Why does a hydrometer measure and how does it work?

Answer 1

• A hydrometer is an instrument that measures the density of a liquid. How this works may be easiest to understand by using a hypothetical example with a human as the hydrometer.

Freshwater Pool

• Let's say you're at a resort that features two Swimming Pools. One pool is freshwater, the other saltwater. You go into the freshwater pool and take with you a waterproof marker. No one else is in the pool, so the water is still. You lower yourself into the deep end, slowly and carefully so you disturb the surface as little as possible. You tread water slightly and only make enough movement to keep yourself upright. Once you're not bobbing up and down at all, you make a mark on your skin with the marker right at the water level. We'll assume that, because of your particular body mass and your general makeup, the mark ends up on your chin.

Saltwater Pool

• Now you go into the saltwater pool with all the same conditions and repeat the whole process. The mark you make in the saltwater pool will be somewhere lower on your body than the mark from the freshwater pool. Why? Because saltwater is denser than freshwater. You---in this hypothetical example---just demonstrated that fact by serving as a human hydrometer. A hydrometer will not sink as deeply in a dense liquid as it will in a liquid that is less dense.

Hydrometer Features

• Actual manufactured hydrometers vary somewhat in design, but a typical example would be one about the size of a pen. It's a sealed glass tube with a ballast weight in the bottom (Mercury or lead possibly). The upper part of the hydrometer---the part that will bob up above the surface of the liquid---has a graduated scale on the inside. This allows the user to place the hydrometer into the liquid, wait for it to come to rest, then take a reading that will indicate the liquid's density.

Applications

• Hydrometers are useful in many applications. One interesting one is the finishing of maple syrup. After maple sap has been boiling and has reached just the right density to be called syrup, a hydrometer placed in it will float at a specific level. If it floats too low, the sap hasn't boiled off enough water and is still too light and thin. If the hydrometer floats too high, the syrup has become too thick and dense and the maple producer will probably have to make something else, like maple sugar candy, instead of syrup. The hydrometer has to give a reading that's just right for it to be the proper density for maple syrup. Hydrometers are used in brewing and wine making and in many other industries. Hydrometers are also used to test the acid in a lead acid battery (like the ones used in automobiles): if the battery's charge is too low, so also will be the concentration or density of the acid.