To check the specific gravity of the electrolyte, it is possible to use a hydrometer (also called an “aerometer”) or a digital density meter (also called a “digital hydrometer”).
Using a hydrometer
A lead acid battery hydrometer is a special type of hydrometer which looks like a syringe with a bulb. Inside the bulb there is a float which is calibrated for measuring the Specific Gravity (SG). To use the hydrometer, you suck some of the battery acid (H2SO4) out of the battery up into the bulb and read off the value indicated by the float floating in the sample.
Using a digital density meter
A digital density meter (sometimes called a digital hydrometer) can be used to measure the specific gravity of the sulfuric acid electrolyte as long as the measuring cell withstands aggressive acids. The result is typically converted into the right temperature and displayed in the desired unit like SG (Specific Gravity) 80/80 on the digital display.
Density meters (or digital hydrometers) have some advantages over hydrometers: As well as being quicker and giving the result on a digital display already converted to the temperature, cleaning is simpler. Between measurements with a digital density meter cleaning of the cell is not required because the samples are similar, only a rinsing prior to the next measurement is performed. Density measurement on a density meter only requires handling of a small volume of 2 mL.
By following a few simple guidelines it is possible to ensure good density measurement results.