. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. pipe was threaded into the wall of the tank 1 inch above the bottom to indicate the level of the free vmter surface within the tank. Forty holes 1-1/8 inches in diameter, and arranged in four vertical rows of 10 holes each, were drilled through the walls of each tank to check bulk density of the porous medium. These vertical rows were spaced equally around the tank and the holes within each row were spaced equally from top to bottom. The holes were sealed by expanding plugs made f
. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. pipe was threaded into the wall of the tank 1 inch above the bottom to indicate the level of the free vmter surface within the tank. Forty holes 1-1/8 inches in diameter, and arranged in four vertical rows of 10 holes each, were drilled through the walls of each tank to check bulk density of the porous medium. These vertical rows were spaced equally around the tank and the holes within each row were spaced equally from top to bottom. The holes were sealed by expanding plugs made from black rubber stoppers having washers on each face and pierced by bolts. The plugs were com- pressed and expanded when the bolts were tightened. The dimensions for the test voids were selected after an examination of field data from six access tubes. These tubes had been installed 6 feet deep and showed an extremely large sea- sonal fluctuation in soil moisture. Three of the tubes yielded neutron counts that seemed "reasonable," , percent moisture by volume did not exceed soil pore space when soil was saturated. The remaining three tubes yielded questionable data, , data indicating that the percent moisture by volume exceeded the pore space of the soil. Castings were made of the access holes. Be- fore this could be done, the rubber stoppers sealing the bottoms of the access tubes had to be removed. Cement was poured into the tubes and 3/8-inch reinforcing rods were used to work the mortar gently out the bottom of the tubes. As the cement flowed out into each of the access holes, the access tube was raised slowly and the reinforcing rod was left in the hole. When the access hole was filled complete- ly with cement, the access tube was removed; and the cement was allowed to harden for 2 weeks. Then the castings were excavated (fig. 2) and measured. The three access holes yield- ing questionable data had voids, as determined by the castings, that ranged in siz
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