用于滤膜称重的饱和氯化镁溶液恒湿系统搭建与评估

目前中国大气颗粒物污染严重,许多颗粒物测量工作使用基于滤膜采样的重量分析法,滤膜称重过程的质量保障至关重要.本文搭建了一套具有恒湿能力的滤膜平衡称重箱(1.2 m×0.6 m×0.8 m)并自制流量控制系统用于平衡和称量滤膜.通过测试空白滤膜和负载大气颗粒物的滤膜在不同相对湿度下的质量变化确定了30%~40%的平衡相对湿度并选用饱和氯化镁溶液控制称重箱内的相对湿度.用20 L·min~(-1)的干燥气通过装有饱和氯化镁溶液的恒湿箱,再将平衡后的气体通入平衡称重箱使其相对湿度达到稳定,随后用5 L·min~(-1)的流量维持.通过一个月的测试表明,系统在称重箱外相对湿度变化较大时,仍可维持称重箱内相对湿度在30.1%~34.0%之间.使用该恒湿系统称量60张滤膜,恒湿3次后称量的偏差均值不超过±0.02mg.相比于其他恒湿系统,这套系统具有简单、稳定、易于维护和成本低的特点.

Evaluation and Development of a Weighing Chamber by Using Saturated MgCl2 Solution

In recent years, China has suffered a lot from atmospheric particles. Many studies of particles are based on filters. As a result, the accuracy of filter weighing is of great importance. A weighing chamber (1.2 m×0.6 m×0.8 m) was developed and evaluated using saturated MgCl₂ solution with a self-made flow control system to maintain constant relative humidity (RH). By evaluating the mass change of blank and aerosol-enriched filters after weighing in different RH, we selected RH of 30%-40% as the range for the proper constant RH. To reach a constant RH, 20 L·min^-1 dry air was put through a RH-constant chamber with MgCl₂ solution in it. Then, the RH-constant air was put continually into the weighing chamber. After the weighing chamber reached stable RH, the flow rate was adjusted to 5 L·min^-1 to maintain the RH. Throughout a one-month test, the weighing chamber maintained 30.1%-34.0% RH while the outside RH changed a lot. We weighed 60 filters with this weighing chamber after equilibration for 24 hours. The standard deviation after three times'' weighing was no more than 0.02 mg. compared to other methods, the RH-controlling method of this weighing chamber was simple, stable, easy to maintain, and cost effective.