SPECIFIC GRAVITY Fuel oils are commonly sold and described as of a certain specific gravity or else as of a certain degree Baume. Through out the oil-burner' industry the Baume reading is generally used. The specific gravity of fuel oil is the relation by weight of a given volume of distilled water to the same volume of fuel oil when both are weighed at a temperature of 60° F. The specific gravity of fuel oil can be determined by the hydrostatic balance, by hydronieters, and by the specific gravity bottle. Throughout the oil industry the gravity as determined by the hydrometer is uni versally referred to. The principle of operation of the hydrom eter is based on the law that a solid body floating in a liquid will displace a quantity of the liquid equal in weight to the floating body. Hence a body of constant weight and proportion will al ways sink to the same extent into a liquid of a certain density and will sink to a greater or less extent as the density decreases or increases. Because when a liquid expands or contracts with tem perature, the density of the liquid varies accordingly, therefore, when the hydrometer is constructed the scale must be standard ized for a certain temperature. As it is not always convenient to have the liquid at the temperature for which the scale of the hydrometer is arranged, it is often necessary to apply a correc tion for temperature variation.
The standard hydrometer in the oil industry was evolved by Baume. Baurne's hydrometer has an arbitrary scale. For liquids lighter than water, Baume took for zero the point on the stem to which the hydrometer sank in a solution parts of salt and 90 of water. For the point 10 in the scale he took the level to which the hydrometer sank in distilled water. The space between the two marks he divided into 10 equal parts and called each space a defzrec and he continued the scale with the same inter vats between the marks. The proper method of manipulating a hydrometer must be adhered to if accurate results are desired.
The following instructions are in line with those given by the U. S. Bureau of Standards.
It is essential that before it is used the hydrometer shall be thor oughly cleaned and dried. The liquids to be tested should be contained in clear, smooth, glass vessels of suitable size and shape. Thorough mixing of the liquids is requisite, before the hydrotn eter test is made, by means of a stirrer that reaches to the bottom of the vessel, so that! the liquid will be uniform in density and temperature throughout. A perforated disc or a spiral at the end of a sufficiently long rod will give the best results as the up and down motion serves to disperse layers of the liquid of dif ferent density. The temperature of the surrounding atmosphere
should be taken into account also and the temperature of the liquid being tested should be the same as the atmosphere, as otherwise its temperature will be changing during the test, thus causing not only differences in density, but also doubt as to the actual temperature. The temperature of the hydronieter itself should also be the same as that of the liquids being tested. When immersing the hydrometer it should be slowly sunk into the liquid slightly beyond the point where it floats naturally and then al lowed to float freely. Surface tension effects on hydrometer observation are a consequence of the downward force exerted on the stem by the curved surface of the liquid or'"meniscus" which rises on the stem and which affects the depth of immersion and consequent scale reading. The liquid for which the hydrometer is intended must be specified, therefore, because a hydrometer will indicate differently in two liquids having the same density, but different surface tensions. Hydrometers may be compared with each other if they are of equivalent dimensions, however, even if the liquid used differs in surface tension from the specified liquid, but comparisons of dissimilar instruments, in such liquid, must be corrected for the effect of surface tension. Spontaneous changes in surface tension occur in many liquids, due to the for mation of surface films of impurities, which may come from the apparatus, the liquid, or the air. Errors from this source may be avoided by the purification of the surface by overflowing imme diately before making the observation. Air bubbles must be allowed to disappear from the surface of the liquid before taking the scale reading. In reading the hydrometer scale, the eye is brought to the height of the level' surface of the liquid and the point on the scale read, which appears to coincide with the level surface. In reading the thermometer scale, the errors of parallax are avoided by so placing the eye that near the'end of the mercury column the portions on either side of the stem and that seen through the capillary appear to lie in a straight line. (See fig. 7.) The line of sight is then normal to the stern. The readings of the Baume hydrometer may be changed to those of absolute specific gravity as determined by a hydrostatic balance by the following formulas which hold for oil and for all other liquids lighter than water.