Paper industry measurement: continuous (on-line) measurement of moisture content in paper webs is commonly achieved using which sensing approach?

Introduction / Context:
Paper moisture strongly affects quality, strength, and runnability. Mills use continuous, non-destructive sensors to monitor moisture across the web and adjust dryer steam or machine speed accordingly.

Given Data / Assumptions:

• Continuous, on-line monitoring is required.
• Contact or non-contact electrical methods correlate sheet moisture with dielectric or resistive properties.

Concept / Approach:
As paper moisture changes, its electrical resistance and dielectric constant change markedly. High-resistance Wheatstone bridge circuits (or RF/dielectric sensors) detect these changes in situ, enabling continuous moisture profiling. Psychrometers or hair hygrometers read ambient humidity, not sheet moisture; weighing is offline and discontinuous.

Step-by-Step Solution:
Select an electrical property sensitive to moisture (resistance/dielectric).Implement as a high-resistance bridge with electrodes/capacitance heads.Calibrate output versus % moisture using lab references.

Verification / Alternative check:
Modern scanners combine microwave/infrared absorption with electrical methods; classic curricula cite high-resistance bridge techniques as foundational.

Why Other Options Are Wrong:
Psychrometer/hair hygrometer: Measure air humidity, not sheet moisture.Weighing: Discrete lab method, not continuous.Thermocouples: Monitor temperature, not moisture.

Common Pitfalls:
Poor electrode contact, contamination, and temperature variation can bias readings; temperature compensation and proper electrode design are essential.

Final Answer:
High-resistance Wheatstone bridge across the sheet