. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. METHODS AND PROCEDURES The controlled conditions implicit in a labo- ratory study of the effects of voids on accuracy of neutron soil moisture measurements require that: 1. The volume of the porous medium be large enough to model a natural soil of infinite lateral extent. 2. The porous medium have a constant density throughout. 3. There be a reliable method for measuring the depth to the water table within the porous medium. 4. There be a reliable method for measuring the amount o
. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. METHODS AND PROCEDURES The controlled conditions implicit in a labo- ratory study of the effects of voids on accuracy of neutron soil moisture measurements require that: 1. The volume of the porous medium be large enough to model a natural soil of infinite lateral extent. 2. The porous medium have a constant density throughout. 3. There be a reliable method for measuring the depth to the water table within the porous medium. 4. There be a reliable method for measuring the amount of water stored within the porous medium. The effects of voids were measured within two 25-cubic foot plastic tanks; one tank served as a control, and the other tank was used for tests (fig. 1). Each tank had walls inch thick and was 67-1/2 inches deep and 29 inches in diameter (inside dimensions). These dimen- sions were selected because a study by Marston (1965) showed that a radius of approximately 14 inches in drained soil is sufficient to contain the sphere of influence for a neutron soil mois- ture meter for readings taken at depths greater than 12 inches. In our study, a tank radius of 14-1/2 inches was considered to represent a vol- ume of infinite lateral extent. A concave steel bottom equipped with a strainer and drain was fastened to each tank. A steel lid with a 2-inch hole to accommodate an aluminum soil moisture access tube was placed over the tank to prevent evaporation of water from the interior. Eight steel tension rods were spaced equally around the tank to clamp the top and bottom securely. A manometer made of plastic Figure 1. — Measurement of the effects of voids were made within two 25-cubic-foot plastic tanks. The sample tank contained built-in "voids" while the control tank had 2. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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