A climate chamber that measures the moisture buffer capacity of materials
This article and the linked files describe in detail a research apparatus designed to give direct measurements of the effects of linked diffusion and sorption through materials in a continuously changing environment. These measurements are essential to the prediction of moisture and heat buffering by materials in places with a significant air exchange, such as museum stores and galleries. The unique feature that sets this apparatus apart from orthodox climate chambers is that it measures the rate of water vapour loss or gain in the chamber, while controlling the RH in the chamber, or the moisture flux through the experimental material that forms the lid of the chamber.
Both the moisture content and the temperature of the chamber can be independently controlled to constant values or to cycles of change. The chamber can be fitted with a climate controlled upper chamber but is more conveniently put in a climate controlled room. The chamber can be controlled remotely, via the internet.
The motive for building this chamber is the failure of the current models of moisture transport through building envelopes to treat correctly the absorption of moisture by materials subjected to the typical daily rythm of both the outdoor and the indoor climate. The models make the assumption that moisture equilibrium between air spaces and the surrounding material is instant. This is only correct for very fine fibres or for materials that hardly absorb water at all.
The standard data for permeability to water vapour of building materials are mostly derived from the steady state 'cup method' in which one measures water loss, or gain, from the cup through the specimen which covers it, while the RH is controlled to constant, but different, values on each side.
For highly porous materials, such as fibrous insulation, the value measured in this way is mostly attributable to diffusion through the linked air spaces and takes no account of moisture absorption during changing gradients of temperature and water vapour concentration, which is the natural environment of building materials.
The other material constant used in the models is the water sorption curve. This also is measured after attainment of a steady state, and therefore does not include any information about the rate of approach to equilibrium.
A description of the climate chamber is presented here at three levels:
A general introduction, in the form of a paper submitted to the 2002 Symposium on Building Physics in the Nordic countries. (0.5Mb pdf)
A more detailed description, from which a similar chamber can be constructed. (0.6Mb pdf)
A zipped collection of programs (8Kb zip file) for operating the chamber through a Hewlett Packard data logger. This part is really meant as a backup collection for this particular chamber in the Technical University of Denmark's department of Civil Engineering, but may give useful hints on how to connect HP data loggers to computers running the Linux operating system, a combination which HP does not officially support. However, the Python programs will also run under Windows; the only change needed is to the serial communication module.
This work is funded by the Danish Energy Agency. The research team in the Department of Civil Engineering of the Technical University of Denmark is Carsten Rode, Kurt Kielsgaard Hansen and Ruut Peuhkuri.
In December 2008 the weighing device in the chamber was rebuilt to increase its precision. The changes are described in the supplement tinman_rebuild2008.pdf
This mechanical change caused some changes in the control program 2008 concerning the timing and sequence of relays controlling current to the motor driving the beam position.
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.