PRINCIPLES OF NAVAL ENGINEERING 



Liquid-in-glass thermometers may have 

 graduations etched directly on the glass stem 

 or the graduations may be carried on a separate 

 strip of material which is placed behind the 

 stem. Many thermometers used in shipboard 

 engineering plants have the graduations marked 

 on a separate strip, since this type is in general 

 easier to read than the type which has the 

 graduations marked directly on the stem. 



Liquid- in-glass thermometers are made in 

 various designs. The stem may be straight or 

 it may be angled in various ways, depending 

 upon the requirements of service. The thermom- 

 eters may be armored or they may be partially 

 enclosed by a metal case, if such protection is 

 necessary. Several types of angle-stem liquid- 

 in-glass thermometers of the type commonly 

 used aboard ship are shown in figure 7-3. These 

 thermometers are used in 5-inch, 7-inch, and 

 9-inch scale lengths. However, bimetallic ther- 

 mometers are currently being substituted aboard 



-EXPANSION CHAMBER 

 -STEM 



-CAPILLARY BORE 



> GRADUATIONS 



-LIQUID COLUMN 



ship for the 5-inch scale liquid-in-glass ther- 

 mometers. 



Most liquid - in - glass thermometers used 

 aboard ship are provided with wells or separable 

 sockets. The well is installed in the piping sys- 

 tem or equipment where the temperature is to 

 be measured, and the thermometer glass bulb 

 and part of the glass stem are fitted into a thin 

 metal protection tube, packed with a heat- 

 transfer material, and fastened in place in the 

 well. The well is made of metals that will 

 withstand the temperatures, pressures, and 

 fluid velocities without damage; it protects the 

 glass sensing bulb against damage and also 

 eliminates the need for closing down a system 

 or securing a piece of machinery merely in 

 order to replace a thermometer. 



One disadvantage of the well type of instal- 

 lation is that a certain amount of time is re- 

 quired for the thermometer to reach thermal 

 equilibrium with the system in which the tem- 

 perature is being measured. To some extent, 

 the time lag can be decreased by filling the space 

 around the bulb in the well with a heat transfer 

 medium such as graphite. Where rapid response 

 to temperature changes is a vital requirement, 

 however, bare bulb thermometers are used in- 

 stead of the well type of installation. Bare bulb 

 thermometers have very much faster response 

 to changes in temperature, but they cannot be 

 removed for replacement or servicing while 

 the machinery is operating or the line is under 

 pressure. 



45 RECLINED AN6LE 



45 INCLINED 

 ANCLE 



LEFT SIDE ANGLE 



RIGHT SIDE ANGLE 



^==mi^ eoapi=- 



33.11(147A) 

 Figure 7-2. — Liquid-in-glass thermometer. 



61.26 

 Figure 7-3.— Angle-stem liquid-in-glass 



thermometers. 



130 



