What type of refrigerant does not allow moisture content to exceed 50 PPM by weight?

The refrigerant R-134a is designed to have a moisture content limit that does not exceed 50 parts per million (PPM) by weight. This characteristic is crucial because moisture in refrigerants can lead to various problems, such as corrosion, reduced efficiency, and the formation of acids within the refrigeration system. R-134a's development included properties that minimize issues related to moisture contamination, making it essential in applications where maintaining low moisture levels is critical.

In contrast, other refrigerants listed may not have the same stringent moisture requirements or could face different challenges related to moisture management. R-22, R-12, and R-404A have different operational considerations and moisture tolerance levels that don't align with the specific threshold that R-134a presents. Thus, R-134a is the correct choice for refrigerants that prevent moisture content from exceeding 50 PPM by weight.