Moisture Sensing: A key process in Industrial Process Optimization

Moisture sensing by the use of Near and Mid-Infrared Spectroscopy has been established as a powerful technique for process monitoring in a wide variety of industrial applications. The industrial settings range from the paper industry through the food industry into the pharmaceutical industry and beyond. The power of the technique rests in the ability to perform a "standoff" detection, where the sensing head is located typically several inches away from the material being measured. This sensing at a distance capability allows meaningful measurement of the moisture content of a product to be be performed continuously and non-invasively at points distributed throughout a process flow line by measuring the spectrum of the scattered light from the product as it is conveyed through a factory.

The Companies in the Industry

Foundational work on instruments using this approach was performed to address the paper industry by Chiba et.al. at Yokogawa[1]. Today, several companies produce instruments to address this industrial sensing application, with units commonly priced in the range of $10,000 to $30,000 USD. Some of the industry leaders in this market are Grecon (www.grecon.com ), MoistTech (www.moisttech.com) , NDC ( www.ndcinfrared.com ) Process Sensors Corporation ( www.processsensors.com ) Yokogawa ( www.yokogawa.com ) and Kett ( www.kett.com ). Several of these companies currently deploy a platform which is well described by the Grecon IR 5000 Moisture Meter Analyzer below.

The measurement performed is basically a multi-wavelength spectral analysis at two or perhaps more wavelengths. One of the wavelengths corresponds to an absorption peak of water, typically 1400nm or 1940nm, and a nearby reference wavelength is selected at which water absorption is weak.

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Moisture Sensing Industry Standard The output of a halogen lamp is collected to generate the desired wavelengths. Filters mounted on a rotating wheel are used to alternately illuminate the material under test with the probing and reference wavelengths. Light scattered from the sample is detected, and information on the moisture content of the sample is extracted from the ratio of the scattered light intensity at the probing and reference wavelengths. From www.grecon-us.com/html/moisture.htm

The advantages of the halogen lamp/rotating filter wheel based system are that the components are simple, extremely well understood, and can be engineered to have acceptable reliability and precision. However, there is no escaping the realities that halogen lamps decay over time and motors wear out. These and other limitations of the current systems create a driving force for alternate solutions to the moisture sensing process.

Development of Light Emitting Diodes as a Light Source Alternative

Replacement of the Halogen lamp and rotating filter wheel with an electronically modulated solid state light source would potentially result in a moisture sensing system with substantially improved sensitivity, reliability and immunity to operating environment.

Mid infrared Light Emitting Diodes have been recently evaluated for this purpose by Aikio et. al of VTT Technical Research Center of Finland[2]. In their work, they have demonstrated that proper design can produce a moisture sensing system with performance comparable to that of the halogen lamp based system.

However, even this advanced, state of the art LED based approach has it’s shortcomings. Referring to the VTT work, it requires the development of a custom LED module integrated onto a thermal electric cooler (TEC) in order to provide the light sources. The custom LED sources, plus the integration of these sources into the temperature controlled package, will negatively impact the cost of the LED based solution. The limited power available from the LEDs limits the accuracy at which product may be monitored using the LED based approach. Additionally, the broad spectral output of the LED extends beyond the water absorption peak, to the extent that the reference LED and the signal LED have overlapping spectra. This spectral overlap further compromises the sensitivity of the measurement.

Fiber Laser Based Alternative

The UBQ-7810-1925 has been designed as a light source for robust, compact, reliable next generation solutions for the moisture sensing industry. Providing high spectral output centered on the peak of the water absorption band, all of the photons from this device are put to good use in extracting moisture information. Furthermore, the industry leading insensitivity to temperature of the UBQ-7810-1925 can completely eliminate the need for external temperature control of the unit using power consuming thermal electric coolers. Being a laser light source, the beam may be readily directed, efficiently collected, and adjusted with simple optics to any appropriate illumination beam size down to a diffraction limited spot.

Moisture Sensing System Comparison

We can compare the performance of moisture sensing systems based on the use of Halogen lamps, LEDs, and Fiber Lasers. Kett Produces commercial units using both Halogen lamps and LEDs, we incorporate the VTT results from [2], and, largely extrapolating from these LED results, we include the theoretical performance improvement with a high brightness fiber laser source. Key noteworthy items are the extremely fast refresh rates enabled by both LED nd fiber laser sources relative to halogen lamps. Note however, the compromise in accuracy of the VTT prototype driven primarily by the limited brightness of the LED source. By contrast, the high brightness coherent laser source enables industry leading 1.4 ppm level accuracy, while enabling substantial reduction in measurement diameters to 5mm or below.

 Conclusions

We conclude that, while present LED based solutions offer certain substantial advantages in terms of refresh rate relative to Halogen lamps, several opportunities exists to even further enhance system performance with the use of fiber lasers.

See the product page for the UBQ-7810-1925 for details on pricing and availability.

Note:   This Application Note is available for comment and discussion.

References

 [1]          H. Hara, R. Chiba, K. Isozaki, and T. Yamada, “Development of an infrared-ray moisture meter for the paper-making process,” Industrial Electronics, Control, Instrumentation, and Automation, 1992. Power Electronics and Motion Control., Proceedings of the 1992 International Conference on, 2002, pp. 1604–1607.

[2]          R. Aikio, H. Lindström, P. Suopajarvi, J. Malinen, and M. Mantyla, “Low-noise moisture meter with high-speed LED techniques,” Next Generation Spectroscopic Techniques III, Applications of Novel Spectrometers",  Orlando, Florida, USA: SPIE Proceedings Volume 7680 , 2010, pp. 768008-768008-10.