水蒸气渗透系数干湿杯实验系统性误差的来源及修正

Origin and Correction of the Systematic Errors in Cup Test for Water Vapor Permeability

水蒸气渗透系数是多孔建筑材料最重要的湿物性参数之一,通常借助干湿杯实验获得。该实验中有盐溶液表面到被测试件下表面所夹空气层的阻力、被测试件上表面空气层阻力和测试装置的空气渗漏3种常见的系统性误差因素。本文分析了上述3种系统性误差因素对8种常见多孔建筑材料水蒸气渗透系数测试结果的影响,并提出相应的修正方法。结果显示,不考虑下、上空气层阻力时,水蒸气渗透系数的测试值偏小,且试件的水蒸气渗透阻越小,空气层阻力的影响越大。而不考虑测试装置的空气渗漏时,水蒸气渗透系数的测试值偏大,且试件的水蒸气渗透阻越大,空气渗漏的影响越大。3种因素的影响明显,产生的最大相对误差分别高达60%、18%和20%,有必要通过增加测试和计算修正。若某影响因素未知时,建议从试件预估的水蒸气渗透阻、下部空气层厚度、装置的渗漏系数和上部风速等方面控制系统性误差,以获得较为可靠的结果。

Water vapor permeability is one of the most important hygric properties of porous building materials,and it is usually obtained by cup tests.There are three common factors generating systematic errors in cup tests,including the resistance of the air layer between the surface of the salt solution and the lower surface of the test specimen,the resistance of the air layer above the upper surface of the test specimen,and air leakage of the test device.This paper analyzed these three factors on the results of the cup tests for eight common porous building materials,and proposed corresponding correction methods.Results show that the measured water vapor permeability is smaller than the real value when the lower and upper air layer resistances are ignored,and the impact of the air layer resistances increases with decreasing vapor resistance of the specimen.In contrast,the calculated water vapor permeability is larger than the real value when the air leakage of the test device is ignored,and the impact of the air leakage increases with the increasing vapor resistance of the specimen.All these three factors are exerting significant impacts,producing maximum relative errors up to 60%,18%,and 20%,respectively,and should therefore be corrected through adding tests and calculations.If some influencing factors are unknown,it is suggested to control the systematic errors from the estimated water vapor resistance of the specimen,the thickness of the lower air layer,the leakage coefficient of the device,and the upper wind speed,to obtain more reliable results.