tailieunhanh - Water Vapor Measurement: Methods and Instrumentation

Written by an expert with over 20 years of experience in the field, this timely volume provides - in a single source - all aspects of humidity measurement and instrumentation, including rudiments and theory, common applications, advantages and limitations of frequently used sensors and techniques, and guidelines for installation, maintenance, and calibration. Containing references, tables, equations, drawings, and photographs, this practical volume is essential for food scientists; analytical chemists; industrial, process, meteorological, and environmental engineers and scientists specializing in humidity measurement or control; companies engaged in calibrating, manufacturing, selling, distributing, and servicing humidity instrumentation; and undergraduate and graduate students in. | 1 INTRODUCTION I. What Is Humidity The term humidity refers to water vapor . a gas. It is water in gaseous form. This book does not cover moisture which relates to water in liquid form that may be present in solid materials or liquids. However the term moisture is frequently used in this book and in practice relating to measurements that are in fact water vapor measurements. For example the term trace moisture is commonly used rather than trace humidity or trace water vapor. There are also many industrial applications where moisture in a solid material or its surface is measured by calculating the water content from humidity measurements in the immediate vicinity of the object such as moisture in a container filled with pellets and in grain silos. Humidity is present everywhere in the earth s atmosphere. Even in extremely dry areas and in boil-off from liquefied gases there are traces of water vapor which in some applications could cause problems. Measurement of humidity is more difficult than the measurement of most other properties such as flow temperature level and pressure. One reason for this is the extremely broad dynamic range which could start from 1 part-per-billion or less -112 c frost point representing a partial vapor pressure of about X 10 6 mm Hg to saturated steam at 100 C 212 F representing a partial vapor pressure of 760 mm Hg. This amounts to a dynamic range of about 109. Another reason is that measurements may have to be made in widely varying atmospheres for example from temperatures of -80 C -112 F to 1000 C 1832 F in the presence of a wide range of gases which could be corrosive or non-corrosive and in the presence of a variety of contaminants of particulate and or chemical nature. Humidity measurements play an ever-increasing role in industrial laboratory and process control applications by allowing improvements in quality of product reduction of cost or increasing human comfort. In the tobacco industry increasing proper humidity .