromagnetic waves); scanning capacitive sensors (with a measuring head that traverses the width of the machine); and NMR sensors (Nuclear Magnetic Resonance, for very high in-line accuracy). The data are integrated with the machine control system for automatic regulation of speed, steam and drying.

Applications

The applications include: the paper mill itself (moisture control on the paper machine to regulate dryers and calenders); quality control of finished reels before shipping; acceptance of recovered paper deliveries for recycling (moisture is a contractual matter – a damper bale weighs more but is worth less); printing and converting (checking paper moisture before printing – the wrong moisture causes print defects, distortion and register problems); storage warehouses (checking moisture to plan climate control); and the packaging industry (moisture in corrugated board for boxes, where it affects compression strength).

Test standards

The reference standards are: UNI EN ISO 287 (gravimetric method, the international reference – moisture in paper by drying); TAPPI T412 (equivalent, the US reference for board); TAPPI T550 (moisture by capacitive method); UNI ISO 638 (for pulp). The standardised test conditions are a temperature of 23 °C and 50% RH, with the sample conditioned for at least 4 hours in a controlled atmosphere. Acceptance moisture specifications depend on the product: typically 5-7% for printing paper, 6-9% for board and 7-10% for corrugated board.

Selection criteria

The choice of paper moisture meter depends on the application: formal laboratory quality control uses thermogravimetric balances to ISO 287; quick checks in production call for portable capacitive meters with a paper database; continuous in-line measurement uses IR or microwave sensors built into the machine; and for acceptance testing of deliveries, portable instruments with good accuracy (typically ±0.5%) are adequate.