“Wrapping stone blocks in plastic sounds odd? Give it a try…”

– Heather Viles, co-author and SEAHA CDT Co-Director

SEAHA CDT Alumnus Scott Allan Orr and supervisors from the University of Oxford, Historic Environment Scotland, and the Consarc Design Group has published a paper that presents a more accurate method for understanding how the output of non-destructive moisture measurement methods relate to the actual amount of water present.

Non-destructive techniques can be an informative part of understanding how a building is functioning in relation to its environment. They are based on measuring a physical property or phenomena, such as the interaction with electromagnetic energy, as a proxy for the amount of moisture present. However, since these measurements are influenced by several factors, such as the material, they are converted into arbitrary units to indicate relative levels of moisture.

The process of gravimetric calibration involves taking measurements with a non-destructive technique of a sample of the material of interest at several moisture contents (measured by mass) to characterise the relationship between them. Ideally, this is a directly proportional relationship (a straight line, more or less)—however, this is rarely the case. The samples need to be an appropriate size. For devices that measure within a large area, this means that the samples need to be suitably large as well.

Previous work has developed calibrations with samples that are first saturated, and then monitored while they dry out. While this is suitable for smaller samples (i.e. for calibrating devices with a small measurement area), this paper demonstrated that for devices with large measurement areas the samples of necessary size take much longer to dry out, resulting in a significant unequal distribution of moisture within the sample. This wreaks havoc on the calibration results.

To address this challenge, this paper validates that the calibrations are more accurate when using an approach described as ‘isolated diffusion’. This involves using several samples of the same material, and sealing them with impermeable plastic so that the moisture has time to evenly distribute throughout the sample. This results in more accurate and more consistent understanding of the device output from non-destructive techniques for moisture measurement.

The technique does not require any specialist equipment except for a suitable balance, meaning that it can be used by practitioners and researchers alike. It is particularly suitable for electromagnetic devices such as microwave-based tools and radar, since these do not direct contact with a surface to provide a consistent and accurate measurement.

Read the Open Access article on the website of the Journal of Applied Geophysics.


Scott Allan Orr is a researcher within the Oxford Rock Breakdown Laboratory (OxRBL) and a Stipendiary Lecturer in Physical Geography at St Catherine’s College, Oxford. He is currently working with Historic England to produce guidance on moisture measurement and monitoring in historic buildings and building materials. He is a Member of the Royal Society of Chemistry and part of the Management Group of the Council on Training in Architectural Conservation (COTAC).

Scott holds a BASc in chemical and environmental engineering from the University of Toronto, where he contributed to projects incorporating environmental monitoring and urban spatial distribution of atmospheric aerosols. From 2014 to 2018 he was a member of the EPSRC Centre for Doctoral Training in Science and Engineering in Arts, Heritage, and Archaeology, undertaking an MRes and DPhil at University College London and the University of Oxford, respectively. This project was undertaken in collaboration with Historic Environment Scotland and the Consarc Design Group, a leading heritage conservation architecture firm based in Belfast but working throughout the UK. In 2016, Scott was a visiting researcher at the German Federal Institute for Materials Research and Testing in Berlin.