Campbell Scientific DOT600 Roadbed Water Content Meter - Manual (Page 26)

Appendix A. Design and Calibration
A-6
weighted in a 10 mph wind there will be some error, so taring the scale and
weighting the sample should be performed in the same environment.
The enclosure should also be leveled, using the bubble level next to the scale,
to have accurate weight measurements.
Displacement Sensors/Sample Thickness/Compression Pressure
Magnetic displacement sensors provide position of both the compression piston
and the plate above the compression springs. With the information both
sample chamber volume and compression force can be derived.
Testing of the displacement sensor, used for measuring sample thickness, was
performed by placing spacers of known length in the sample chamber and
measurements were taken. The spacers were custom made with tolerance ±
0.0005 inches. The typical error from .400 inches to 1.00 inch was found to be
± .005 inches, with a maximum of ± .009 inches. Testing showed a positive
error bias for thicknesses of .400 inches to .600 inches and a negative error bias
for thicknesses of .600 inches to 1.00 inch. The error for sample thicknesses
between .600 inches to 1.00 inch also tends to be minimized.
Testing of the displacement sensor, used for measuring spring compression,
was done by using a load cell to determine compression pressure. Repeated
measurements showed an error of ± 1.7 PSI with no bias.
Soil water content
Soil water content tests were conducted on multiple soil textures. This was
done by measuring sample gravimetric water content using the DOT600. The
cylindrically-shaped sample was removed from the sample chamber intact. It
was then oven dried at 105C for 24 hours to determine sample dry weight, bulk
density, gravimetric water content. Three repetitions were used for each soil
and water mixture. Testing showed accuracy of ± 1.5% gravimetric water
content. For example, a measured gravimetric water content of 10% has an
actual value in the range of 8.5% to 11.5% gravimetric water content. Test
bias for gravimetric water content was positive.