PRINCIPLES OF NAVAL ENGINEERING 



MEASUREMENT OF SPECIFIC GRAVITY 



The specific gravity of a substance is defined 

 as the ratio of the density of the substance to 

 the density of a standard substance. The stand- 

 ard of density for liquids and solids is pure 

 water; for gases, the standard is air. Each 

 standard (water or air) is considered to have a 

 specific gravity of 1.00 under standard condi- 

 tions of pressure and temperature. -"^^ For a 

 solid or a liquid substance, then, we may say 

 that 



Sp. gr. of solid or liquid= 



density of substance 

 density of water 



Density is sometimes defined as the mass 

 per unit volume of a substance and sometimes 

 as weight per unit volume. In engineering, 

 fortunately, this difference in defintions rarely 

 causes confusion because we are usually inter- 

 ested in relative densities— or, in other words, 

 in specific gravity. Since specific gravity is the 

 ratio of two densities, it really does not matter 

 whether we use mass densities or weight den- 

 sities; the units cancel out and give us a pure 

 number which is independent of the system of 

 units used. 



Aboard ship, it is sometimes necessary to 

 measure the specific gravity of various liquids. 

 This is usually done by using a device called a 

 hydrometer . A hydrometer measures specific 

 gravity by comparing the buoyancy (or loss of 

 weight) of an object in water with the buoyancy 

 of the same object in the liquid being measured. 

 Since the buoyancy of an object is directly re- 

 lated to the density of the liquid, then 



Sp. gr. of liquid: 



buoyant force of liquid 

 buoyant force of water 



"built in" by the calibration of the hydrom- 

 eter. 



For fuel oil and other petroleum products, 

 it is customary to measure degrees API, rather 

 than specific gravity, in accordance with a scale 

 developed by the American Petroleum Institute. 

 The relationship between specific gravity and 

 API gravity is given by the formula 



Sp. gr.= 



141.5 



131.5 -t- degrees API 



A hydrometer of the type normally used 

 aboard ship to measure the degrees API of fuel 

 oil is shown in figure 7-37. The major differ- 

 ence between this hydrometer and others used 

 aboard ship is that this one is calibrated to read 

 degrees API rather than specific gravity. 



MEASUREMENT OF VISCOSITY 



The viscosity of a liquid is a measure of its 

 resistance to flow. A liquid is said to have high 

 viscosity if it flows sluggishly, like cold mo- 

 lasses. It is said to have low viscosity if it 

 flows freely, like water. The viscosity of most 

 liquids is greatly affected by temperature; in 

 general, liquids are less viscous at higher tem- 

 peratures. 



A hydrometer is merely a calibrated rod 

 which is weighted at one end so that it floats in 

 a vertical position in the liquid being measured. 

 Hydrometers are calibrated in such a way that 

 the specific gravity of the liquid may be read 

 directly from the scale; in other words, the 

 comparison between the density of the liquid 

 being measured and the density of water is 



13 



For most engineering purposes, the standard pres- 

 sure and temperature conditions for water as a 

 standard of specific gravity are atmospheric pressure 

 and 60° F. 



4.135 



Figure 7-37.— Hydrometer. 



150 



