Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGMEASUREMENT OF SPECIFIC GRAVITYThe specific gravity of a substance is definedas the ratio of the density of the substance tothe density of a standard substance. The standardof density for liquids and solids is purewater; for gases, the standard is air. Eachstandard (water or air) is considered to have aspecific gravity of 1.00 under standard conditionsof pressure and temperature. -"^^ For asolid or a liquid substance, then, we may saythatSp. gr. of solid or liquid=density of substancedensity of waterDensity is sometimes defined as the massper unit volume of a substance and sometimesas weight per unit volume. In engineering,fortunately, this difference in defintions rarelycauses confusion because we are usually interestedin relative densities— or, in other words,in specific gravity. Since specific gravity is theratio of two densities, it really does not matterwhether we use mass densities or weight densities;the units cancel out and give us a purenumber which is independent of the system ofunits used.Aboard ship, it is sometimes necessary tomeasure the specific gravity of various liquids.This is usually done by using a device called ahydrometer . A hydrometer measures specificgravity by comparing the buoyancy (or loss ofweight) of an object in water with the buoyancyof the same object in the liquid being measured.Since the buoyancy of an object is directly relatedto the density of the liquid, then"built in" by the calibration of the hydrometer.For fuel oil and other petroleum products,it is customary to measure degrees API, ratherthan specific gravity, in accordance with a scaledeveloped by the American Petroleum Institute.The relationship between specific gravity andAPI gravity is given by the formulaSp.gr.=141.5131.5 -t- degrees APIA hydrometer of the type normally usedaboard ship to measure the degrees API of fueloil is shown in figure 7-37. The major differencebetween this hydrometer and others usedaboard ship is that this one is calibrated to readdegrees API rather than specific gravity.MEASUREMENT OF VISCOSITYThe viscosity of a liquid is a measure of itsresistance to flow. A liquid is said to have highviscosity if it flows sluggishly, like cold molasses.It is said to have low viscosity if itflows freely, like water. The viscosity of mostliquids is greatly affected by temperature; ingeneral, liquids are less viscous at higher temperatures.Sp. gr. of liquid:buoyant force of liquidbuoyant force of waterA hydrometer is merely a calibrated rodwhich is weighted at one end so that it floats ina vertical position in the liquid being measured.Hydrometers are calibrated in such a way thatthe specific gravity of the liquid may be readdirectly from the scale; in other words, thecomparison between the density of the liquidbeing measured and the density of water is13 For most engineering purposes, the standard pressureand temperature conditions for water as a4.135standard of specific gravity are atmospheric pressureand 60° F. Figure 7-37.— Hydrometer.150