Hydrometers

A hydrometer is an instrument used to measure the density of a liquid as compared to that of water. Hydrometers usually consist of a calibrated glass tube ending in a weighted glass sphere that makes the tube stand upright when placed in a liquid. The lower the density of the liquid, the deeper the tube sinks. Depending upon the intended use hydrometers can vary in size and will feature different types of scales.

All hydrometers are standardized, but not all hydrometers are calibrated. In order to manufacture a hydrometer, the instrument must be compared to a traceable reference standard. Standardization during manufacturing establishes the guidelines for scale placement, thus the hydrometer has been standardized. If a hydrometer is calibrated (in the past, the term “certified” may have been used) this means that, after the instrument is manufactured, it’s compared to an instrument that was certified by an outside body at a specific point or points along the scale. The results of this calibration are recorded on an official report. This report of calibration accompanies the calibrated instrument after the calibration process.

There are several types of hydrometers, each designed for a specific application:

• Specific Gravity hydrometers measure the ratio of a material’s density to the density of water
• Baume hydrometers measure specific gravity on evenly spaced scales, with one scale for liquids heavier than water and one for liquids lighter than water
• Brix (Bx) hydrometers determine the percentage of weight by sucrose; traditionally used in the wine, sugar, fruit juice, and honey industries
  API hydrometers measure how heavy or light a petroleum liquid is compared to water, used to compare the relative densities of petroleum liquids
• Alcohol Proof hydrometers are used for distilling and rectifying, and for waste liquors
• Isopropyl Alcohol hydrometers measure percent by volume of isopropyl alcohol
• Sodium Chloride hydrometers measure saturation and concentration of sodium chloride
• Salt Brine hydrometers are graduated in percent of either saturation of sodium chloride in water or by weight of sodium chloride.
• Calcium Chloride Salometers determine the percentage of saturation, specific gravity, or freezing point of CaCl2
• Draft Survey hydrometers determine the apparent density of sea/fresh water