tailieunhanh - Ebook Fundamentals of industrial instrumentation and process control: Part 2

(BQ) Part 2 book "Fundamentals of industrial instrumentation and process control" has contents: Humidity, density, viscosity, and pH, other sensors, actuators and control, signal conditioning, signal transmission, process control, documentation and symbols. | This page intentionally left blank. Chapter 9 Humidity, Density, Viscosity, and pH Chapter Objectives This chapter will introduce you to humidity, density, viscosity, and pH, and help you understand the units used in their measurement. This chapter will also familiarize you with standard definitions in use, and the instruments used for their measurement. The salient points covered in this chapter are as follows: I Humidity ratio, relative humidity, dew point, and its measurement I Understanding and use of a psychrometric chart I Instruments for measuring humidity I Understand the difference between density, specific weight, and specific gravity I Instruments for measuring density and specific gravity I Definition of viscosity and measuring instruments I Defining and measuring pH values Introduction Many industrial processes such as textiles, wood, chemical processing and the like, are very sensitive to humidity; consequently it is necessary to control the amount of water vapor present in these processes. This chapter discusses four physical parameters. They are as follows: 1. Humidity 2. Density, specific weight, and specific gravity 141 Copyright © 2005 by The McGraw-Hill Companies, Inc. Click here for terms of use. 142 Chapter Nine 3. Viscosity 4. pH values Humidity Humidity definitions Humidity is a measure of the relative amount of water vapor present in the air or a gas. Relative humidity (Φ) is the percentage of water vapor by weight present in a given volume of air or gas compared to the weight of water vapor present in the same volume of air or gas saturated with water vapor, at the same temperature and pressure, ., amount of water vapor present in a given volume of air or gas Relative humidity = × 100 maximum amount of water vapor soluble in the same volume of air or gas ( p and T constant) () An alternative definition using vapor pressures is as follows: Relative humidity = water vapor pressure in air or gas × .