Soldering terminology

Measuring vessel used for determining the specific gravity of a liquid. During the quality control of soldering flux, specific gravity is used for easy measurement. The device is a glass bottle with a stopper which passes through a vertical capillary (thin tube) open at both ends. Upon filling the container, the stopper is inserted and the excess liquid overflowing the stopper is removed. As a next step, the temperature and weight of the liquid is measured in a container.
If the mass of the pycnometer is w when empty, W when filled with water and W' when filled with the test fluid, the specific gravity of the fluid can be calculated from the following formula: W'-wW-w