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 0.50 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 none. 



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