Vivax Metrotech vLocDM, Руководство пользователя

Страница: 44 / 54

Page 38 of 47
8.4 Interpreting Graphs
There are two types of graphs that can be plotted. These can either be plotted.
1. Linear
2. Logarithmic
In either case, it is important to look at the trend of the graph rather than individual points. This is because the
signals radiating from a pipe can be affected by many external influences such as:
•
passing cars
• ground currents from stray currents
• ground currents from the transmitter
• slight movements of the receiver
This is just a few examples, the list is probably endless.
8 Interpreting Results
1
2 3 4 56 78
9
10
11
12
13
14
15
16 17 18 19 20
mA
Graph A
Distance
4
5
6
7
Section Points 4-7 of Graph A
Also note the step response at points 1 to 2. This is typical of readings near the application point and may not
represent a fault. The step is very often caused by the concentration of ground currents at the application point
or the cable leading to the ground bed. If in doubt, check the section using the A-frame.
Whether Logarithmic or Linear scales are used is a matter of personal preference. However, when looking for
defects along a pipe, similar defects, further along the pipe can appear smaller than those near the application
point. This is because there is less pickup at distance as the signal from the transmitter is reduced. Using a
logarithmic scale has the effect of compensating for this effect and is more likely to represent the true magnitude
of the defect which is a long way down the pipeline.
The intention is to identify changes in current which signify a defect. So if we were to look at a small section of
graph it is, incorrectly, possible to interpret fluctuations as a defect. See diagram below.
Just taking points 4/5/6/7 is possible to think that there is a defect at this point. However, looking at points 1 to
20 shows that there is in fact a trend of signal loss at points 10-13, which is the more likely fault location.