Different Kinds of Hydrometers
What is a hydrometer?
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.
What are the different kinds of hydrometers and when do you use them?
1. Specific Gravity hydrometers can be used for almost any liquid. Specific Gravity is a dimensionless unit defined as the ratio of density of the material to the density of water. If the density of the substance of interest and the reference substance (water) are known in the same units (e.g., both in grams/centimeter3 or pounds/feet3), then the specific gravity of the substance is equal to its density divided by that of the reference substance (water =1 grams/centimeter3).
2. Baume hydrometers are calibrated to measure specific gravity on evenly spaced scales; one scale is for liquids heavier than water, and the other is for liquids lighter than water.
3. Brix (Bx) hydrometers is for determining the percentage of weight by sucrose. One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by weight (% w/w) (strictly speaking, by mass). If the solution contains dissolved solids other than pure sucrose, then the °Bx only approximates the dissolved solid content. The °Bx is traditionally used in the wine, sugar, fruit juice, and honey industries.
4. API hydrometers, also known as The American Petroleum Institute index, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is thus an inverse measure of the relative density of a petroleum liquid and the density of water, but it is used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically, API gravity has no units (see the formula below), it is nevertheless referred to as being in “degrees”. API gravity is gradated in degrees on a hydrometer instrument. The API scale was designed so that most values would fall between 10 and 70 API gravity degrees.
5. Alcohol Proof hydrometers are used for distilling and rectifying and for waste liquors.
6. Isopropyl Alcohol hydrometers measure percent by volume of isopropyl alcohol.
7. Sodium Chloride hydrometers measure saturation and concentration of sodium chloride.
8. Salt Brine hydrometers are graduated in percent of either saturation of sodium chloride in water, or by weight of sodium chloride.
9. Calcium Chloride Salometers (a hydrometer for indicating the percentage of salt in a solution) are for determining the percentage of saturation, specific gravity or freezing point of CaCl2.
10. Draft Survey hydrometers determine the apparent density of sea/fresh water.