A Mechanism for the Promotion of Biodiversity Conservation
4 Findings and Discussion
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Costing far less than relative humidity sensors they could still be connected to a data logger to record the moisture content, the disadvantages however may be that they require lengthy calibration tests and their fragility would be a concern.
1.2 The rig
To test these monitoring methods an outdoor test rig was constructed towards the end of 2010 and monitoring has been setup to record interstitial, internal, and external environments. The rig comprises of walls 225 mm thick and stands two meters tall with a roof footprint of 3000 by 3200 mm accommodating an overhang of 500 mm. The bales are raised 250 mm from ground level on rammed earth car tyres to prevent splash back of rainwater. There are 108 monitoring points within the rig walls which provides a wealth of information; this paper is concerned with only one of those points ‘B5.7’ as it had the highest overall moisture content prior to construction.
The Rig is not as it stands complete, it has been rendered with two applications of lime render externally to a depth of 20 mm; internally it has had one coat of lime and all 108 monitoring points remain open to the air due mainly to the nature of the experimentation being conducted. The internal environment is influenced directly by the external as there are major air leakage paths yet to be secured. It has no heat source and no roof or floor insulation. The completion of the rig is due to take place after this wave of experiments has ended.
1.3 Data collection
Point ‘B5.7’ in the rig stands 300mm above ground level, is monitored at a depth of 112 mm. Adopted monitoring techniques include a Maxim iButton sensor, a straw probe, a wood disc, and a Protimeter Timbermaster with Balemaster Probe and temperature probe attachment. It is worth noting at this point that all measurements in this paper will be presented in wet percentage terms.
A hole was drilled into the wall of the rig, the wood disc and iButton were located at the end and the straw probe was then inserted to plug the hole, measurements were taken once a week.
During measurements the straw probe is extracted and weighed in its entirety and the temperature compensated moisture content confirmed with a Timbermaster meter held across the metal rods. The diameter along the grain of the wood disc is then measured with a digital scale rule and the Timbermaster meter with temperature compensation is used to verify the moisture content of the wood. The moisture content of the straw in the wall is then checked with the Timbermaster, and Balemaster probe and temperature sensor attached. Finally the iButton data is downloaded.
Utilising these methods will help to verify the advantages and disadvantages of each method and produce results that can be reviewed and analysed.
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the Timbermaster with bale master probe attached has monitored the rig from the beginning of the construction.
Moisture Content Comparisons of B5.7
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0
28/03/2011 00:00 07/04/2011 00:00 17/04/2011 00:00 27/04/2011 00:00 07/05/2011 00:00 17/05/2011 00:00 27/05/2011 00:00
Date
Moisture Content (%dry)
Straw Probe: Timbermaster conductivity Straw Probe: Weight converted to MC Straw Probe: Negative Error Straw Probe: Positive Error Wood Disk: MC derived
Timbermaster and Bale Master probe Relative Humidity converted to MC
Figure 7. Moisture Content Comparisons
The ‘weight converted to MC’ (Moisture Content) refers directly to the mass of the straw probe converted using the formula laid out in section 3.1.1 with a negative and positive error obtained from the restrictions of the equipment used.
The ‘Straw Probe: Timbermaster conductivity’ shows a similar trend to the converted weight results but field observations have shown that results can be influenced by applying different pressures to the rods and by poor contact with the straw. The same conclusion can be drawn from the ‘Timbermaster and Balemaster probe’ at around 13.5% moisture there may be a lack of contact between the straw and the probe as it drops off from a steady decline that reflects the straw probe weight conversion data. The contact issues will not be investigated further in this paper.
The wood disc shows that the bale is drying out at a steady rate, it follows within the error boundaries of the ‘weight conversion’ method.
The relative humidity obtained from the iButton sensor converted with Lawrence’s formula confirms that the moisture content is dropping and that it is similar to the results found in the error band of the mass of the straw and of the wood disc.
4.1.1 Rig study analysis
The benefits of the iButton together with the formula and the amount of data that can be collected would prove a tantalising option for a long-term study, but the overall cost and complication of maintaining it over the lifetime of a building may prove inhibitive.
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The Balemaster probe has its limitations as is demonstrated by this example as contact is lost a lower reading is obtained reducing confidence in the method. It also requires a lot of effort to take multiple results and is not suitable for long-term studies.
A straw probe offers a very cheap way of manually taking moisture measurements through mass conversion, but conductivity cannot be relied upon as the connection between straw and metal rods cannot be verified and accuracy suffers. Mass conversion may not be a solution to a long term monitoring system over the entirety of a building but could provide a risk assessment tool for a few sample points without jeopardising efficiency of the building.
The expansion of the wood disc may hold a very cheap solution to monitoring at multiple points within a building. With the addition of a strain gauge it should be possible to record data via a network avoiding the sacrifice of an expensive relative humidity sensor every seven years.
5 Conclusion and Further Research
Human populations are set to increase putting greater pressure on the natural environment. Straw bale building may provide a way to alleviate the pressure slightly, utilising a locally produced, abundant, and natural by-product that can be grown to benefit a local ecosystem. Producing an organically farmed crop or employing methods such as field boundaries can help provide extra habitat and prevent pollution of watercourses and therefore encourage an ecosystem.
Straw bale building may be able to spearhead Biodiversity Conservation in the built environment by instilling the notion of Nature as a holistic intertwined and interconnected part of the build over the total life cycle of the material. However to make this possible the method will have to be proved viable. This will in turn requires a skill set to be developed amongst many professions including agricultural, planning and surveying, but above all one capable of proving the health of a build. Presently the lack of credible scientific research is restraining straw bale building from becoming more widely accepted.
The relationship of straw and moisture is of most concern, and in developing a monitoring method capable of demonstrating the history and potential risks in a build will help verify this method of construction. In conclusion, the straw probe and wood disc show encouraging initial results however the need for further study in both methods are required. Further research will be laboratory based to prove performance and reliability and will subject the two new methods to rigorous testing including environmental simulations and cycles of temperature and moisture.
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