Online paper moisture measurement in industry For continuous measurement of the moisture content of paper on a running web, which physical property is most commonly monitored?

Introduction / Context:
Paper machines require tight control of moisture to ensure print quality, caliper stability, curl control, and energy efficiency in dryers. Continuous, non-destructive online sensors are mounted across the web to provide real-time moisture profiles. Understanding which property correlates reliably with moisture helps in selecting and maintaining the correct instruments.

Given Data / Assumptions:

• Measurement must be continuous on a moving paper web.
• Options include thermal, electrical, and magnetic properties.
• We consider widely adopted industrial practice.

Concept / Approach:
The electrical resistance (or, conversely, conductance) of paper changes significantly with moisture because water increases ionic conduction pathways within the fibers. Many continuous sensors therefore use resistance (or capacitance at radio frequency) to infer moisture. Thermal conductivity methods are less common online due to response time and installation complexity, while magnetic susceptibility bears little relation to moisture content in paper fibers.

Step-by-Step Solution:

Verification / Alternative check:
Mill experience and vendor datasheets show resistance/capacitance-based moisture meters and microwave/IR sensors as the common choices for online paper moisture monitoring.

Why Other Options Are Wrong:

• Thermal conductivity: Can correlate with moisture but is not the mainstream online method for moving webs.
• Magnetic susceptibility: Not a primary indicator of water content in paper.
• None of these: Incorrect because resistance-based systems are widely used.

Common Pitfalls:
Ignoring temperature compensation; resistance readings require algorithms to correct for paper temperature and basis weight.

Final Answer:
Electrical resistance through the paper