As an illustration of this there are given below a series of figures illustrating the beginning of the table that could be constructed from Figure 4. If a large sheet of coordinate paper be taken the line shown in Figure 4 may be extended so that a table may be constructed for pulp of all concentrations.
| Weight of
flask and pulp | Specific gravity |
| 180.25 | 1.0300 |
| 180.31 | 1.0305 |
| 180.37 | 1.0310 |
| 180.43 | 1.0315 |
| 180.49 | 1.0320 |
| 180.55 | 1.0325 |
| 180.61 | 1.0330 |
| 180.67 | 1.0335 |
The highest degree of accuracy can be secured by filling the flask and making the weighing at exactly 68° F. This is obviously not practicable under factory conditions, however, and satisfactory results can be secured by taking the temperature of the pulp at the time of weighing and correcting for temperature by the use of [Table 8]. This table gives the correction to be added to the specific gravity when the pulp is taken at temperatures between 68° and 86° F., and the correction to be deducted from the specific gravity for temperatures, between 55° and 68° F. As a matter of principle, correction factors should be avoided as far as practicable, and the smaller the correction factor the more accurate the results will be. This table will be found especially useful in determining the specific gravity of the partly concentrated pulp, as is directed on page 50.
As stated above, in determining specific gravity by this method it is advisable that the reading be made to the second place of decimals. For this purpose an assay pulp balance is suggested. An assay pulp balance carrying a maximum load of 300 grams is listed by dealers in chemical apparatus at $52.50. This balance may be obtained from dealers in chemical apparatus by designating it as “Assay pulp balance E. & A. No. 292, capacity 300 grams.” The same balance, more heavily built, and preferable for that reason, carrying a maximum capacity of 600 grams, is listed at $63.
A satisfactory set of weights, suitable for weighing a cup similar to that shown in Figure 1, may be obtained from any dealer in chemical apparatus by designating it as E. & A. No. 516, “Metric brass weights in wooden box, 200 grams to 1 centigram.” This is listed at $5.50.
For convenience, all of the apparatus necessary for using this method of determining specific gravity is listed below. With the exception of the specific gravity flask this apparatus may be purchased of any dealer in chemical supplies. The specific gravity flasks have not heretofore been available except through this laboratory, which purchased a considerable quantity of them and supplied them to manufacturers of pulp as long as this supply lasted. At the urgent request of the writer, Eimer & Amend and Emil Greiner & Co., both of New York City, have finally stocked this item and stand ready to supply it to those wishing to secure it.
- Specific gravity flask for tomato pulp, of Pyrex glass, 1½ × 6¼ inches (outside measurements), capacity about 125 cc.
- Two-bottle Babcock milk tester, with 2 brass holders, E. & A. No. 1883.
- Assay pulp balance, maximum load 300 grams, E. & A. No. 292.
- Metric brass weights in wooden box, 200 grams to 1 centigram, E. & A. No. 516.
- Chemical thermometer, 50 to 212° F. [17]
(b) COLD PULP WITHOUT CENTRIFUGING
A method frequently employed for determining the specific gravity of cold pulp is to fill the cup by pouring, strike off with a straight edge, wash the outside, dry and weigh. As ordinarily practiced, this determination is attended by considerable error. If the balance is arranged for reading specific gravity directly, weights should be at hand for determining the accuracy of the balance and the weight of the flask, and both should be checked from time to time. The pulp on being poured into the flask or cup carries with it air bubbles to such an extent as to materially reduce the weight. Attempts to remove these air bubbles without the use of a centrifuge have not been successful. This is shown in [Table 7], in the column headed “Pouring cold and whirling by hand.” The figures given in this column were obtained by weighing the sample after it had been whirled vigorously in the cup shown in Fig. 5 until air bubbles appeared to be eliminated. From 50 to 175 revolutions were given the cup in each of the determinations whose results are shown in this column. Even then it will be noted by comparison with Column 1 that the results are low. As the method is ordinarily practiced in the plant, without any attempt to remove the air bubbles by whirling, the results obtained are likely to be less accurate than those shown in the column just mentioned.
(c) SPECIFIC GRAVITY OF HOT PULP