In the early hours of the morning, while the rest of us are safely tucked up in bed, an unusual predator prowls London's streets.
Night after night 300 technicians patrol the city looking for the leaking pipes that waste London's valuable water. Rob Gomez and Graham Dobbs work for Mouchel Parkman, which is contracted to carry out detection work for Thames Water.
As the rest of us are turning in for the night, Gomez and Dobbs are gearing up to start their day.
By 10pm they are meandering through the deserted streets of Islington looking for leaks in the ageing mains water network.
Armed with a map of specic zones or District Meter Areas (DMAs) suspected of containing serious leaks, the pair then use listening sticks and ground microphones to detect and locate serious problems. Each DMA covers approximately 3,000 properties (see box).
On arrival at their first potential trouble spot - a quiet residential street in Islington - they check that the meter readings from the ow monitors into the DMA match measurements recorded at head ofce. An average DMA has three or four detectors recording the quantity of water supplied into it.
Historic measurements show that the ow rate into the area is increasing. But as there has been no growth in consumers and the pressure has remained constant, the probability of a leak is high.
Gomez reads the meter, first checking the rate of ow and then checking the pressure in the main, while Dobbs compares the results to the log.
If the pair believe there might be a leak, the men will check by attaching a metal pipe called a swan neck to a re hydrant and releasing a specied amount of water from the mains. This tests the integrity of the DMA boundary as the other monitors should then show an increase in ow equivalent to this 'leak' as the system attempts to maintain its equilibrium.
Satised that water is escaping in this area, Gomez and Dobbs begin placing noise loggers strategically.
Listening for unusual activity is the key method used for locating a leak. During the day, people flush toilets and run taps, causing lots of noise in the mains but at night there is very little activity so unusual sounds are likely to betray the escaping water.
'A noise logger is a small piece of monitoring equipment about the size of a hand grenade. Its got a magnetic end so it can just be dropped on top of a valve, then a few times a night it will wake up and listen for noise and record it, ' explains Thames Water network strategy manager Alf Pierce.
Now that the noise loggers are in position Gomez and Dobbs use a laptop to record their measurements and interpret the data. Data is extrapolated and an approximate location for the leak is calculated.
To narrow it down further, the pair pull out a piece of kit known as a listening stick. Gomez places an ear on the plastic disc at one end and places the tip of the steel rod at a mains valve (4).
Hearing the distinctive 'whoosh' of a leak, Gomez takes the stick and follows the line of the pipe either side of the valve to discover where the leak is located.
Because assessment is partially based on the experience of the men listening in, there are always occasions where they are unable to establish whether a sound is a leak or a running tap.
So at this point they use a microphone that is run along the ground. Resembling someone listening to an old school tape player, Dobbs dons headphones and walks along the line of the main, adjusting the controls to lter background noise and narrow down the leak location.
Next an £8,000 leakage noise correlator is employed to pinpoint the site where repair is required. Gomez measures out the distance between two valves on either side. Meanwhile Dobbs places a microphone called an accelerometer on the end of the rst valve and feeds it into a red amplier; he then repeats the exercise on the second valve this time using a blue amplier.
Additional information is also fed into the correlator, such as the distance between valves, pipe diameter and material.
Both ampliers then feed information to the correlator which calculates the distance from each valve to the leak.
Finally, the men pace out the distance to the leak and mark up the road for the repair team which will move in within days.
Depending on the severity of leakage in the area the DMA may become part of Thames mains renewal programme where all of the pipe work in an area is ripped out and replaced.
Over the next 4 years £500M has been set aside by Thames to pay for this work.