Geokon GK-604D, User Manual

Page: 78 / 117

66
A+
A-
Bottom Cushion
Accelerometer Housing
Wheel Assembly
Wheel Assembly
Cable Fitting
Cable
Figure 75 - Inclinometer Probe
Inclinometer probes usually contain two accelerometers with their axes oriented at
90
°
to each other. The A axis is in line with the wheels (Figure 75 illustrates) with
the B axis orthogonal to it. Thus, during the survey, as the A + , A − readings are
obtained, the B
+
, B
−
readings are also recorded.
During the data reduction these two sets of readings (A + , A − and B + , B − ) are
combined (by subtracting one set of readings from the other) in such a way that the
effect of any zero offset of the force balance accelerometer is eliminated. [This zero
offset is the reading obtained from the inclinometer probe when it hangs vertical.
Ideally the offset (or bias) would be zero, but usually there is a zero offset which
can change during the life of the probe due to drift of the transducer, wear and
damage of the wheels or most likely due to a sudden shock to the transducer
caused by dropping or allowing it to hit too hard against the bottom of an installed
inclinometer casing.]
Subsequent surveys of the inclinometer casing, when compared with the original
survey, will reveal any changes of inclination of the casing and locations at which
these changes are taking place. Analysis of the change of inclination is best
performed by computing the horizontal offset of the upper wheels relative to the
lower wheels which has produced the tilting (
θ
) over the reading interval (L) of the
survey (usually the wheel base of the probe, 2 feet for English systems, .5 meter for
Metric). At each position of the inclinometer the two readings taken on each axis (A +
, A
−
and B
+
, B
−
) are subtracted from each other leaving a measure of sine
θ
. This
value is then multiplied by the reading interval (L) and the appropriate factor to
output horizontal deflection in engineering units (inches for English, centimeters or
millimeters for Metric) (see Figure 76).