Zonescan – Histogram Interpretation Guide
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| The Zonescan logger can be programmed
to record decibel samples at 1 second intervals. It can also be programmed
to log over a period of 23 hours 59 minutes.
These logged results are then presented in a graphical format for interpretation by a leakage engineer. There are a number of schools of thought when it comes to interpreting “Leak Noise” histograms. This guide is only to be used to get you started on the road to building your own knowledge and experience in sorting out the wheat from the chaff, so to speak. 1. Ambient or Background
Noise. |
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| The above graphs are showing typical background noise levels and patterns. The Zonescan software is a good guide to the significance of your histogram. The “Leak Value” is a statistical evaluation of the logged decibel levels based on over ten years of logging experience. But the “Leak Value” is not an absolute calculation, so bear that in mind when analysing your graphs. | 2. Low Noise Low Volume Leaks. These are typically passing valves, leaking hydrants, pinholes in customers pipes, etc, etc. The Zonescan software will normally rank this kind of noise pattern between 30 and 45, and display it as below. So you can filter out this kind of leak by setting your warning threshold above 45. |
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| 3.
High Pressure Low Volume Leaks. These types of leak are usually physically the same as the above leaks but at much greater supply pressure, hence they are much “sharper” in appearance. The Zonescan software will normally give these results a “Leak Value” of anywhere between 65 and 95. These leaks can be very localised to the logger, so correlation is not always successful. A couple of examples are shown above. |
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Typical Distribution Mains Leaks. This type of leak is becoming more and more rare as leakage levels are get closer to their economic level, nonetheless they are still occurring regularly throughout the network. The pattern generated by a “typical” mains leak can also be generate by a “good” leak just off the distribution main, eg: supply pipe, communication pipe, etc. This type of leak is normally the easiest to spot as the histogram is reasonably sharp and is usually between the middle and the top of the decibel range. |
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| 5.
“A Leak or Not A Leak” – That is the question. Sometimes due to environmental and/or operational factors a “good” leak gives an unusual histogram pattern. This can be misleading to the experienced leakage engineer and to the Zonescan software. These are the situations when the leakage engineer’s situational awareness really comes into it’s own. Always ask the question: “What could be giving me this unusual result ?.” Remember that the Zonescan logger is not that clever, it just listens to what is there, through whatever material it is attached to or placed on, it cannot make any judgements on it’s location. Here are two examples on the right. |
This pattern was generated by a good leak that was
just 36 metres away. It is loud, but wide. So the software reduces
it’s “Leak Value” to below 50, but only just. The
decibel level should make you suspicious. The logger was just sitting on a plastic MSM meter blanking screw in an APLAS box. |
Believe it or not this is a good leak. But it is disguised
behind a lot of usage. The little sharp peak just to the left of the
main peak in actually the leak noise. It looks like there were two
periods of usage during this logging period, a sustained draw off
and a short loud draw off. |
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 Logger 100194 |
 Logger 100191 |
| This shows the above leak the following night, minus
the usage noises. Although by logging for an extended period you should
be able to find any leak noises in one night. By logging for 2 to 3 nights continuously you will have a better chance of being right first time. |
6. Unaccounted For Water (UFW).
This Zonescan logger was located on a pipe supplying
a non-metered public toilet. You can clearly see the usage, which
was happening every night. |
The graphs above were logged on a Fire Main of a large commercial property in a major UK city. The water company supplying this customer suspected that they were utilising their un-metered fire main to supply the property with water for commercial purposes. It was not operationally possible to fit a metering device so two Zonescan loggers were attached to the fire main pipe, one on a straight length of pipe, the other on a “T” junction of the pipe. The loggers were set to record from 00:01 to 23:59 to establish a daily profile of the suspected illegal usage. | |
Logger 100194 indicates a very loud and continuous noise on the fire main throughout the logged period, i.e. all day long. This could be caused by very loud ambient/background noise in the property, e.g. air conditioning units located close to the fire main pipe. But when examined in conjunction with the logged results from logger 100191 doubt can be cast on this assumption. Logger 100191 was located on a “T” junction about 25 metres from logger 100194. It can be seen that the noise logged at this point is so loud that it goes off the scale of the logger at some point during the day. If 100194 was indeed logging background noise then 100191 would have an identical graph. This is obviously not the case. The only reasonable interpretation that can be made is that water is continuously flowing through the pipe generating a great deal of noise as it passes through the “T” junction. |
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