through all operations exactly the same as the crucibles used for the tests. The tare crucible must have practically the same weight and surface area as the test crucible.
Heat the crucibles and covers in a muffle furnace at approximately 925°C. Cool in a desiccator and weigh to the nearest 0.1 milliD gram. Dry about 6 grams of the paper to conD stant weight at 100° to 105°C.,and weigh to the nearest 0.01 gram. Wet the paper with distilled water containing not more than 10 parts per million of igneous solids, then roll it into a compact mass with clean fingers. Put the test specimen in a crucible, cover it, and place it in a cold muffle furnace. Start the ignition and after smoking has ceased remove the cover and continue the ignition for 2 hours at 925°C. Replace the cover, cool, and weigh the crucible and ash. The ignition is considered to be complete when the weight of the ash becomes constant after reignition for 30 minutes. All the cooling must be done in a desiccator, and the test crucibles and tare crucible must be ·allowed to remain on the balance pans for the same length of time. The ash must be protected from air currents at all times.
Duplicate determinations shall be made, and they shall agree within 0.003 percent of the weight of the dry paper. Report the averD age of the values to the nearest. 0.01 milliD gram per 11-centimeter circle of the paper.
4.3.3 Bursting strength, wet. - Clip to-· gether a pack of fivecircles of the filter paper, and immerse the pack in distilled water at
23°C. for 5 minutes. Remove the pack, allow
to drain suspended for 10 seconds, then imD
mediately determine the bursting strength in
accordancewith method 112 described in FedD
UU-P-236a
tested, a volume of prefiltered water equal to three or four times the volume of the cone will usually be sufficient. If a considerable quantity of water is to be prepared, the filters should be renewed frequently, because they lose their effectiveness with continued flow of water through them.
Carefully fold a filter circle in the usual
way to form a 60° cone. Place it in a 60°
funnel and fill it with water prepared in the
manner described above. Press down the folds
to expel all air pockets and to make the three plies smooth and in good contact. Allow about three-fourths of the water. to filter through,
then pour off the excess. Save the water to wet the next specimen. Remove the empty cone and suspend it freely over a burette by supporting the cone in a loop of wire of such
size that it will support the cone at about two-thirds of the distance from its apex.
Using prefiltered water at a temperature
of 23° ± 2°C., pour into the cone, all at once, a measured volume which is approximately
two-thirds of the volume of the cone. When one-fifth of the water has filtered through into the burette, start a stopwatch. When
half the remaining volume has filtered
through, stop the stopwatch and record the
time in seconds. Test at least 10 circles, using the same water over and over and adding:to it from the prepared supply as found neces-
sary. .
The time required to filter:half· the volume. of water remaining in the· eonical filter fs substantially ·the same· for· any given paper, irrespective of the. si.Ze:_ of.the test specimen. Suitable volumes of. water initially · poured in for filter paper circles of the sizes most
generally used are as follows:
eral Specification UU-P-31. Use enough packs to obtain not less than 10 values. ReD port the bursting strength to the nearest 1A. point.
4.3.4 Time of flow of water.-Prepare disD
Diameter of circle
Centimeters
11
9
7
5.5
Initial 11olume
Milliliters
25
15
7.5
3.5
tilled water by prefiltering it through filter paper that filters at least as slowly as the paper to be tested. Use two or more filters in series. For each kind of filter paper to be
Report the filtration time in seconds and
specify the diameter in centimeters of the filter paper used for the test. ·
Note 1.-For some reason that has never
s
155o
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