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TEMPERATURE   MEASURING DEVICES
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Figure 11-11.—Effect of unequal expansion of a bimetallic strip. has a bulb and a very fine-bore capillary tube. The tube contains alcohol or some other liquid that uniformly expands  or  contracts  as  the  temperature  rises  or  falls. The selection of liquid is based on the temperature range for which the thermometer is to be used. Almost  all  liquid-in-glass  thermometers  are  sealed so atmospheric pressure does not affect the reading. The space above the liquid in this type of thermometer may be a vacuum, or this space maybe filled with an inert gas, such as nitrogen, argon, or carbon dioxide. The capillary bore may be round or elliptical. In either case, it is very small; therefore, a relatively small expansion or contraction of the liquid causes a relatively large change in the position of the liquid in the capillary tube.  Although  the  capillary  bore  has  a  very  small diameter, the walls of the capillary tube are quite thick. Most  liquid-in-glass  thermometers  have  an  expansion chamber at the top of the bore to provide a margin of safety  for  the  instrument  if  it  should  accidentally overheat. Liquid-in-glass   thermometers   may   have graduations etched directly on the glass stem or placed on  a  separate  strip  of  material  located  behind  the  stem. Many  thermometers  used  in  shipboard  engineering plants  have  the  graduations  marked  on  a  separate  strip because this type is generally easier to read. You  will  find  liquid-in-glass  thermometers  in  use  in the oil and water test lab for analytical tests on fuel, oil, and  water. Bimetallic  Expansion  Thermometers Bimetallic expansion thermometers make use of different  metals  having  different  coefficients  of  linear expansion.  The  essential  element  in  a  bimetallic expansion thermometer is a bimetallic strip consisting of  two  layers  of  different  metals  fused  together.  When such a strip is subjected to temperature changes, one layer expands or contracts more than the other, thus tending to change the curvature of the strip. Figure   11-11   shows   the   basic   principle   of   a bimetallic expansion thermometer. One end of a straight bimetallic strip is fixed in place. As the strip is heated, the other end tends to curve away from the side that has the greater coefficient of linear expansion. When used in thermometers, the bimetallic strip is normally wound into a flat spiral (fig. 11-12), a single helix, or a multiple helix. The end of the strip that is not fixed in position is fastened to the end of a pointer that moves over a circular scale. Bimetallic thermometers are easily adapted for use as recording thermometers; a pen is attached to the pointer and positioned so that it marks on a revolving chart. Filled-System   Thermometers Generally,  filled-system  thermometers  are  used  in locations  where  the  indicating  part  of  the  instrument must be placed some distance away from the point where the temperature is to be measured. For this reason, they are often called distant-reading thermometers. However,   this   is   not   true   for   filled-system thermometers. In some designs, the capillary tubing is very  short  or  nonexistent.  Generally,  however, filled-system   thermometers   are   distant-reading thermometers.  Some  distant-reading  thermometers have capillaries as long as 125 feet. Figure 11-12.—Bimetallic thermometer (flat, spiral strip). 11-6






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