12.1.10 When the specimen reaches the mark on the pipet,
stop the stopwatch and turn the stopcock to its initial position
to vent the pipet and so allow the specimen to return to the test
jar.
12.1.11 If the time taken to reach the mark exceeds 60 s on
the first filtration, abandon the test and repeat it on a fresh
portion, starting at a higher temperature.
12.1.12 Repeat the operations (see 12.1.9 to 12.1.10) for
each 1°C decrease of the specimen temperature until the
temperature is reached at which the pipet is not filled to the 20
mL mark within 60 s. Record the temperature at which this last
filtration was commenced as CFPP (see Section 13).
NOTE 8—A small minority of samples may exhibit anomalous aspiration behavior, which can be detected by examining the observed aspiration
times. This behavior is marked by an unexpected reduction in the time
taken to fill the pipet, after which aspiration time again continues to
increase progressively, until the failure limit of 60 s is reached.
12.1.13 If the filter has not plugged when the temperature of
the specimen reaches –20°C, continue the test by using a
second cooling bath maintained at –51 6 1°C, quickly transferring the test jar and filtration assembly to a new jacket
placed on the second cooling bath. Alternatively, for single
bath apparatus, adjust the refrigeration unit to –51 6 1°C. The