An antifreeze hydrometer is a tool that tests the concentration of your engine coolant. Coolant is a mix of water and antifreeze (glycol). If the mixture has too much water, it can freeze in winter, cracking the engine block. If it has too much glycol, the engine can overheat because pure antifreeze does not transfer heat as well as water. The hydrometer features a clear plastic body, a rubber squeeze bulb at the top, a hose at the bottom, and a series of floating plastic discs or a pointer needle inside.
To test the fluid, squeeze the bulb, insert the hose into the radiator or overflow tank, and draw coolant into the chamber. The floating discs or pointer will rise based on the liquid's specific gravity. A scale printed on the side shows the freezing and boiling points of the mix. Wash the tool with clean water after use to prevent dried coolant from sticking the pointer or discs together.