Its an instrument consisting usually of a glass flask, with a thermometer?

Question

...for determining the relative density or specific GRAVITY OF LIQUIDS? What is this scientific instrument?

Answer 1

The answer is a hydrometer. It is a direct-reading instrument for indicating the density, specific gravity, or some similar characteristic of liquids.

Almost all hydrometers are made of a high-grade glass tubing. The main body is the float section in the bottom of which ballast, such as small shot, is secured. A small-diameter tube, the stem, extends from the upper end of the float section. Inside the stem is the scale, printed on heavy-grade paper, and well-secured within the stem so its position will not change. When the hydrometer is placed in a liquid, the stem extends vertically above the surface for a portion of its length.

Hydrometers may be classified according to the indication provided by graduations of the scale as follows:

(1) density hydrometers, to indicate densities at a particular temperature, and usually for a particular liquid;
(2) specific gravity hydrometers to indicate specific gravity of a liquid, with reference to water, at a particular temperature;
(3) percentage hydrometers to indicate, at a particular temperature, the percentage of a substance such as salt, sugar, or alcohol dissolved in water (alcoholometers are an example); and
(4) arbitrary scale hydrometers, indicating the density, specific gravity, or concentration of a liquid in terms of an arbitrarily defined scale, at a defined temperature.

The last group includes the saccharimeter (indicates percentage of pure sucrose solutions); the Baumé hydrometer (measures specific gravity of liquids lighter than water); the lactometer (tests milk); and the barkometer (tests tanning extracts).

Brewers( I would know because I am a Brewer by Profession), wine-makers and distillers use hydrometers extensively to measure the percentage of alcohol in their products.

Answer 2

It is not a flask it is a graduated cylinder and a thermometer is used in conjunction with a Hydrometer.
(A hydrometer does not measure density .. density is a mass per unit volume and has units (like g/litre, lbs/gallon ..etc)

The Hydrometer shows the S.G. (or R.D.), (or API gravity) at the specific temperature noted.
Correction tables are then used to determine the S.G. or API gravity at STP (Standard Temperature and Pressure).

(S.G, R.D. & API have no units.. they are a comparison between gases against air, or liquids/solids against water).

Answer 3

Hydrometer!

Answer 4

Yep, its a Hydrometer - often used by the department of weights and measures to test alcohol content of spirits and beer.

See link....
http://en.wikipedia.org/wiki/Specific_gravity#Relative_density_and_hydrometers

Answer 5

hydrometer