16 and to derive two entirely conflicting moisture-content results. Extreme caution must also be applied where timbers are damp stained (the situation where a. moisture meter is most likely to be used) because the presence of salts will em1ticallychange the electrical resistance ofthetimberproducingerroneousreadings. Most moisture meters cannot give a percentage moisture reading in materials other than timber and should not be used to measure wall moisture levels. The standard procedure is to collect mortar samples with a slow speed drill and to calculate their absolute moisture content from weight loss after controlled oven drying. The main disadvantage with this method isthat mortar, masonry and brick are inherently variable materials and results may not be directly comparable, particularly where further readings are required, for example to monitor the progress of long-term drying. Wooden dowel sensors are a simple, cheap and consistent alternative to overcome these disadvaritages. Timber dowels are inserted into pre-drilled holes and allowed to equilibrate with the structure for 3-4weeks. The equilibrium moisture content of the dowels is calculatedbythe oven/balance method and represents the wetting potential ofthe wall. These dowels serve a dual purpose as bait rods to detect concealed fungal activity. In the presence of active dry rot Ramin dowels are readily attacked within four weeks. It must be noted that decay will reduce the density of the wood and proportionally increase its percentage moisture content. For long term monitoring the sensors need to be regularly replaced and redundant sensors should always be removed. Variations on these themes ofmoisture measurement have been developed into COMPUTERNEIWORKS WfffiOUTCABUNGFOR IJSTEDBIBLDINGS Conservation Officers are often faced with the problem of the demand by owners oflisted buildings in commercial use for computer installations which may require damaging alterations. Recently, a wireless local area computer network (IAN), has been launched which enables users of listed buildings to gain the benefits of computer networking without the need for physical cabling. The Wireless LAN provides great flexibility as it enables existing networked computers to be moved and allows new computers to be added to a networked system easily and quickly, allwithout the need for physical wiring. The system can Ramin 'bait' dowel attacked by active dry rot Serpula lacrymans. permanent monitoring systems designed to provide early warning if leaks recur in vulnerable areas. As part of essential building maintenance the manually interrogated or fully automated sensors provide greater security against future decay than paper guarantees which are nullified by water ingress. Evaluating the level of wood-boring beetle activity can be rather subjective unless there are large numbers of adult beetles present. Other visual clues may be inconclusive. Our recent research has demonstrated that death watch beetle do reuse existing emergence holes, so the absence of fresh holes may not eliminate the possibility of activity. Old frass is readily dislodged from damaged timber by vibration and heaps of pellets may not therefore provide confirmation of current activity. However, damage caused by insect infestations progresses at a relatively slow rate and monitoring is the also save users considerable amounts of money as computer network management, maintenance and change costs are minimal as cabling, which is expensive to install and update, is not required. For instance, it is estimated that best approach to determine whetl1er a population isthriving or declining before proceeding with expensive, and often ineffective treatments. Elevated timber moisture levels required to sustain wood-boring beetle infestations may be attributable to ambient conditions. Single relative humidity readings are of limited use as the environment is likely to fluctuate through daily or seasonal cycles which are most effectively evaluated by aperiod of continuous data logging. Estimates of beetle population levels may, if practical, be made by regular beetle counts, but often cleaning regimes remove the evidence, or fresh beetles cannot be distinguished amongst years of accumulated debris. Alternatively the number of emergence holes used in a flight season can be quantified by pasting tissue paper over flat surfaces, or by filling existing holes with beeswax. The latter technique is particularly useful for monitoring carved, moulded timbers. DETAILEDDIAGNOSIS Correct diagnosis of the cause, nature and extent of decay is essential to minimise destructive intervention and maximise conservation of historic fabric, with the added incentive of reducing costs. There are many techniques available, using complex technology and simple robust equipment where appropriate. In experienced hands, all are useful tools to assist with problem analysis, but these form only a part of a thorough independent survey applying scientific knowledge of the performance of decay organisms within the limitations of the built environment. Chris Cullen DipBldgCons (RICS) ABIAT is a conservation scientist for Timber Specialists, Ridout Associates. for five to ten users, the Wireless LAN would reduce the cost by up to 20%over an equivalent hard-wired system. One computer is needed to act as a base station, with up to 50 remote computers using 'frequency hopping' radio technology to communicate with one another via the base station. The system is robust and immune to interference or snooping and the use of radio waves means that communication can be made through walls and between buildings. Setting up takes just two hours and installations can range in size from just five to 1,000 users. Security can be assured by activating a data-encryption facility. Further information can be obtained from Azlan Ltd, Mulberry Business Park, Fishponds Road, Wokingham RGI I 2GY, tel: 01734 894400,fax: (01734) 894300. CONTEXT49
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