Campbell Scientific DOT600 Roadbed Water Content Meter - Manual (Page 25)

Appendix A. Design and Calibration
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
15 20 25 30 35 40
Water content sensor output (microseconds)
volumetr
ic wat
er content
(m^
3
m^-3)
clay calib
sand calib
loam calib
silt calib
FIGURE A-3. Partial calibration set showing calibration data and equation.
The effect of clay content on sensor response is apparent.
A.7 Description of the Test Performed and Factors
Which Could Have Effect on Results of DOT600
Measurements
The scale/load cell
Weights from 100 grams to 1000 grams were placed at various locations of a
symmetrically defined grid superimposed on the scale surface. The CR850
measures an average weight over 2.5 seconds. There were no significant
differences in error among any of the positions. The total specified error of the
load cell is ± 0.32g. From our testing the maximum error of the load cell was ±
0.22g, giving us a maximum error of .22% for a typical 100g soil sample. The
load cells also appear to not have an error bias. It is concluded that placing the
sample chamber anywhere on the scale surface such that it is not extending
over any edge of the scale will result in no significant error contribution to the
measurement.
Also during field testing of the scale we found wind to have an effect on scale
accuracy. To remove the error caused by wind we added programming to
determine if there was a rate of change due to wind. This code will prevent any
spikes in weight, but if the scale is tared in a wind of 5mph and the sample is
A-5