There are three key components to a web gauging system: scanner, sensor, and software platform. Together these components determine what can be measured, how accurately and reliably, ease of use, maintenance requirements, and process relevance. Choosing the best of each, for a specific application, is the key to maximizing return on investment.
Beta transmission sensors are critical for helping to ensure the efficiency, quality, and cost-effectiveness of web gauging processes. Beta transmission sensors are devices that measure the thickness or basis weight of materials by emitting beta particles from a radioactive source and detecting the number that pass through the material. They are used in various industries, such as paper, plastic film, metal foil, and textiles, to provide precise, non-contact, real-time measurements essential for maintaining product quality and consistency. These sensors are versatile, reliable, and allow for immediate process adjustments, making them crucial for efficient and cost-effective web gauging. Despite using radioactive sources, they are designed to be safe with minimal risk to operators when responsibly managed. This article will discuss web gauging systems and what to look for when choosing a sensor.
What are Web Gauging Systems?
Web gauging systems are measurement systems used in manufacturing processes to monitor and control the thickness, basis weight, or coating weight of continuous web materials such as paper, plastic films, metals, textiles, and nonwovens. These systems employ various sensor technologies, including beta transmission, X-ray, infrared, and laser sensors, to provide real-time, non-contact measurements across the width of the material. The data collected is used to help ensure product quality, optimize material usage, reduce waste, and enhance overall production efficiency by allowing for immediate adjustments during the manufacturing process.
For manufacturers operating continuous web processes, an effective web gauging system is the key to meeting stringent product quality specifications with high efficiency. Offering continuous, in-line thickness and basis weight measurement, such systems help enable the precise process control required for competitive economics. Web gauging solutions can be used to achieve thinner and more uniform layers, to implement layer-by-layer (LbL) assembly and create precise multi-layer structures with controlled basis weight/thickness, and to innovate with new materials.
Web Gauging Technologies
Web gauging technologies are crucial to many applications in the polymers and plastics industry. The diversity of products that benefit from such technology is considerable; from extruded plastic films and coatings to food packaging, building materials, non-wovens and Li-ion batteries. Selecting a sensor that will reliably measure the material in question is the first step towards a robust web gauging system. A wide range of measurement technologies are available to help ensure an optimal match. Consider the features of different technologies to identify the best option.
- Beta (nuclear) sensors – which we discuss below — determine basis weight from measurements of the absorption of beta particles emitted from an isotope. Typical applications include film and sheet extrusion, extrusion coating, roofing and other building products, gum, and vinyl calendaring.
- Infrared sensors determine thickness from measurements of the absorption of light, operating in either transmission or reflectance modes. Typical applications include coatings, films, non-wovens, multilayer products, and biaxial extrusion.
- Direct thickness sensors measure multiple physical properties to monitor materials with a variety of finishes, with minimal sensitivity to product color, opacity, or transparency. Typical applications include extruded and calendared sheet, foam, rubber, composites, and coated substrates.
- X-ray sensors determine basis weight and thickness from measurements of the absorption characteristics of X-rays from an electrical source. These powerful sensors can measure the broadest range of materials and products on the same line with a single sensor making them a simple and cost-effective choice. Typical applications include paper and cardboard, textiles, noncast films, non-wovens, plastics and packaging, composite biax films and insulating materials, coatings, and laminates.
Critical Factors in Choosing the Most Appropriate Beta Transmission Sensors
As noted, Beta transmission sensors are used in the web gauging industry to measure basis weight and control processes in the polymers and plastics industry. Beta sensor measurement is based on the absorption of beta particles emitted either from a Kr-85 or Sr-90 source (depending on the application) by the web to determine the resulting basis weight of the web.
For the best measurement and therefore the best control, there are a number of design elements that are critical to delivering superior sensor performance. As with any measurement technique, the amount of electronic signal generated by the sensor provides the best results.
Here are six critical factors for predicting beta sensor performance in web gauging:
- Source strength (Number of beta particles emitted)
- High source strength provides the highest signal and lower ratiometric noise, thus improving the quality of the measurement.
- Physical geometry of the source
- Unique slot source geometry (instead of the common circular geometry) of the source capsule improves measurement resolution for optimal edge and gauge band detection. In most cases, better edge measurement leads to better edge control, reducing defects and the amount of edge trim.
- Alignment of the source to the detector
- Misalignment of the source and detector due to wear-and-tear, mechanical runout or vibration contributes significantly to measurement noise. Special source and detector optics are used to make the sensor nearly insensitive to X-Y-Z misalignment of up to 6X the normal scanning path run-out.
- Web movement or “sheet flutter” between the source and the detector
- Sheet flutter —the up-and-down movement of the material during a normal operation— causes measurement noise and error. Beta transmission sensors fitted with a special detector optic reduce this measurement error and improve measurement repeatability.
- Measurement impact from operating temperature and pressure variation
- Temperature and Pressure fluctuation throughout the day affects the measurement of the web because it changes the density of air molecules found in between the source and detector (PV = nRT). Therefore, a change of 10°C is equal to a change of about 1gsm of air mass, thus temperature and pressure compensation are necessary, especially in thin film application.
- Full range calibration
- Does your production mix change frequently? Each sensor is calibrated for its full measurement range capability to ensure linearity along the full product range.
We’ve created an infographic to easily see the 6 Critical Factors For Predicting Beta Sensor Performance in Web Gauging.
Sensors are one of the key technologies for weight and thickness measurement to meet process, material, and cost constraints. The right sensor will help ensure accurate and robust process measurement 24/7 even for multilayered products, in the face of material variability, and/or in harsh process environments.
Additional Resources
- Infographic: 6 Critical Factors For Predicting Beta Sensor Performance in Web Gauging.
- FAQs, brochures, comparison charts and other resources on our Web Gauging Resource website page
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