Accuracy of determining density or relative density is determined by the combined accuracy of measuring the volume and weight of a given quantity of the substance. While measuring relative density of liquids, two common method of measuring the volume are a measuring cylinder and a density bottle.Among these two method the density bottle gives more accurate measurement of the volume, resulting in more accurate determination of density or relative density.
Volume is measured using a measuring cylinder by pouring a given quantity of the liquid in the cylinder and reading out the quantity against the graduations provided in the cylinder for the purpose. In this method errors can occur due to the limitation of the human eye to differentiate between minor variations in the level of the liquid in the cylinder. Also generally the graduations in the measuring cylinder are not marked too accurately. In contrast the density bottles are manufactured to contain a known exact volume of liquid with much higher accuracy of measurement. Also the bottle with the stopper are so designed that human limitation do not affect the quantity of liquid filled in it.
The density bottle is used to find the density of the liquid, where you need to fill up the density bottle with a specific loquid and put its stopper or lid in a firm fixed postion.Any extra liquid could pass through the lid. The mass M of the bottle along with the content liquid could be found. If you subtract the mass m of the empty bottle from M you get the liquid mass of a known volume. Then the mass of the liquid divided by its volume gives the density of the liquid. Similarly it could be found the density of any othe liquid. The ratio of the density of two liquids is the relative density of the first liquid to the 2nd liquid.
The handy part here is the accuracy is more if you go by density bottle, as it has a narrower neck, than the cross section area of a jar. The accuracy may be less while judging the level of the liquid in jar.