The internal structure of traditional water meters can be divided into three parts: shell, sleeve, and inner core from the outside to the inside. The shell is made of cast iron, and after water comes out of the inlet, it passes through the lower annular space of the shell, which is called the "lower annular chamber". Above this circular space, there is an "upper chamber" connected to the water outlet. There is a filter with a small hole at the bottom of the sleeve, which filters out impurities in the water. There are two rows of circular holes on the side of the sleeve, located exactly opposite the upper and lower ring chambers of the shell. Obviously, the lower row is the inlet hole and the upper row is the outlet hole. It is particularly noteworthy that both rows of holes are drilled diagonally along the tangent direction of the circle. Note that the directions of the upper and lower rows of holes are opposite. Water flows in along the tangent direction from the lower drainage hole, inevitably forming a rotating water flow, which is very important for the operation of the water meter. The inner core is divided into three layers: upper, middle, and lower. From the glass window, the upper layer is visible, with only the pointer and dial. The most crucial thing is actually the lower layer, which has a plastic wheel with many plastic blades on the edge, called the "impeller".