The work reported in this paper addresses this problem, focusing on a real need. The Swedish warship Vasa is a beautiful (though somehow negative, see its history) example of naval architecture. After being recovered from the sea bottom, it is now under study in order to protect its wood from deterioration. It's well known that wooden samples, after having been taken out of water or mud, run a high risk of disintegration. Gas diffusion across wood is a critical issue in this case, which must be verified in-field, with non- (or at least low-) invasive methods.
The proposed technique requires drilling two small (about one cubic centimeter), dead-end holes in the wood, spaced by about one centimeter. One hole is used to flush a known gas or mixture, the second one for the probe. That is the very volume in which the interaction path between light and gas must be realized. The authors have vast experience in measurements in scattering media, which have been exploited also for medical purposes. Some of the materials that in other works of theirs were under investigation, now become probes themselves: a porous material of about a half cubic centimeter volume is inserted into the second hole; it allows an interaction path of about 26 times its thickness. The diffusion time within the porous material is around thirty seconds, short enough for the wood diffusion measurements. A standard spectroscopic technique (wavelength modulation spectroscopy) is then used to manage the signals.
The authors demonstrate that for the measurement of the concentration of oxygen these features, despite the apparent opacity of the porous medium, are sufficient to obtain a minimum detectable level of 0.3% at room conditions of temperature and pressure.
This Applied Optics article is a proof of the importance of the development of specific diagnostic techniques. Rigorous measurements yielded quite a counterintuitive result, namely that the denser the wood, the more rapidly diffusion occurs.
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