The next piece of equipment we will discuss is the Bubble Counter.
A bubble counter allows us to easily determine the flow rate of the CO2 that is going into the aquarium. We often refer to the flow rate as "bubbles per second." The bubble counter is filled with fluid (it can be water, glycerin, or even mineral oil. The latter two are sometimes preferred because bubbles flow through the liquid slower, making it easier to count. In addition, they do not evaporate like water does), and as gas flows through, bubbles are generated so that you can count your bubble rate.
Many different types of bubble counters exist, from in-line ones, to DIY ones, to commercially bought bubble counters. They all perform the same function, at different costs. Some people prefer one over the other due to aesthetics, size constraints, etc., but in the end, they perform the same function.
Not much else to say on this piece of equipment