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Appendix C – Frequently Asked Questions
Q: What is the advantage of measuring cable marks from the top
wheels?
A: The Digital Inclinometer Pro system displays readings as mm or inches
of tilt, and this reading applies at the depth of the top wheels. The Soil
Standard Systems cable markers are measured from the middle of the
probe, but this has some unexpected effects when the readings are
plotted:
Metric Plots:
Digital Inclinometer Pro system:
A reading taken at 10 m is correctly
plotted at 10 m, as you would expect.
Soil Standard System:
A reading taken at 10 m is correctly plotted at
9.75 m, which you would not expect.
Imperial Plots:
Digital Inclinometer Pro system:
A reading taken at 30 feet is correctly
plotted at 30 feet, as you would expect.
Soil Standard System:
A reading taken at 30 feet is correctly plotted at
29 feet, which you would not expect.
The drawing shows a probe in
tilted casing. We can overlay the
probe with a right triangle. The
vertical side of the triangle is the
gravity vector. The hypotenuse
of the triangle is the gauge length
- the distance between the top
and bottom wheels of the probe.
You can see the tilt angle at the
bottom of the triangle.
The tilt angle applies anywhere
along the length of L. The tablet
displays readings in sine units
(the sine of the angle of tilt),
which also apply anywhere along the length of L.
The tablet records sine units, but when it displays a reading, it
multiplies the sine value by L, the gauge length of the sensor (500mm
or 24 inches) to display a reading in mm or inches. This value applies
only at the top of the triangle, as in the illustration, which is the
elevation of the top wheels of the probe. Thus we measure cable
marks from the top wheels of the probe, so that the reading is shown
at the proper depth.
