Thermal Variance
Dimensional change in measurement frames results from the fluctuation of ambient temperature in the mill or testing laboratory. A c-frame thermal expansion describes the physical growth or contraction of the arms that hold thickness sensors over a paper web. This movement introduces a bias in the readings.
Measurement Error
Precise thickness gauging becomes impossible when the support structure moves independently of the material being measured. During a c-frame thermal expansion, the sensor heads drift apart, leading the control system to report an increase in caliper. This false data triggers the automatic profile control to reduce the pulp flow or increase the nip pressure at the calender stack.
Such corrections result in a product that is actually thinner than the target specification.
Structural Mitigation
High performance gauging systems use materials with a low coefficient of expansion or active temperature compensation to maintain accuracy. Minimizing c-frame thermal expansion involves circulating water through the frame or using internal reference rods made of specialized alloys. These methods ensure the distance between the sensors remains fixed regardless of the heat generated by the drying cylinders.
Reliable data allows the mill to run closer to the lower limit of the weight specification.
