Principles of naval engineering - Historic Naval Ships Association

.PRINCIPLES OF NAVAL ENGINEERINGelements. 1^ There are two major classes ofmechanical pressure elements: (1) liquid-columnelements, and (2) elastic elements.Liquid-Column ElementsVACUUMrr^Liquid-column pressure measuring elementsinclude the devices commonly referred to asbarometers and manometers. Liquid-columnelements are simple, reliable, and accurate.They are used particularly (although not exclusively)for the measurement of relatively lowpressures or small pressure differentials.Liquids commonly used in this type of pressuregage include mercury, water, and alcohol.One of the simplest kinds of liquid-columnelements is the fixed-cistern barometer (fig. 7-14) which is used to measure atmospheric pressure.Mercury is always used as the liquid inthis type of instrument. Atmospheric pressureacts upon the open surface of the mercury inthe cistern. Since the tube is open at the cisternend, and since there is a vacuum above themercury in the tube, the height of the mercuryin the tube is at all times an indication of theexisting atmospheric (barometric) pressure.A simple U-tube liquid-column element formeasuring absolute pressure is shown in figure7-15. The liquid used in this device is mercury.There is a vacuum above the mercury at theclosed end of the tube; the open end of the tubeis exposed to the pressure to be measured. Theabsolute pressure is indicated by the differencein the height of the two mercury columns.Manometers are available in many differentsizes and designs. Some are installed in sucha way that the U-tube is readily recognizable,as in part A of figure 7-16; but in some designsthe U-tube is inverted or inclined at an angle.The so-called single-tube or straight-tubemanometer (part B of fig. 7-16) is actually aU-tube in which only one leg is made of glass.MERCURY-ATMOSPHERICPRESSUREATMOSPHERICPRESSURE ININCHES OFMERCURY3025