User's Manual Part 10

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Good practice tips
Accelerometers can be used for sensing leak noise in metallic pipes; however, hydrophones
should be used for plastic and large-diameter pipes. Low-frequency vibration sensors may
be used as an alternative to hydrophones.
For metal pipes, attach sensors directly to the pipe. To ensure good coupling with the pipe, clean
the pipe’s surface with a steel brush. If the pipe cannot be accessed directly, sensors can be
attached to fire hydrants. Attaching accelerometers to fully charged and de-aired fire hydrants
yields superior results. Sensors can also be attached to underground valves. Preferably, they
should be lowered into access chambers and attached directly to valves. However, chambers
may be filled with debris and many users prefer to attach accelerometers to valve keys.
If low-frequency vibration sensors (or accelerometers) are used instead of hydrophones
for plastic pipes, they should be mounted vertically on top of fully charged and de-aired
fire hydrants.
To maximize the similarity between leak noise signals, mount the sensors on similar fittings and in
the same direction. For example, two sensors on valve keys but not one on valve key and the
other on fire hydrant (unless necessary). Different fittings filter leak noise differently. Also, if the leak
position is less than 20% of the sensors spacing, it’s good practice to re-position the near sensor
farther or the far sensor closer. Leak noise signals arriving at sensors that are positioned in a
significantly unsymmetrical manner about the leak will be attenuated in significantly different
degrees.
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