Remote Health Monitoring for Asset Management

Where is the sample mean and n is the sample size. The standard deviation is a measure of
how widely dispersed values are from the average. For sensors exposed to air, the measurements
vary with the diurnal temperature variations, and as such the standard deviation is high. For
sensors in sound earth, the temperature varies very little over time, such that the standard
deviation would be expected to be low. To evaluate the behavior of the sensor array, data was
examined under three conditions; the standard deviation over a 24 hour period, the average of 24
hr standard deviations over 1 week, and the standard deviations for 1 week of data. Figure 5.16
shows the standard deviation from the sensor array under each of the three conditions. Analysis
of data in this manner reveals if such data analysis would be adequate over a short time period
(24 hours), when ambient temperature variations may be small, as well as over a longer time
period (1 week) when ambient temperature variations would be expected to be larger. For
evaluation of the effects of scour, a shorter time period is desired, such that rapid notification of
a change in condition at the pile site would be feasible.
STANDARD DEVIATION (ºC)
7
6
5
4
3
2
1
Typical 24 hrs
Average - 24 hrs
7 - day period
12
10
8
6
4
2
STANDARD DEVIATION (ºF)
0
0 50 100 150 200
HEIGHT (in.)
Figure 5.16. Standard deviation of temperature outputs for three time intervals: 24 hr. period, average of 24
hr. periods for 7 days, and standard deviation over a 7 day period.
0
Figure 5.17 shows the average standard deviations for the 24 hr periods, as well as the standard
deviation of those results, to indicate the scatter between separate 24 hr periods. As the figure
indicates, the dispersion of standard deviations is higher for the sensor in air and in the transition
region relative to those sensors in sound earth. The results shown in this figure indicate that
examining the standard deviations may provide a robust tool for estimating the height of sound
earth along the pile. The standard deviations for three sensors are shown in Table 1. This
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