. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. on neutron measurements made at a given depth was the difference, measured horizontal- ly, between the two curves. This difference is referred to as measurement error and is ex- pressed in percent moisture by volume. Measurement errors were determined for each inch of depth for each graph and were plotted on total error graphs as the horizontal deviation from a vertical axis (figs. 7 through 10). The area under the error curve is the sum- mation of the products of each measurement
. Effects of air gaps and saturated voids on accuracy of neutron moisture measurements. Soil moisture Measurement. on neutron measurements made at a given depth was the difference, measured horizontal- ly, between the two curves. This difference is referred to as measurement error and is ex- pressed in percent moisture by volume. Measurement errors were determined for each inch of depth for each graph and were plotted on total error graphs as the horizontal deviation from a vertical axis (figs. 7 through 10). The area under the error curve is the sum- mation of the products of each measurement error and the inch of depth which each repre- sents. Total error can be thought of as the total error in measurement (in inches of water) if neutron measurements had been made at each inch through the zone of influence around each void. The purpose of the computation of total error is to quantify the relative difference in the effect of a void as the water level changes. Small Void-Coarse Sand Figure 7A shows the results of measure- ments from the two tanks when the water levels were near the tops of the tanks; , the area around the small void is saturated completely. The error in measurement at 24 inches below the surface was + percent by volume cal- culated by subtracting percent for the control tank from percent for the sample tank. From the surface of the sand down to a depth of 13 inches there is no measurement error due to the void. If readings were to be taken at each inch from 13 inches to 33 inches, the total error would be + inches. Figure 7B shows the two curves when the water level had dropped to inches below the surface. The void was still saturated and 1 1 1 1 1 1 1 1 r. qI I I 1 I I I I I 0 10 20 30 40 50 60 70 80 90 100 SOIL MOISTURE-PERCENT BY VOLUME Figure 6. — Troxler calibration curve for a model 104 probe. 7. Please note that these images are extracted from scanned page images that may have been digitally enhanced for rea
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