In this work we presented the issues related to mass measurement and the dependencies that apply to this process. A special attention was given to the comparison process of weights specifying the elementary sources of measurement uncertainty. The mass comparators were divided due to their design with a focus on their advantages and disadvantages. The research demonstrated the differences between automatic and manual comparison process. Also the importance of ambient conditions for calibration was discussed determining the potential areas of increased risk. Several places for mass comparator installation were presented as well as users opinions concerning operation of this type of equipment. Two projects on kilogram redefinition were described: Watt balance and Avogadro project (silicon sphere).