As paper machines widened, production tolerances increased, paper thickness decreased and throughput ramped up, the marketplace and scale of production demanded more efficiency. Newer, faster online technologies that could deliver greater accuracy were required. Initially this was achieved by measuring the reel hardness or using the paper’s caliper. Further developments including electrical arcing, which was complemented by non-contact capacitance-based devices used to measure paper weight. This was then taken one step further with systems based on radioactive attenuation. One company that commercialized this technology was Industrial Nucleonics, founded in 1950, and a forerunner to ABB in the paper industry.
Eventually, technology more akin to what we see today was developed. Using non-contact sensors, which deploy visible light, microwaves, ionizing radiation, and infrared, coupled with online scanners that could scan the full width of the web, mills could get about 60 measurements per second. It is these technologies that underpinned the modern renaissance in measurement capabilities and accuracy.
So why was this development such a game changer at the time? In operation, the organic materials within paper webs exhibit strong infrared absorption. As a result, the moisture concentration can be estimated by measuring the differences in absorption.
Signal processing used in this older generation of infrared moisture sensors exploits what is known as amplitude modulation, which is used to suppress any background phenomena that can influence the measurements. It was a major stepping stone to keeping moisture at desired levels.
Plus, the eventual emergence of digital control technologies enabled the first big leap in moisture controls. The decades that followed focused on the development of Machine Direction (MD) and Cross-Machine Direction (CD) Controls with a continuous stream of enhancements that could react to the always improving measurements, enabling faster start-ups, reduced variability, and faster grade changes.