制冷与空调专业英语第2版 林慧珠 主编 2-2新.pptVIP

Unit Two Lesson 5 Fluid Lesson 6 Level Measurement Lesson 7 Measuring Rate of Fluid Flow Lesson 8 What Is Humidity Lesson 6 Level Measurement New Words Expressions Text Notes to the Text Exercise Lesson 6 Text In the control of many industrial processes quantity and level are often unconsciously confused with each other. Probably because one presumes that the dimensions of a tank are constant, one considers that the fluid level in that tank are the image of the quantity it contains.1 Text: Now this hypothesis is not always confirmed especially when the tank section is not uniform from top to bottom of the tank. We shall therefore draw the reader’s attention straightaway to the end result of the measurement he wishes to make; the measurement of the level will not always be the one which is most appropriate. Text: Methods of measuring levels are very varied, both in conception and realization. Nevertheless they can be divided into four categories; direct measurement, measurement by float, measurement of pressure and special measurement processes. In any one application the choice of a measurement method will depend on many factors - is the tank open or closed? is it pressurised or under vacuum? is the fluid to be measured solid or liquid? Corrosive or non-corrosive? heated or cold? etc. 2 Text: The measuring principle of direct measurement equipment is extremely simple. It consists of introducing into the tank a rigid graduated rule , of taking it out and visually recording the graduation mark to which the instrument is wet. 3 An experienced operator may obtain a reading which is accurate to 1/10 of a millimeter. The disadvantages of this method are obvious. Text: It involves an isolated (and not continuous) reading of the level, it is limited to moistening non-volatile liquids, and most of all it involves only a visual reading therefore, not one which is converted into an electric or pneumatic signal which ca