ings. However, in a sceptical world it clearly requires proof of concept.

Heriot Watt University has undertaken initial laboratory testing, and a two-year research program that commenced in January 2006 was agreed to with funding from the UK Engineering and Physical Sciences Research Council.

Identification of persistent trap seal depletion would allow local installation of active control devices to prevent further failures.

Air admittance valves (AAVs) could be used to limit trap seal loss due to negative transients, or variable volume containment devices installed to limit trap seal loss due to positive pressure transients.

Comparison of a defect-free simulation with the measured response of the network to an applied transient will identify the presence of a defect.

Simulation or measurement of the response at two monitoring stations will identify the defect location in terms of distance and travel time for a transient from the surge generator to the termination and back to the monitoring location.

The accompanying chart on page 83 illustrates a simple laboratory demonstration of the principle to be developed in this research, showing the defect-free system response with an AAV or an open end as the stack termination.

As the applied transient is positive, the AAV closes and becomes a closed end.

Depletion of one trap seal yields a quite different system response, with the reflection returning to the transducer earlier than previously, representing the shorter travel time at the acoustic velocity in air.

The way forward

The proven mathematical simulation of transients, together with the evidence provided by initial small-scale testing, indicates that this methodology has potential as a means of identifying defective system components.

Following further laboratory testing that will include the development of a suitable transient generator, extensive site testing will be undertaken using buildings made available by industrial research collaborators.

During this phase, detailed consultation with facilities management practitioners will ensure that the final methodology proposed meets their practical requirements and does not infringe any Code regulations on system components.

It will be necessary to ensure that the pressure monitoring equipment and transient generators do not themselves present any cross-contamination routes to habitable space.

Finally, suggested facilities management procedures for introducing transient identification will be presented for discussion and consultation through the relevant professional bodies, including the recently announced Consortium to Address the Role of Drainage in Infection Spread (CARDIS) working group of CIBW062.