=========+==========
Temp. °F.|Correction
---------+----------
190 | .0401
191 | .0406
192 | .0411
193 | .0416
|
194 | .0421
195 | .0426
196 | .0431
197 | .0436
|
198 | .0441
199 | .0447
200 | .0452
201 | .0457
|
202 | .0462
203 | .0466
204 | .0472
205 | .0477
|
206 | .0482
207 | .0487
208 | .0492
209 | .0498
|
210 | .0504
211 | .0510
212 | .0515
---------+----------
With a materially larger cup or flask (which should be of metal) a
heavier balance and heavier weights should be used than suggested on
page 40. In using a specific gravity cup similar to that shown in
Figure 5 but holding about 1,000 grams of pulp an assay pulp balance
with a capacity of 1,500 can be employed, or owing to the increased
accuracy of the larger sample a less accurate and cheaper scale such as
the “Howard trip scale,” or better a box scale such as is listed as E.
& A. 338, may be employed. In working with a cup of this size a set of
weights ranging from 1000 grams to 1 centigram is necessary.
The determination of specific gravity in hot pulp is attended by
considerable error. Even if the flask or cup be carried directly to
the kettle, and filled as quickly as possible, the pulp is materially
cooled in transferring, and by the time the surface is “struck off”
sufficient contraction may occur to increase the weight of the contents
of the flask and cause material error.
When a pail of hot pulp is carried to another room or building for the
determination of specific gravity, the error caused by cooling may be
increased. Again, notwithstanding the fact that the pulp is hot, enough
air bubbles become incorporated into it in pouring into the cup to
make a considerable difference in the weight. These two errors counter
balance each other to some extent, but it is impossible to control the
manipulation with sufficient uniformity to secure satisfactory results.
[Illustration: 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]
=====================================================================
|Specific gravity by different methods of filling cup or flask.
Sample+---------------+---------------+---------------+--------------
Number| Centrifuging | Pouring cold | Pouring at | Dipping at
| at 68°F. | and whirling | boiling | boiling
| | by hand | temperature | 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
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
------+---------------+---------------+---------------+--------------
[18] Table 7 gives the results obtained in the determination of
specific gravity of several samples of pulp by different methods. The
specific gravity given in the first column under the head “Centrifuging
at 68° F.” has been proved to be correct by other analytical methods.
It will be noted that where duplicate determinations are given in this
column they agree with each other very closely. The errors in the other
methods in determining specific gravity are shown in the remaining
columns. It will be noted that the duplicates given in these columns
vary materially from each other.
It should be stated that the results given under the heading “Pouring
cold and whirling by hand” were obtained by much more careful work than
is practicable in the factory. The flasks in which the determination
was made were equipped with a bail, as shown in Fig. 5. page 44, and
the samples were whirled by hand until air bubbles were eliminated
as far as practicable by that method. The results, in this column
are therefore much more accurate than are obtained by the method as
ordinarily practiced.
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.
In Table 8 are given a series of corrections making it possible to use
this method at any temperature between 50 and 80° Fahrenheit. The more
closely the readings are taken to 68° F. the more accurate the results.
Moreover, when it is attempted to strain the insoluble solids from
hot pulp or cyclone juice, considerable evaporation occurs, causing
concentration of the product and producing an error in the results.
When hot pulp is handled, therefore, it must be strained as quickly
as possible, and more accurate results may be obtained if the pulp
is cooled quickly before straining. This may be done by placing in a
large can and stirring vigorously while the can stands in ice water, or
shaking under water in a large flask.
There are several forms of hydrometer which may be used for determining
the specific gravity of the filtrate. The ordinary specific gravity
hydrometer is the most logical form to use, since it gives the specific
gravity directly. Unfortunately, specific gravity hydrometers with the
particular marking required for this work are not a stock article, and
would, therefore, have to be made to order. For this reason they would
be difficult to obtain and not easily replaced if broken.
The Brix hydrometer appears to solve the difficulty. This hydrometer
has no direct relation to specific gravity, but Brix readings can,
of course, be converted to the specific gravity readings by a table
arranged in parallel columns. Table 9 gives the specific gravity of
tomato pulp and the corresponding Brix reading of the filtrate. The
Brix hydrometer gives directly the per cent of sugar in a solution of
cane sugar, one degree Brix being equivalent to one per cent sugar at
the temperature for which the hydrometer was calibrated. This fact and
the ordinary purpose for which the instrument is manufactured are of no
interest to us in this connection, however. The Brix hydrometer of the
range desired for the examination of cyclone juice and pulp is a stock
article and can be secured readily.
The instrument can be used with the same accuracy as the specific
gravity hydrometer, and the results obtained by it, after correcting
for temperature by Table 8, are converted into terms of specific
gravity by means of Table 9. The determination of the specific
gravity of pulp by means of the hydrometer reading of the filtrate
obtained from the pulp has several advantages over the ordinary method
of weighing a measured quantity of the pulp. When applied to pulp
manufactured from whole tomatoes, the method is reasonably accurate.
It is also very rapid and the equipment required is inexpensive. This
method is especially applicable to the examination of pulp manufactured
from whole tomatoes. It is less applicable to trimming stock pulp,
although even with that product the method will be of value, especially
for the examination of cyclone juice for the purpose of controlling
concentration. With pulp manufactured from trimming stock, the relation
of the specific gravity of the pulp to the specific gravity of the
filtrate obtained from it will vary according to the nature of the
raw material used and also according to the method of manufacture. It
seems probable, therefore, that after a manufacturer has determined
this relation as applied to his own product, he may be able to use this
method with reasonable accuracy even in connection with trimming stock
pulp.
The method is adapted especially to the examination of cold pulp or
cyclone juice.
The following apparatus is used in this method:
1 Brix hydrometer, graduated at 20.0° C., with a range of 1–10°,
graduated in 1/10°.