Lesson 26　INSTRUMENTATION　(仪器, 仪表) 轮机英语课件.pptVIP

LESSON 26 INSTRUMENTATION (仪器, 仪表) All machinery must operate within certain desired parameters. Instrumentation enables the parameters—pressure, temperature, and so on—to be measured or displayed against a scale. The various display devices used for measurement of system parameters will be examined here. PRESSURE MEASUREMENT Two basic types of instrument may be used to measure absolute, gauge pressure, draught(draft) or vacuum pressure and differential pressure. One is the liquid column, in which the density and liquid height are used to measure pressure. The other (is) the metallic formed elastic(弹性的) type such as the Bourdon tube(包端管，波登管), diaphragm(膜片) or bellows(波纹管), using in some cases an opposing spring. MANOMETER(压力计) Liquid placed in connecting legs of a U-tube manometer will indicate the difference in the pressure acting on the top of two legs. One end is connected to the pressure source; the other is open to atmosphere. The liquid in the tube may be water or mercury(水银). The excess of pressure above atmospheric--gauge pressure--will be shown as the difference, in liquid level. It is usually used for low pressure reading such as air pressure. BOURDON TUBE This is probably the most commonly used gauge pressure measuring instrument. It is made of an elliptical(椭圆的) section tube formed into a C-shape and sealed at one end. The sealed end, which is free to move, has a linkage arrangement which will move a pointer over a scale. The applied pressure acts within the tube entering through the open end, which is fixed in place. The pressure within the tube causes it to change in cross section and and attempt to straighten out(弄直, 伸直) with a resultant(合成的) movement of the free end, which registers(指示、显示) as a needle movement on the scale. While the reference or zero value is usually atmospheric, to give gauge pressure readings, this gauge can be used to read vacuum pressure values. OTHER DEVICES Diaphragms(膜片, 振动膜) or bellows(波纹管) may be used f