PRINCIPLES OF NAVAL ENGINEERING 



elements. 1^ There are two major classes of 

 mechanical pressure elements: (1) liquid-col- 

 umn elements, and (2) elastic elements. 



Liquid-Column Elements 



Liquid-column pressure measuring elements 

 include the devices commonly referred to as 

 barometers and manometers. Liquid-column 

 elements are simple, reliable, and accurate. 

 They are used particularly (although not exclu- 

 sively) for the measurement of relatively low 

 pressures or small pressure differentials. 

 Liquids commonly used in this type of pressure 

 gage include mercury, water, and alcohol. 



One of the simplest kinds of liquid-column 

 elements is the fixed-cistern barometer (fig. 7- 

 14) which is used to measure atmospheric pres- 

 sure. Mercury is always used as the liquid in 

 this type of instrument. Atmospheric pressure 

 acts upon the open surface of the mercury in 

 the cistern. Since the tube is open at the cistern 

 end, and since there is a vacuum above the 

 mercury in the tube, the height of the mercury 

 in the tube is at all times an indication of the 

 existing atmospheric (barometric) pressure. 



A simple U-tube liquid-column element for 

 measuring absolute pressure is shown in figure 

 7-15. The liquid used in this device is mercury. 

 There is a vacuum above the mercury at the 

 closed end of the tube; the open end of the tube 

 is exposed to the pressure to be measured. The 

 absolute pressure is indicated by the difference 

 in the height of the two mercury columns. 



Manometers are available in many different 

 sizes and designs. Some are installed in such 

 a way that the U-tube is readily recognizable, 

 as in part A of figure 7-16; but in some designs 

 the U-tube is inverted or inclined at an angle. 

 The so-called single-tube or straight-tube 

 manometer (part B of fig. 7-16) is actually a 

 U-tube in which only one leg is made of glass. 



VACUUM . 



rr^ 



MERCURY- 



ATMOSPHERIC 

 PRESSURE 



ATMOSPHERIC 



PRESSURE IN 



INCHES OF 



MERCURY 



30 



25 



10 



69.86(147B) 

 Figure 7-14.— Simple Barometer (fixed cistern) 

 for measuring atmospheric pressure. 



gages. Bourdon-tube elements are suitable for 

 the measurement of very high pressures, up to 

 100,000 psig. The upper limit for bellows ele- 

 ments is about 800 psig and for diaphragm ele- 

 ments about 400 psig. Diaphragm elements and 

 bellows elements are commonly used for the 

 measurement of very high vacuum (or very low 

 absolute pressure) but Bourdon tubes can be 

 used for such applications. 



Elastic Elements 



Elastic elements used for pressure mea- 

 surement include Bourdon tubes, bellows, and 

 diaphragms. All three types of elastic elements 

 are suitable for use in pressure gages, vacuum 

 gages, and compound (both pressure and vacuum) 



Strain gages and other electrical pressure measur- 

 ing devices are not included in this discussion; they 

 are rarely used aboard ship. 



BOURDON- TUBE ELASTIC ELEMENTS.- 

 Bourdon-tube elements used in pressure gages 

 are essentially the same as those described for 

 use in filled-system thermometers. Bourdon 

 tubes for pressure gages are made of brass, 

 phosphor bronze, stainless steel, beryllium- 

 copper, or other metals, depending upon the 

 requirements of service. 



Bourdon-tube pressure gages are often clas- 

 sified as simplex or duplex, depending upon 

 whether they measure one pressure or two. A 

 simplex gage such as the one shown in figures 



138 



