Fig. 5. Specific Gravity Cup for Hot Pulp.

The figures obtained in the second column of [Table 7] (under the heading “Pouring at boiling temperature”) show the error of this method with carefully calibrated apparatus and working under the best conditions. By comparison with the first column, it will be noted that the results are always low, and that the difference between individual determinations is so great that a correction factor cannot be established. It should be borne in mind that these results were obtained by chemists. When the method is employed even by careful operators in the plant, still greater discrepancies may be expected.

Table 7.—Comparison of Different Methods of Determining Specific Gravity [18]

Sample NumberSpecific gravity by different methods of filling cup or flask.
Centrifuging
at 68°F. 		Pouring cold and whirling by hand Pouring at boiling temperature Dipping at boiling temperature
1477 1.0610... 1.0464...
Do 1.0610... 1.0449...
Do...... 1.0600...
1484 1.0423 1.0330 1.0380...
1485 1.0347... 1.0336...
1483 1.0464 1.0437 1.042...
Do... 1.0420 1.0446...
Do... 1.0430......
Do... 1.0442......
1482 1.0441 1.0416 1.0360...
Do 1.0444 1.0419 1.0410...
Do 1.0447 1.0424 1.0413...
1481 1.0449 1.0410 1.040...
Do 1.0449 1.0410 1.041...
Do... 1.0418 1.0397...
1480... 1.0430 1.036...
Do... 1.0420 1.040...
Do... 1.0429 1.044...
Do...... 1.0407...
1496 1.0340 1.0330......
Do 1.0341 1.0326......
Do... 1.0326......
Do... 1.0346... 1.0299
Do... 1.0341... 1.0303
Do... 1.0341... 1.0343
Do......... 1.033
1515 1.0351...... 1.036
Do 1.0352...... 1.035
1519 1.0380...... 1.0377
Do......... 1.0383
Do......... 1.0329
Do......... 1.0350
1521 1.0440... 1.0410 1.0439
Do...... 1.0439 1.0448
1522 1.0500... 1.0428 1.0493
Do...... 1.0455 1.0508
1524 1.0519...... 1.0504 [46]
1524......... 1.0529
Do......... 1.0510
1526 1.0519... 1.0472 1.0529
Do...... 1.0463 1.0525
1528 1.0519... 1.0509 1.0514
Do...... 1.0485 1.0514
1530 1.0252... 1.0281 1.0260
Do......... 1.0264
Do......... 1.0269
1531 1.0291... 1.0312 1.0294
Do......... 1.0311
Do......... 1.0313

It was thought that better results might be secured by modifying the construction of a cup in such a manner as to permit it to be filled by dipping below the surface of the pulp in the kettle. A bail made of 3/16-inch wire was, therefore, soldered to the opposite side of the cup (see Fig. 5). By means of the bail the cup was lowered into the kettle. After it was filled with the pulp the attempt was made to remove air bubbles by repeatedly giving the bail a quick twist or circular motion with a sudden stop. The cup was then brought quickly to the surface of the kettle and “struck off” with a straight edge, the outside of the cup and bail washed quickly with water, dried, and the cup and contents weighed.

In using this method the steam is turned off, and as soon as the foam subsides the cup is sunk well below the surface of the pulp. At this time the heat in various portions of the kettle is of course uniform, by reason of the thorough mixture caused by the vigorous boiling. Owing to the large mass of rather viscous material, and the heat of the kettle itself, the contents of the kettle cool slowly, and even after 10 minutes the temperature does not decrease more than 1° F., except at the very surface of the pulp. As a result of several observations, it was found that a thermometer bulb held 3 inches below the surface of the pulp showed a lowering of temperature of not more than 1° F. in 10 minutes and a lowering of only 0.5° F. in from 5 to 7 minutes.

The bail employed was about 6½ inches wide and 8 inches long. There was some difficulty, owing to the pulp spattering on the hands of the operator because of the air escaping from the cup. This might be diminished by the use of a longer bail, or by wearing suitable gloves. When evaporating tanks are used it will probably be necessary to attach the bail to a stick or support of some kind. In addition to permitting this method of filling, the bail has the additional advantage that the cup full of pulp may be handled for washing and conveying to the balance much more conveniently and with less danger of spilling than with the handle on the side of the cup. Again, the bail does not heat when the cup is filled with hot pulp, and for that reason is easier to handle.

(d) HYDROMETER METHOD

Hydrometers are of little value in determining the specific gravity of tomato pulp. With cold pulp they cannot be used at all. With hot pulp a relatively slender hydrometer comes to rest and readings can be taken with more or less accuracy. The value of the reading is relative to the specific gravity of the pulp and varies with the shape of the hydrometer and with the character of the pulp. It is necessary therefore to obtain the relation between the reading of the hydrometer in the hot pulp and the specific gravity (obtained by an accurate method) of the same pulp cooled without evaporation. In the hands of a careful operator some manufacturers have found hydrometers (used with hot pulp) helpful in making pulp of uniform specific gravity.

The hydrometer gives much more accurate results with the filtrate of pulp. As shown on page 31, there is a direct relation between the specific gravity of tomato pulp and of the liquor obtained by filtering or straining the same, so that when the specific gravity of the latter is known that of the former may be ascertained readily by means of a table. This method is peculiarly applicable to the examination of cyclone juice and light pulp from which the insoluble solids may be removed quickly by straining through a cloth, and it therefore affords the most rapid method that is available to the average factory for determining the specific gravity of cyclone juice.