Valmet IQ Web Caliper Sensor Assembly A420734 – High-Accuracy Thickness Measurement

The Valmet IQ Web Caliper Fixed Coil Head Sensor Assembly, model A420734, is a precision measurement device designed for on-line thickness measurement of moving webs in paper, board, and tissue machines. This sensor utilizes a non-contact eddy-current principle with a fixed coil head that generates a high-frequency magnetic field. The interaction of the field with the conductive web causes changes in impedance, which are proportional to the distance between the coil and the web surface. By comparing measurements from two opposing sensors or using a single-sided configuration with a reference plate, the system calculates the web caliper with exceptional accuracy, typically within ±0.1 µm.

The A420734 sensor assembly consists of the fixed coil head, a preamplifier unit, a cable harness, and a mounting frame that integrates into the IQ scanner beam. The scanner traverses across the web width at speeds up to 10 m/s, providing full web profile measurements in less than 30 seconds. The fixed coil head is encapsulated in a rugged housing that protects against dust, moisture, and mechanical damage, with an IP67 rating for washdown environments. The preamplifier unit is located near the coil head to minimize signal degradation and is equipped with automatic temperature compensation to maintain measurement stability over a wide ambient temperature range of 5°C to 50°C.

Electrical specifications for the A420734 include a power supply of 24 V DC, with a current consumption of 0.5 A. The output signal is a 4-20 mA analog current proportional to the gap distance, or a digital serial data stream via RS-485 for direct integration with the IQ system's measurement computer. The sensor's measurement range is 0.5 to 5.0 mm, with a resolution of 0.05 µm and a repeatability of 0.1 µm. The frequency response is 1 kHz, enabling capture of fast variations in web thickness caused by defects or process instabilities. The system includes a self-calibration routine that references a built-in standard of known thickness, ensuring long-term accuracy without manual intervention.

Installation of the A420734 sensor assembly must be performed with careful alignment to the web path. The sensor is mounted on the scanner beam such that the coil head is perpendicular to the web surface, with a gap of approximately 2 mm during operation. The mounting frame includes adjustment screws for precise positioning and locking mechanisms to prevent drift due to vibration. The cable harness should be routed away from high-voltage lines and motor drives to avoid electromagnetic interference. It is recommended to install a cooling fan or air purge to keep the coil head free of dust and condensate, especially in humid or dirty environments.

Commissioning the sensor involves setting the initial gap, performing a zero calibration with the scanner at the reference position, and verifying the measurement accuracy using certified shims. The IQ system's software provides a calibration wizard that guides the operator through the steps and stores the calibration coefficients in the sensor's non-volatile memory. After calibration, the system automatically compensates for temperature variations and aging of the coil through ongoing reference measurements. The sensor's health is continuously monitored, and any deviation from expected values triggers an alarm, prompting maintenance action.

Routine maintenance of the A420734 includes cleaning the coil head surface with a soft cloth and isopropyl alcohol to remove oil and fiber deposits that could affect the measurement. The preamplifier unit's status LED indicates normal operation, with flashing patterns signaling faults such as coil damage, cable disconnection, or overheating. Periodic verification of the calibration using traceable thickness standards is recommended to maintain ISO compliance. In case of a malfunction, the sensor can be replaced as a complete assembly, minimizing downtime. Spare sensors and cables are available through Valmet's service network.

Typical applications of the Valmet IQ Web Caliper Sensor A420734 include basis weight control, moisture correction, and formation analysis in papermaking. The sensor is also used in plastic film, nonwovens, and metal foil production. Its fixed coil head design offers advantages over traditional mechanical contact gauges, including non-contact operation, high speed, and low maintenance. The sensor's data is used by the IQ system's advanced control algorithms to adjust refining, headbox, and press section parameters, improving final product uniformity and reducing grade change waste. The A420734 is a trusted component of Valmet's quality control suite, delivering the measurement performance required to achieve premium product quality in demanding web processes.