reached. The water content in percent of the dry
filter papers and tares weighed at one time should be
weight is subsequently determined, and the soil suc-
kept small (nine or less) to minimize error caused by
tion is found from a calibration curve.
water evaporating from the filter paper.
(1) Apparatus. Materials consist of 2-inch-dia-
(c) The tare is opened and placed in an oven for
meter filter paper, 2-inch-diameter tares, and a gravi-
metric scale accurate to 0.001 g. A filter paper is en-
5 degrees Centigrade. The ovendry weight of the filter
closed in an airtight container with the soil specimen.
paper is then determined, and the water content as a
(2) Procedure.
percent of the dry weight is compared with a calibra-
(a) The filter paper disc is pretreated with 3 per-
tion curve to determine the soil suction.
cent reagent grade pentachlorophenol in ethanol (to in-
(3) Calibration. The ovendry water content of the
hibit bacteria and deterioration) and allowed to air
filter paper is dependent on the time lapse following
dry. Reagent grade pentachlorophenol is required be-
removal from the drying oven before weighing.
cause impurities in the treatment solution influence
the calibration curve. Care is required to keep the fil-
were determined for various elapsed times following
ter paper from becoming contaminated with soil from
removal from the oven. The calibrations are given for
the specimen, free water, or other contaminant (e.g.,
Fisherbrand filter paper, Catalog Number 9-790A, en-
the filter paper should not touch the soil specimen,
closed with salt solutions of various molality for 7
particularly wetted specimens).
days. Calibration curve No. 1 resulted from weighing
(b) Seven days are required to reach moisture
the filter paper 5 seconds following removal from the
equilibrium in the airtight container. At the end of 7
oven. Time lapses of 15 minutes and 4 hours lead to a
days, the filter paper is transferred to a 2-inch-clia-
similar calibration curve (No. 3) of significantly small-
meter covered tare and weighed immediately on a
er water contents than the 5-second curve for identi-
cal suctions. Calibration curve No. 2 was determined
gravimetric scale accurate to 0.001 g. The number of
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