comparison, and then converting this into degrees Baumé. This roundabout method once again emphasizes the uselessness of employing the Baumé scale. If the moisture content of the oil has been ascertained, a computation is then made in order to arrive at the actual specific gravity or Baumé reading for the moisture free oil. The Method of the Westphal Balance for Exact Measurement. Let us then examine in detail such a method. The Westphal FIG. 150.-A commercial balance for determining specific gravity of oil. The common hydrometer is not of sufficient accuracy to determine the specific gravity of oil used in fuel oil tests. A simple and accurate method for such determination is accomplished by the employment of a Westphal Balance as shown in the illustration. The specific gravity is first ascertained by comparison of the oil with a water standard and then by means of the mathematical relationship connecting specific gravities and Baumé readings, the latter gravity reading is ascertained. balance is a convenient and accurate method by which the specific gravity of fuel oil may be obtained to four decimal points. As shown in Fig. 150, the apparatus necessary consists of a balance arm, supported on knife edges, from one end of which is hung a glass bulb, the other end being counter-weighted. Along the balance arm are nine notches, the hook supporting the glass bulb being in the position of the tenth notch. The glass bulb has a displacement of exactly five grams of pure water at 4°C., which is the point of maximum density of water, the density for which scientific gravity comparisons are made. Hence if the bulb above described were so immersed in water at 4°C. a five gram weight would establish equilibrium if hung from the hook. This would indicate a specific gravity of 1.0000. The zero point of the balance is adjusted by turning a thumb screw, which forms one point of the three point support shown in the figure, until the pointers are opposite each other before the bulb is immersed. For specific gravities less than 1.0000 the five gram rider called the unit weight is hung in a notch such that equilibrium is nearly reached, never exceeded. This gives the first decimal place. The 110, 100, and 1000 unit weights are then hung respectively in notches so that equilibrium is finally established. The specific gravity is then read directly to four decimal places by noting the notches in which the riders hang, commencing with the largest rider. Thus when the unit weight hangs in the ninth notch, the 10 weight in the sixth notch, the 100 weight in the seventh notch, and the 11000 weight in the third notch, the specific gravity is evidently 0.9673. Details of Procedure. Before proceeding with a gravity determination, the oil sample should be allowed to stand in the laboratory several hours in order that any drops of water in the oil may settle. A small quantity is then poured from the sample can into a suitable glass jar. The Westphal balance, having been dusted with a soft brush, is then adjusted to equilibrium and the specific gravity of the sample obtained. The temperature is also ascertained by means of the thermometer inserted in the oil sample. Since specific gravities of fuel oil are by common practice referred to at a temperature of 60°F., it is now necessary to make a second determination at a temperature differing by 15° to 20°F. from the first, in order that we may have sufficient data with which to compute what the gravity would be at 60°F. temperature. To take this second reading the temperature of the sample in the jar may be raised by immersion in a water bath. In doing this great care must be taken to allow no water to get into the oil. Computations Involved. Let us next illustrate the computations involved in a gravity determination. Let us assume that by means of the Westphal balance, the oil sample is seen to have a specific gravity (S1) of 0.9644, at a temperature (t1) of 68.9°F., and a specific gravity (S2) of 0.9587 at a temperature (t2) of 86.6°F. Since the specific gravity has changed (S1 - S2) over a tempera(S1-S2) ture change of (t1 t2) the change for 1°F. would be (t1-12) This change in specific gravity for 1°F. is the coefficient of expansion (C.), for the oil and may be expressed by the formula The coefficient is thus seen in this case to represent an intermediate value, for in practice we find that in different oils C. varies from (-0.00027) to (-0.00042). From the fundamental definition of the coefficient of expansion it is now seen that at 60°F., the specific gravity becomes SS1+C.(60-t1) = (5) Consequently by making the proper substitutions for the case cited we find that the numerical value of the specific gravity of this oil sample for 60°F. is = 0.9673 S = 0.9644+ [− 0.000322 X ( - 8.9)] In order to convert this specific gravity to the Baumé scale we now, by substituting in formula given above for such conversion, find that Assuming that this particular oil sample has been found to contain 0.5 per cent. by weight of moisture and 0.484 per cent. by volume, let us now see how we should find the specific gravity of the dry oil. Let V represent the percentage of water by volume and Sw, So, Sm represent respectively the specific gravity of the water, dry oil, and moisture. Then we may write the following relationship: 20 Sm (6) From scientific tables we find that S at 60°F. has a value of 0.9990, and from the Westphal balance Sm has been found to be 0.9673. By transforming the formula above it is seen that If it is desirable to ascertain the Baumé reading for the dry oil, we next ascertain its value from the above relationship of specific gravity and the Baumé scale from equation (2). According to formula (3) this Baumé reading would of course be computed as follows: When a large quantity of oil is to be purchased and it is desirable to carry the Baumé reading to still further decimal points, the two formulas will not of course check; hence, one or the other of these formulas should be agreed upon prior to a purchase of any magnitude. CHAPTER XXVII MOISTURE CONTENTS OF OILS From our previous discussion of steam generation in the modern central station it was found that something over a thousand heat units are necessary to convert one pound of water at ordinary temperatures into saturated steam. When moisture appears in the oil used for heat generating purposes in the furnace it is evident, then, that large heat losses may thereby be involved. For, not only must this moisture be converted into saturated steam, but this steam itself must be superheated to the temperature of the outgoing chimney gases, thus dissipating energies that should go toward steam generation in the boiler. FIG. 151.-An electrically driven oil centrifuge. In this centrifuge the four arms-two plain and two grad uated-are caused to rotate by electric power and the water thus caused to separate from urement of the moisture pres the oil. The consequent meas ent is then easily ascertained. Hence the water involved in fuel oil composition is a dead loss which should. be avoided as far as possible. Settling tanks accomplish much in drawing off the water content, but when the water appears in the oil as an emulsion it is almost impossible to commercially segregate it from the oil. Since, then, all fuel oils contain a certain amount of moisture, the careful determination of its exact proportions often becomes an important problem in efficient steam engineering performance. Summary of Methods Employed in Determining the Moisture Content. There are ten methods by which the moisture content of oil can be ascertained with approximate accuracy. For detailed information on this subject the reader is referred to Technical Paper No. 25 of the United States Bureau of Mines entitled, "Methods for the Determination of Water in Petroleum and its Products." These methods may be briefly summarized as follows: The moisture content of heavy oils and greases may be approximately ascertained by the loss of weight due to heating. |