Handbook of Humidity Measurement, Volume 1

Spectroscopic Methods of Humidity Measurement

Ghenadii Korotcenkov

• Publisher: CRC Press
• Year: 2018
• Topic: Technology & Engineering
• eBook ISBN:
  9781351400640
• Subtopic: Chemistry
• Index: Chemistry

The first volume of The Handbook of Humidity Measurement focuses on the review of devices based on optical principles of measurement such as optical UV, fluorescence hygrometers, optical and fiber-optic sensors of various types. Numerous methods for monitoring the atmosphere have been developed in recent years, based on measuring the absorption of electromagnetic field in different spectral ranges. These methods, covering the optical (FTIR and Lidar techniques), as well as a microwave and THz ranges are discussed in detail in this volume. The role of humidity-sensitive materials in optical and fiber-optic sensors is also detailed. This volume describes the reasons for controlling the humidity, features of water and water vapors, and units used for humidity measurement.

Section III

Optical and Fiber-Optic Humidity Sensors

10	Introduction in Humidity Measurement by Optical and Fiber-Optic Sensors

10.1 INTRODUCTION

In Chapters 3, 4, and 8, we also have considered optical-based instruments designed for humidity measurements. The operation of these optical hygrometers is based on the absorption and fluorescence properties of water molecules. This is a so-called direct sensing scheme. However, studies have shown that for development of the optical humidity sensors one can use another approach. Instead of studying the properties of air, containing water vapor, we can investigate the effect of water vapor on the physical properties of humidity-sensitive materials.

Experiment has shown that water can have a comprehensive effect on the properties of different materials (Section 2.4). Under the influence of the water volume, properties of materials, such as the refractive index (RI), density, and the phase composition can change. Surface properties such as the composition of the adsorbed molecules, activation energies of adsorption and desorption are also sensitive to the presence of water vapor in the atmosphere. Condensation of water modifies the dielectric properties of the environment and thus affects the interaction between the nanoparticles. This means that for building the solid-state optical humidity sensors one can use almost all the variety of optical effects observed in the solid bodies—transmittance, reflectance, luminescence (fluorescent), interference, scattering, and the plasmon resonance (Wang et al. 1991; Xu et al. 2004; Wolfbeis 2006, 2008). This approach uses the so-called indirect or indicator-based sensing scheme. In many cases, this approach proved to be more effective, because it allowed reducing significantly the size of devices and increase their sensitivity. In addition, fiber-optic and planar optical humidity sensors, despite of the optical nature of measurements, are quite different structurally from the optical hygrometers discussed in Chapters 3, 4, 5. For this reason, a consideration of these sensors we have allocated in the individual chapters.

It should be noted that the appearance of solid-state planar optical and fiber-optic sensors is a result of three critical factors that have been instrumental toward development of this technology (Davis et al. 1986; Udd 1991; Culshaw 2008; Sharma and Wei 2013). First, it was the invention of laser that provided a high-intensity light source such as light-emitting diodes (LED) and laser diodes (LD) with strong spatial and temporal coherence properties. Second, it was the advancements in optoelectronics industry that facilitated the development of optical detectors to realize sophisticated low noise detection techniques to record optical interference effects with high sensitivity. And finally, it was the development of low-loss single-mode fibers that made it possible to guide light in a flexible fiber-optic waveguide and realize compact, low cost, robust, and versatile fiber-optic interferometers that would otherwise be impractical to achieve with bulk-optic components. The availability of rapid data acquisition and data-processing techniques also contributed to the fast development of this sensing technology.

As it was indicated earlier, solid-state optical and fiber-optic sensors can be described as devices, which can be employed for the detection and determination of physical and chemical parameters through measurements of optical property of humidity-sensitive materials. In general, solid-state optical sensors incorporate optical fibers as light guides. That is, in optical sensors optical fibers perform a passive role. At the same time in the fiber-optic sensors, optical fibers serve as the active element. Although we must admit that in many cases this separation is very conditional, because mechanisms, providing the sensitivity of the majority of solid-state optical and fiber-optic sensors to humidity are the same.

