恒重是指两次称量所得质量之差不得超过一定的允许误差。《火电厂排水水质分析方法》（DL/T 938-2005）对恒重的定义：连续两次烘干或灼烧后的质量，其差值不超过0.0003g。
用已烘干至恒重的G4玻璃坩埚过滤。
Filter the mixture with a G4 glass crucible pre-dried to a constant weight.
结果采用烘箱两次干燥法比直接干燥法容易达到恒重。
Results the oven-twice drying was easier than the direct drying to achieve constant weight.
发现恒重后的双锍盐含有二个结晶水这一新现象，并确认最终产物为聚对苯乙炔。
Based on our determined results, we had found a new phenomenon that the disulphonium salt contains two crystal-waters and the last product is PPV.
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