Measurement Of Moisture Content In Porous Insulation Materials Via Temperature Matching

Porous materials are widely used for thermal insulation. However, porous materials can easily absorb moisture when exposed to high-humidity conditions. The gained moisture degrades the insulation performance and induces microbe growth. If the water content inside the porous insulation material can be routinely monitored, the maintenance and treatment to the insulation material can be appropriately taken to minimize the adverse impacts caused by the moisture accumulation. Numerous methods have been developed to detect moisture contents in soils. So far there is still lack of a rapid, inexpensive, nondestructive and accurate method for moisture measurement. As compared with the soils, the porous insulation materials are non-hygroscopic and light. The insulation materials cannot hold moisture with a quantity as large as that in the soils. The distribution of the moisture in a non-hygroscopic material can be highly non-uniform. Measuring the moisture in insulation materials is hence more challenging.This investigation proposed to measure moisture contents in porous materials based on transient temperature responses for a short while. An electric heating wire and two temperature sensors are embedded into the test material. The transient temperatures caused by the sudden constant heating of the heating wire were monitored. Based on the composite heat conduction theory, the impacts of the finite radius and heat capacity of the heating element to the monitored temperature responses are considered. The volumetric heat capacity (product of the density with the specific heat) of the material is obtained by matching the transient temperature responses through enumerating all possible thermal properties. According to the addition of volumetric heat capacities, the moisture content is inferred from the change of the volumetric heat capacity with respect to its dry status. In this investigation, light sponge blocks were chosen as the test material, whose volumetric moisture mass ranges from 0 to 80 kg/m3. The separating distance between the heating wire and the temperature sensors was fixed to 10.5 mm. The temperature responses within the initial 100s were recorded. To evaluate the precision of this method, the measured moisture contents between the proposed method and the gravimetric method using a digital precision balance were compared. In addition, the measurement uncertainties are evaluated by following the ISO and BIPM guidelines. Some strategies to improve the measurement accuracy are summarized.The results of two repeating series of isolated tests show that the maximum gap of the measured volumetric moisture mass between the proposed method and the gravimetric method is within 7 kg/m3 and most of their relative deviations fall within 15%, which is sufficient for engineering applications. Because the gravimetric method indicates only the averaged moisture contents in the whole material while the proposed method measures the moisture contents in the surrounding of the heating wire and the temperature sensors, the results may differ when the distribution of water is not uniform. The uncertainty analysis shows that the accuracies of the measured moisture contents are ascribed to the precision in measuring the heating power rate, the distance between the heating wire and the temperature sensor, and the transient temperatures. The possible moisture contents span widely after accounting for the above three uncertain sources. Fortunately, the moisture contents by the gravimetric method fall within the uncertain scopes. To further improve the measurement quality, a more precise powermeter and a more sophisticated temperature sensor are needed. In addition, maintaining a fixed distance between the heating element and the temperature sensor would help greatly to reduce the uncertainties...