PRINCIPLES OF NAVAL ENGINEERING 



devices may be classified as (1) positive-dis- 

 placement meters, (2) head meters, and (3) area 

 meters. 



POSITIVE-DISPLACEMENT METERS 



Positive-displacement meters are used for 

 measuring liquid flow. In a meter of this type, 

 each cycle or complete revolution of a measur- 

 ing element displaces a definite, fixed volume 

 of liquid. Measuring elements used in positive- 

 displacement meters include disks, pistons, 

 lobes, vanes, and impellers. The motion of these 

 devices may be classified as reciprocating, 

 rotating, oscillating, or nutating, depending upon 

 the type of measuring element used and the 

 general design of the meter read-out or regis- 

 ter. 



A positive-displacement meter of the nutat- 

 ing-piston type is shown in figure 7-28. The 

 flow of oil through the meter causes the piston 

 (also ca disk) to move with a nutating 



motion. Understanding the nature of this motion 

 is the key to understanding the operation of the 

 meter. A nutating motion (fig. 7-29) might be 

 described as a "rocking around" motion; it is 

 similar to the motion of a spun coin just before 

 the coin settles flat on its side. The piston in 

 the meter cannot settle flat on its side like a 

 spun coin, since the piston is nutating (or rock- 

 ing around) on a lower spherical bearing sur- 

 face. 



38.64X 

 Figure 7-28.— Nutating-piston meter for 

 measuring liquid flow. 



38.66X 

 Figure 7-29.— Diagram showing nutating motion 

 of piston and rotary motion of pin in nutating- 

 piston meter. 



The piston cannot rotate because it is held 

 in place by a fixed vane or guide that runs 

 vertically through a slot in the piston. However, 

 the nutating motion of the piston imparts a 

 rotary motion to the pin that projects from the 

 upper spherical surface. The rotary movement 

 of the pin rotates the gears, and the movement 

 of the gears actuates a counting device or regis- 

 ter at the top of the meter. 



Although the action of the nutating piston 

 is smooth and continuous, there is neverthe- 

 less a definite cycle involved in the measure- 

 ment of liquid flow through this meter. The 

 nutating action of the piston seals the measuring 

 chamber off into separate compartments, and 

 these compartments are alternately filled and 

 emptied. The meter is properly classed as a 

 positive-displacement meter, since each com- 

 partment holds a definite volume of the liquid. 



Totalizing meters have a read-out in gallons 

 or pounds of liquid; however, they may also 



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