Figure  11-9.—A.  Standard  U-tube  manometer.  B.  Single-tube manometer. pressure  is  lowered,  the  diaphragm  moves  the  pointer back toward the zero point. MANOMETERS A manometer is perhaps the most accurate, least expensive,  and  simplest  instrument  for  measuring  low pressure or low-pressure differentials. In its simplest form,  a  manometer  consists  of  either  a  straight  or U-shaped glass tube of uniform diameter, filled with a liquid. The most common liquids used are water and oil. One end of the U-tube is open to the atmosphere, and the other end is connected to the pressure to be measured (fig. 11-9). The liquid reacts to the amount of pressure exerted on it and moves up or down within the tube. The amount of pressure is determined by matching the liquid level against a scale within the manometer. TEMPERATURE   MEASURING DEVICES Temperature   is   one   of   the   basic   engineering variables.  Therefore,  temperature  measurement  is essential  to  the  proper  operation  of  a  shipboard engineering plant. As a watch stander, you will use both mechanical  and  electrical  instruments  to  monitor temperature levels. You will frequently be called on to measure   the   temperature   of   steam,   water,   fuel, lubricating oil, and other vital fluids. In many cases, you will  enter  the  results  of  measurements  in  engineering logs and records. THERMOMETERS  (MECHANICAL) Mechanical  devices  used  to  measure  temperature are  classified  in  various  ways.  In  this  section,  we  will discuss  only  the  expansion  thermometer  types. Expansion  thermometers  operate  on  the  principle  that the expansion of solids, liquids, and gases has a known relationship  to  temperature  change.  The  following  types of expansion thermometers are discussed in this section: .  Liquid-in-glass  thermometers s  Bimetallic  expansion  thermometers .  Filled-system  thermometers Liquid-in-Glass Thermometers Liquid-in-glass   thermometers   are   the   oldest, simplest,  and  most  widely  used  devices  for  measuring temperature. A liquid-in-glass thermometer (fig. 11-10) Figure  11-10.—Liquid-in-glass  thermometer. 11-5


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