The current approach based on the development of solid-state optical and fiber-optic sensors is dominant in various fields of analytical sciences, for example, in the development of a variety of chemical sensors and biosensors (Narayanaswamy and Wolfbeis 2004). It was shown that various chemical and biochemical analytes can be qualitatively detected by spot tests using spectroscopic techniques, including colorimetry and photometry. Optical sensors in analytical sciences offer several advantages (Grattan and Sun 2000; Alwis et al. 2013). These sensors are capable of observing a sample in its dynamic environment, no matter how distant, difficult to reach, or hostile this environment is. As it is known, optical fibers allow transmission of light over the great distances; and for chemical sensing typical monitoring distances required the range from 1 to 100 m. These devices are intrinsically safe, involving a low optical power and are nonelectrical at the sensing point. These sensors are electrically passive and immune to electromagnetic disturbances, geometrically flexible and corrosion resistant, capable of being miniaturized, and compatible with telemetry. Furthermore, the possibility of multiplexing several sensors to a single instrumentation unit can afford substantial economic advantages to this type of sensor. In particular, this possibility allows developing devices capable of carrying out a simultaneous control of humidity and the presence of toxic gases.

More detailed information on the features of the fiber-optic sensor functioning can be found in reviews (Davis et al. 1986; Dakin and Culshaw 1988; Grattan and Sun 2000; Yu and Yin 2002; Narayanaswamy and Wolfbeis 2004; Kara 2011; Lou et al. 2014; Rajan 2015). With regard to additional information relating to the use of fiber-optic sensors for measuring humidity, one can refer to the reviews prepared by Moreno-Bondi et al. (2004), Yeo et al. (2008), Alwis et al. (2013), Kolpakov et al. (2014), Noor et al. (2015), Sikarwar and Yadav (2015), Jung et al. (2016).

10.2 TYPES OF OPTICAL HUMIDITY SENSORS—SENSOR CLASSIFICATION

On the basis of the properties of the material, changing under the influence of humidity, currently there can be marked out three basic approaches to the building of optical humidity sensors. The first approach is based on the color change of certain metal–ion complexes, when water is participating in the coordinating sphere (Dacres and Narayanaswamy 2006). So, the absorption band of the humidity-sensitive material undergoes a dramatic hypsochromic shift (Sharkany et al. 2005). This is so-called absorption, photometric, or colorimetric-based humidity sensors (Chapter 12). Other optical humidity sensors, that is, refractometric-based sensors, are based on the physi- or chemisorption of water directly onto the surface of the optical device or in a very thin and porous film, leading to a detectable change in the RI (Chapter 14). The third approach is derived from the phenomenon that water molecules are able to influence the fluorescence emission of certain compounds. Therefore, this type of sensors is called fluorescence-based sensors (Chapter 15). With increasing relative humidity (RH), either an enhancement or a quenching of the fluorescence intensity of an indicator dye is observed (Wang et al. 1991).

In optical sensors, the light may be modulated either inside or outside the optical waveguide, that is, sensing location may be inside or outside the waveguide. Hence, based on the sensing location, optical sensors are classified broadly as intrinsic and extrinsic sensors (read Chapter 11). In intrinsic fiber-optic sensor, the interaction occurs within an element of optical fiber itself and light never leaves the waveguide. External environment acts on the fiber and the fiber in turn changes some characteristics of the light inside the fiber that is measured using the detector. In the extrinsic fiber-optic sensor, the optical fiber is used to couple the light, usually to and from the region where the light beam is influenced by the measurand (or external environment). In this case, the fiber just acts as a mean of getting the light to the sensing location and to detector.

Besides the previously mentioned types of humidity sensors using different properties of the humidity-sensitive materials, it is also necessary to mark the humidity sensors, which are different from others by the specific method of measurement or configuration of the sensor element. These sensors must first of all include interferometric humidity sensors. Most of the components of interferometric fiber sensors use either all-fiber or integrated optic material to provide better stability and compactness. An interferometric sensor works on the modulation in the phase of light emerging from a single mode fiber. The variation in phase is converted into an intensity shift using interferometric schemes (Sagnac forms, ring resonators, Mach–Zehnder, Michelson, Fabry–Perot or dual mode, polarimetric, grating, and Etalon-based interferometers). This type of sensors will be considered in Chapter 16. Developers ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgments
• Author
• SECTION I Introduction to Humidity Measurement
• SECTION II Humidity Monitoring Using Absorption of Electromagnetic Radiation
• SECTION III Optical and Fiber-Optic Humidity Sensors
• Index