Determining the Moisture Content of Soil (Microwave Oven Method) - Soil Properties: Testing, Measurement, and Evaluation - page 54

The procedure for finding moisture content using microwave ovens
is virtually the same as that given in Chapter 4 for conventional ovens,
except for the manner in which soil specimens are dried.
The actual step-by-step procedure is as follows (ASTM D 4643-00 [1]):

PROCEDURE

(1) Determine the mass of a clean, dry container or dish, and record.

(2) Place the soil specimen in the container, and immediately
determine and record the mass.

(3) Place the soil and container in a microwave oven with the heat
sink and turn the oven on for 3 min. If experience with a particular
soil type and specimen size indicates shorter or longer initial dry-
ing times can be used without overheating, the initial and subse-
quent drying times may be adjusted.

Note 3 —The 3-min initial setting is for a minimum sample
mass of 100 g, as indicated in Table 5-1. Smaller samples are
not recommended when using the microwave oven because dry-
ing may be too rapid for proper control. When very large sam-
ples are needed to represent soil containing large gravel
particles, the sample may need to be split into segments and
dried separately to obtain the dry mass of the total sample.

Note 4 —Most ovens have a variable power setting. For the ma-
jority of soils tested, a setting of “high” should be satisfactory;
however, for some soils such a setting may be too severe.
The proper setting can be determined only through the use of
and experience with a particular oven for various soil types
and sample sizes. The energy output of microwave ovens may
decrease with age and usage; therefore, power settings and
drying times should be established for each oven.

(4) After the set time has elapsed, remove the container and soil
from the oven, either weigh the specimen immediately, or place in
desiccator to cool to allow handling and to prevent damage to the
balance. Determine and record the mass.

(5) With a small spatula, knife, or short length of glass rod, care-
fully mix the soil, taking special precaution not to lose any soil.

(6) Return the container and soil to the oven and reheat in the oven
for 1 min.

(7) Repeat (4) through (6), until the change between two consecu-
tive mass determinations would have an insignificant effect on the
calculated moisture content. A change of 0.1% or less of the initial
wet mass of the soil should be acceptable for most specimens.

(8) Use the final mass determination in calculating the water con-
tent. Obtain this value immediately after the heating cycle, or, if
the mass determination is to be delayed, after cooling in desiccator.