Measuring Moisture Content In Porous Insulation Materials By A Hot Film

In a humid environment,porous insulation materials can acquire large amounts of moisture,which worsens the materials’performance and causes corrosion.To develop effective methods for daily monitoring of moisture content in thermal insulation materials is It is of great significance to develop an effective method for daily monitoring of the water content in the thermal insulation material,to provide guidance for the maintenance of the thermal insulation material,and to ensure its optimal working performance.Current thermal methods to measure moisture content in porous materials require that a temperature sensor be placed some distance away from a heating element.As a result,the measurement is very sensitive to the distance between the heating element and the sensor.In addition,the required large heat flux induces unwanted convection and radiation heat transfer that interfere with the measurement.To solve these problems,this paper proposed a hot film method to measure the moisture content of porous materials.A hot film was arranged in the test material as a flat heat source,and adiabatic boundary conditions were created at material boundaries.The thermophysical parameters of the test material were inferred by recording the temperature rise rate of the surface of the hot film.Based on the one-dimensional unsteady thermal conductivity model of a homogeneous material with finite thickness,a linear relationship between the transient rate of temperature rise of hot film’s surface and the reciprocal of the volumetric heat capacity was proposed.Inversely,the volumetric heat capacity of the test material can be calculated from the surface temperature rise rate of the hot film.According to the superposition principle of volume heat capacity,the moisture content of the tested material is finally obtained.According to the above principle,a moisture test sensor was constructed.The electric heating film was heated at a constant power for 90 seconds,and the temperature rise rate was calculated with the temperature data in the last 50 seconds,which was recorded by the thermocouple attached to its surface,and finally the material’s water content was calculated.Lightweight sponge with benchmark gravimetric moisture content between0～20 kg H₂O/m³ was selected as test material.The moisture contents measured by hot film method was compared with those determined by gravimetric weighing,so as to evaluate the precision of hot film method.In addition,in order to quantify the measurement reliability,this investigation conducted an uncertainty analysis complying with the BIPM guidelines.The main sources of measurement uncertainty are identified by the evaluation results.The research results showed that the hot film method accurately measured the moisture in porous materials.The deviation in the measured moisture content between the hot film method and the gravimetric weighing was less than 10%.The standard uncertainty of the experimental results was within 2.66 kg H₂O/m³.Increasing the number of experiments in the calibration process can reduce it.The hot film method can more sensitively indicate low moisture content than high moisture content,because a larger rate of temperature rise at a low moisture content can be more easily detected,and also because the low moisture content is more sensitive to the reciprocal of the volumetric heat capacity.Compared with the existing methods,the measurement method proposed in this study has the advantages of easy operation,low cost,short time,high precision and so on.