CHAPTER7 



PRINCIPLES OF MEASUREMENT 



Measurement is, in a very real sense, the 

 language of engineers. The shipboard engineer- 

 ing plant contains an enormous number of gages 

 and instruments that tell operating personnel 

 whether the plant is running properly or whether 

 some abnormal condition— excessive speed, high 

 pressure, low pressure, high temperature, low 

 water level— requires corrective action. The 

 gages and instruments also provide essential 

 information for the hourly, daily, and weekly 

 entries for station operating logs and for other 

 engineering records and reports. 



This chapter describes some of the basic 

 types of gages and instruments used in ship- 

 board engineering plants for the measurement of 

 important variables such as temperature, pres- 

 sure, fluid flow, liquid level, and rotational 

 speed. Because of the wide variety of gages and 

 instruments used in connection with shipboard 

 engineering equipment, no attempt is made to 

 cover all types that might possibly be encoun- 

 tered; instead, basic principles of measurement 

 and commonly used types of gages are empha- 

 sized. Unusual or highly specialized measuring 

 devices, or ones that have particular application 

 to some one type of rnachinery or equipment 

 aboard ship, are in general discussed in the 

 chapters of this text that deal with the particu- 

 lar equipment; where an unusual type of meas- 

 uring device j^ discussed in this chapter, it is 

 included chiefly as a means of bringing out 

 some interesting or important aspect of mea- 

 surement. Detailed information on most gages 

 and instruments used aboard ship can be obtained 

 from manufacturers' technical manuals and 

 other instructional materials furnished with 

 shipboard engineering equipment. 



THE CONCEPT OF MEASUREMENT 



One of the primary ways in which we extend 

 our knowledge and understanding of the universe 



and of the world around us is by the measure- 

 ment of various quantities. Because we live in 

 a world in which practically everything seems 

 to be in some way measured or counted, we 

 often tend to assume that measurement is 

 basically simple. In reality, however, it maybe 

 quite difficult to develop an appropriate mode of 

 measurement even after we have recognized the 

 need; and, without an appropriate mode of 

 measurement, we may even fail to recognize 

 the significance of the phenomena we observe. 

 Thus the development of scientific and engineer- 

 ing principles has been, and undoubtedly will 

 continue to be, inextricably tied to the concept 

 of measurement. 



Many of our views on the nature of things 

 are profoundly influenced by the procedures we 

 devise for measurement. It is interesting to 

 note how often in the history of science the 

 application of a new instrument or the refine- 

 ment of a measuring technique has led to new 

 ideas about the universe or about the nature of 

 the thing being measured.! Until approximately 

 the middle of the seventeenth century, it was 

 commonly believed that water rose in a suction 

 pump because "nature abhors a vacuum. "2 

 The concept of a "sea of air" surrounding the 



As Sir Humphry Davy (1778-1829) stated, "Nothing 

 tends so much to the advancement of knowledge as the 

 application of a new instrument." (Quoted in Harvard 

 Case Histories in Experimental Science, James Bryant 

 Conant, general editor, and Leonard K. Nash, associate 

 editor, Cambridge, Massachusetts, 1957. Vol. l.page 

 119.) 



9 



"'The explanation that "nature abhors a vacuum "per- 

 sisted for quite some time in spite of the observed 

 fact that water would not rise more than about 32 

 feet in the suction pumps of the time and in spite of 

 Galileo's observation that "Evidently nature's horror 

 of a vacuum does not extend beyond 32 feet." 



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