[17] If it is preferred a chemical thermometer with Centigrade scale
may be used having a range of from 0 to 100 degrees.
(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
Many manufacturers of tomato pulp control the concentration of their
product by determining specific gravity when the evaporation is almost
completed. They therefore desire the results at the earliest possible
moment, and there is no attempt to cool the sample before determining
specific gravity, although in that way much more accurate results could
be obtained.
When necessary to use this method the hot pulp is poured into the
specific gravity flask (Fig. 1 or Fig. 2) by means of a dipper until
the flask overflows. The top is then “struck off” with a straight edge
and the flask placed in a shallow basin of water and the pulp carefully
washed from the outside. The temperature of the pulp remaining in the
dipper is then determined by means of a chemical thermometer.
The flask is then dried with a towel, which operation is greatly
facilitated by the heat of the pulp. The cooling of the contents of
the flask causes contraction, so that after washing the flask is
not entirely full. This should be disregarded, as it is desired to
determine the weight of the amount of pulp that filled the flask
originally. As soon as the outside of the flask is clean and dry the
flask and contents are weighed.
The apparent specific gravity of the hot pulp is ascertained from the
special table prepared for the flask according to the directions given
on page 38, and the correction figure for the temperature of the pulp
obtained from Table 6 is added. _For example_, this method when applied
to a certain sample of hot pulp (without centrifuging) indicated a
specific gravity of 0.9874. The temperature of the pulp was found to
be 201° F. In Table 6 we find that the correction .0457 is equivalent
to 201° F. Adding this to the apparent specific gravity given above,
we have 0.9874 × 0.457 or 1.033 which is as nearly as we can determine
from the hot pulp the specific gravity that would have been determined
by examining the same sample after cooling by method (a). More accurate
results can be obtained by working with larger specific gravity
flasks. For instance, the specific gravity cup shown in Figure 5 may be
made of copper, and may readily be made larger than the glass flasks
shown in Figures 1 and 2. All metal flasks will gradually change in
weight, owing to the solution of metal by the hot tomato pulp, and
their weight should therefore be checked from time to time.