The effects of voids on the accuracy of 

 neutron soil moisture measurements in coarse 

 and fine sand were investigated utilizing two 

 25-cubic-f oot plastic tanks. Test voids 3 

 inches and 6 inches in diameter and 10 inches 

 long were cut from aluminum tubing and 

 fastened to the neutron access tube in each 

 tank. Voids in the control tank were filled 

 with sand; those in the test tank were left 

 empty. The amount of water which was 

 added or subtracted was measured by weigh- 

 ing each tank. The difference in neutron 

 measurements between the two tanks vdth 

 the water at a given level measures the effects 

 of the voids. 



A maximum error of +55 percent moisture 

 by volume was caused by the large void in 

 coarse sand with the void saturated; with the 

 void drained, the error was -12.3 percent 

 moisture. Errors in fine sand were slightly less 

 than those in coarse sand; and errors due to 

 the small void were about one-third the mag- 

 nitude of errors due to the large void. Graphs 

 were developed for use in estimating the size 

 of voids adjacent to access tubes and in cal- 

 culating the probable magnitude of measure- 

 ment errors due to these voids when using 

 field data. 



