Measuring Liquid Water And Ice Content Within Porous Materials Using The Hot-wire Technique

Porous insulation materials can easily acquire a large quantity of moisture when they are exposed to high humidity conditions. The acquired moisture can cause degradation of insulation performance, mould or bacteria growth, corrosion, etc. The moisture problems for buildings in the cold climate regions and for aircraft in the service duty are even severe, in which the moisture may freeze and melt subject to dynamic change of the psychrometric conditions. The cyclic repeation of moisture phase changes can cause damages to materials and even threaten the structural safety. If a method to monitor the moisture accumulation in a wall is available, it will be extremely helpful for guiding the maintenance or replacement of insulation materials upon the exact demands. Then any possible damage caused by the moisture accumulation can be minimized. By far, numerous methods have been developed to measure moisture contents. However, these methods may impose destruction when preparing for test samples, or are inaccurate due to the adoption of empirical formulas, or are too complicated to utilize onsite. There is still lack of an accurate, simple and inexpensive method for in-situ, nondestructive and rapid measurement of moisture content in porous materials.This investigation proposes to measure moisture contents by means of volumetric heat capacity, which is the product of density and specific heat capacity. The mathematical derivation shows there is a certain mapping relation between the acquired moisture content and the change of the volumetric heat capacity before and after moisture absorption. The volumetric heat capacity was proposed to be measured by a transient hot-wire technique, based on which the volumetric heat capacity was expressed into the ratio of the thermal conductivity with the thermal diffusivity. An electric-resistance wire and a temperature sensor are embedded in porous materials during a test. The temporal temperature response was recorded once the hot wire is switched on. The thermal conductivity and the thermal diffusivity of the material are obtained according to the slope and the intercept of the temperature response curve in the linear section. After obtaining the volumetric heat capacity of the dry material and the wet material, respectively, the moisture content can be inferred. The above method was applied to measure both the liquid water and the ice content in sponge blocks, respectively. The acquired moisture contents ranged from10s to70s with an interval of around10%. The inferred moisture contents were compared with those provided by a digital precision balance as a benchmark. To quantify the confidence of the measurements, the uncertainty analysis was performed by following the recommendations of the ISO and BIPM.This thesis finds that the hot-wire method is able to measure both the liquid water and the ice contents with reasonably good accuracy. The deviation between the hot-wire method and the benchmark gravimetric method is less than4%. Neither the measured thermal conductivity nor the measured thermal diffusivity in this investigation correlates well with the moisture content due to the contact resistance. However, their ratio, the volumetric heat capacity is insensitive to the contact resistance and correlates well with the moisture content. The uncertainty analysis shows that the hot-wire method has a low standard uncertainty. Although the slope or the intercept alone contributes a lot to the uncertainty of the measured volumetric heat capacity, their combination contributes a little because the slope and the intercept are correlated with each other. In other words, if the slope is overestimated, the intercept will be underestimated. As such, their overall impact to the inferred moisture content is balanced and hence the uncertainty is minimized. The distance between the temperature probe and the hot wire contributes more than the rest to the uncertainty of the measured moisture content. However, if a fixed distance between the hot wire and the temperature sensor is maintained, it can greatly reduce the uncertainty of measured moisture content. Besides, according to the basic principle, the proposed method should also be workable for detecting any invaded foreign substance into porous materials.