Chapter 7-PRINCIPLES OF MEASUREMENT 



small pressure differences. Both metallic and 

 nonmetallic diaphragms are in common use. 



Metallic diaphragms are made from stain- 

 less steel, phosphor bronze, brass, or other 

 metal. A metallic diaphragm element may con- 

 sist of one or more capsules. Each capsule 

 consists of two diaphragm shells (flat or corru- 

 gated circular disks) which are welded, brazed, 

 or otherwise firmly fastened together to form 

 the capsule. The capsules are all rigidly con- 

 nected so that the applicationof pressure causes 

 all capsules to deflect. The amount of deflec- 

 tion of a diaphragm gage depends upon the 

 number of capsules, the design and the number 

 of the corrugations, and other factors. 



Nonmetallic diaphragms, also called slack 

 or limp diaphragms, are made of leather, 

 treated cloth, neoprene, or some other soft 

 material. Nonmetallic diaphragms are spring- 

 loaded. One common type of nonmetallic dia- 

 phragm pressure gage is shown in figure 7-27. 

 When pressure is applied to the underside of 

 the slack diaphragm, the diaphragm moves up- 

 ward, although it is opposed by the action of the 

 calibrating spring. As the spring moves, the 

 linkage system causes the pointer to move to a 

 higher reading. Thus the reading on the scale 

 is proportional to the amount of pressure ex- 

 erted on the diaphragm, even though the move- 

 ment of the diaphragm is opposed by the cali- 

 brating spring. 



SCALE 



HAIRSPRING 



CONNECTING 

 LINK 



PRESSURE 

 CONNECTION 



PRESSURE GAGE INSTALLATION 



Bourdon tube pressure gages used for steam 

 service are always installed in such a way that 

 the steam cannot actually enter the gage. This 

 type of installation is necessary to protect the 

 Bourdon type from very high temperatures. An 

 exposed uninsulated coil is provided in the line 

 leading to the gage, and the steam condenses 

 into water in this exposed coil. Thus there 

 is always a condensate seal between the gage 

 and the steam line. 



Pressure gage connections are normally 

 made to the top of the pressure line or to the 

 highest point on the machinery in which the 

 pressure is to be measured. Pressure gages 

 are usually mounted on flat-surfaced gage 

 boards in such a way as to minimize virbration; 

 this is a matter of considerable importance, 

 since some ships experience very great struc- 

 tural vibration from screws and machinery. 

 Efforts are currently beingmade to design gages 

 capable of withstanding any vibration that may 



38.212(147B) 

 Figure 7-27.— Nonmetallic diaphragm 

 pressure gage, 



be expected from machinery. Pressure gages 

 designed to withstand shock and vibration fre- 

 quently use small size capillary tubing between 

 the connections and the elastic elements to 

 protect the gage mechanism and the pointer; 

 small size tubing is used between the test con- 

 nection or gage valve and the gage so that piping 

 deflections will not cause errors in the gage 

 readings, 



MEASUREMENT OF FLUID FLOW 



A great many devices, many of them quite 

 ingenious, have been developedfor the measure- 

 ment of fluid flow. The discussion here is con- 

 cerned primarily with the types of fluid flow 

 measuring devices that find relatively wide 

 application in shipboard engineering. These 



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