Measuring Liquid Water Content Within Porous Materials By A Single Heating Tube

One of the commonly used methods to improve the comfort of the building environment is thermal insulation and noise reduction.The insulation materials in the building enclosure can easily accumulate water in the high humidity environment because of their porous characteristics,which leads to its performance degradation.Therefore,the wall will appear mildew,corrosion,wall falling off and so on,seriously endangering people’s life and health.If the moisture content of thermal insulation can be monitored,people can take corresponding maintenance measures in advance.Currently,the heat pulse method is more mature and widely used to measure the moisture content of porous materials,but it still needs to be improved.On the one hand,the distance between heating needle and temperature-measuring needle should be fixed when applying dual-probe heat pulse method.At the moment that the two needles deflect,it’s more complex to correct the deflection distance.Meanwhile,the heating needle needs to provide a larger electric power to ensure that the temperature probe can monitor the effective temperature rise,which causes the generation of latent heat of vaporization at low heterogeneous content and affects the test accuracy.On the other hand,it is difficult to establish a correspondence between the thermal conductivity and the moisture content,so the moisture content estimated by thermal conductivity is too different from the true value through the single-probe heat pulse method.In order to accurately reflect the real value of porous material moisture content,this investigation proposes a new moisture content measurement method based on the single-probe heat pulse method.An electric heating tube,whose surface is fixed with a temperature sensor by thermally conductive adhesive,is embedded into the test material.The transient temperatures caused by the constant heat flux density of the heating tube are monitored.The thermal conductivity and the thermal diffusion coefficient of the material are obtained according to the slope and the intercept of the temperature response curve in the effective logarithmic heating time period.Based on the superposition principle of volumetric heat capacity（i.e.the ratio of material thermal conductivity to thermal diffusivity）,the moisture content of the material is calculated from the change of the volumetric heat capacity concerning its dry and wet conditions.In this investigation,sponge blocks and PET fiber blocks are chosen as the test material,whose volumetric moisture mass range from 0 to 20 kg/m³ with an interval of around 5 kg/m³.The moisture content of the material is obtained by using the recorded temperature data within 500～1200 s.To evaluate the accuracy of the method,the inferred moisture contents are compared with gravimetric results provided by a digital balance.In addition,referring to the"GUM",the standard uncertainty of experimental measurement results are calculated and analyzed to evaluate the precision of the method.The test results show that the accuracy of the heating tube method can satisfy the needs of actual application.Among the range of measured water content,the results of the heating tube method are similar to those of the weighing method,with the error of the single measurement results less than 7%and the error of the average result of multiple measurements less than 0.3kg/m³ H₂O.The main reason for the deviation is that the uneven distribution of water leads to the difference between the overall average moisture content of the material and the local moisture content.Due to the existence of contact resistance,there is no correspondence between measured thermal conductivity,measured thermal diffusivity and moisture content in this investigation.However,the volume heat capacity is less affected by contact resistance and correlates well with the moisture content.According to the uncertainty analysis,the factors affecting the precision of moisture measurement are determined as the heating flux and the slope and intercept of the linear regression line.If a more precise instrument and an accurate model can be boosted,the relative uncertainty can be controlled within 5%.