Page 301 - Water Loss Control

270 Cha pte r S i x tee n

distribution system. For example, if sounding is undertaken every 656 ft (200 m) but the
pipes are plastic with poor sound propagation, there is a very good chance of missing
detection of an existing leak unless the leak happens to be next to a fire hydrant or fit-
ting which is being sounded. Every water distribution system, and every distinct sec-
tion of the system (pipe materials, pressure levels), must be treated on its own merit.
Sounding may be a simple technique, but the planning must be done by someone who
understands the capabilities and limitations of the equipment and personnel, and the
characteristics of the leakage occurrences in their distribution system. Appendix B pro-
vides further information on leak detection equipment and leak detection techniques.
16.4.1 Acoustic Leak Detection Equipment

Mechanical and Electronic Listening Stick
The listening stick, probe rod, or similar name describes a traditional instrument used
to systematically sound all mains fittings and service connection pipes. There are vari-
ous designs, the most common having an earpiece attached to a steel shaft. Alternatives
have a mounted ear piece housing a diaphragm that amplifies sound. However, these
tend to give a “seashell” sound effect, which can be misleading to an untrained ear. Use
of the listening stick is by placement on a fitting, whereby any leak noise is transferred
from the pipe, through the steel shaft and is heard at the earpiece.
The electronic listening stick is used in the same way as the mechanical version, but
has a battery powered sound amplifier attached so that the leak noise is enhanced and
then heard through headphones. The electronic listening stick is utilized in areas of low
pressure, where leak noise is weak and requires amplification. It is also useful for direct
sounding in areas where there may be high noise interference from passing traffic.

Ground Microphone
Ground microphones (Geophones) are listening devices mostly used to listen for leaks
from the surface where contact points such as valves, hydrants, service connection
curbstops, and the like are far apart. Ground microphones are also used to pinpoint the
exact location of a leak. Mechanical listening devices have an appearance and work on
the same principle as the physician’s stethoscope. Today’s electronic devices have sig-
nal amplifiers and noise filters to attenuate the leak noise signal. Ground microphones
(see Fig. 16.9) are usually used in conjunction with other leak detection equipment,
although it can be used alone, especially in areas with few fittings and predominantly
plastic pipe.
Leak Noise Correlator
Just like traditional sonic equipment a leak noise correlator relies on the leak sound
generated by a leak. A leak noise correlator typically consists of a receiver and processor
(correlator unit) unit and two sensors equipped with a radio transmitter. The two sen-
sors are placed on valves or hydrants on each side of the suspected leak. The leak noise
detected by the sensors is converted into electrical signals and then transmitted via the
radio transmitters to the correlator unit. Leak sound travels along the pipe with a con-
stant velocity depending on the pipe diameter and pipe material. The leak noise will
first arrive at the sensor closer to the leak. The correlator uses the time difference
between the two arrival times, information about the pipe material and size, and the
distance between the two sensors to calculate the location of the leak. The calculation
principle is set out in the formula L = TD × V + 2I, where L is length, TD is the time