Hydrometers are used to determine the specific gravity of a liquid. Consists of a long glass tube containing a ballast at the base of the float bulb and graduations on the stem. The ballast is made from steel pellets free from lead and mercury.
API ASTM – The American Petroleum Institute scale is used to measure the specific gravity of a liquid relative to water. A sample with an API gravity value greater than 10 is lighter than water, and an API gravity value less than 10 is heavier than water. Although specific gravity does not have units, the API scale is graduated in degrees, and was designed such that most values would fall between ten and seventy API gravity degrees. According to Perry's Chemical Engineers' Handbook (8th Edition), °API = 141.5/sp gr -131.5.