Hydrometer is a device used for the direct measurement of density of liquids.

Principle

Based on the law of floatation, it makes use of the characteristics of a floating body.

Volume of body × Density of body = Volume of liquid displaced × Density of liquid

Working of a Hydrometer

A simple hydrometer is a glass test tube with lead shots at the bottom. This test tube, along with a scale, is immersed partially and then set free into the liquid whose density has to be measured. The length of the immersed portion of the tube, as indicated by the scale, gives the density of the liquid.

Let the two liquids be water and alcohol. Immerse the test tube along with the scale into water first. Let the length of the test tube under the water be l_l. Therefore, this length gives the density of water. As the density of water is 1 g cm⁻³, this length can be considered as standard.

Now, immerse the tube into the alcohol. Measure the length of the immersed portion. Let the length be l₂ this time.

Therefore,

Commercial Hydrometers

1. Hydrometer for lighter liquids

Construction:

• Long narrow stem

• Big floatation bulb (so that the hydrometer does not sink in the lighter liquid)

• Small gravity bulb

2. Hydrometer for heavier liquids

• Longer narrow stem

• Small floatation bulb

• Small gravity bulb

3. Lactometer

Very small range of densities can be measured using a lactometer. It is used for measuring the amount of water added, if any, to milk.

The relative density of pure milk is 1.045 g cm⁻³. Thus, this hydrometer is used for measuring densities in the range 1 to 1.045 g cm⁻³.

4. Acid-battery Hydrometer

Batteries contain concentrated sulphuric acid. The density of this sulphuric acid is required to be maintained. It has been found that if sulphuric acid density falls below 1.16, then the battery may get damaged permanently.

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Acid-battery hydrometer is specially designed to be put over a battery. This hydrometer consists of an acid sucker, which takes samples from the battery.

Hydrometer is a device used for the direct measurement of density of liquids.

Principle

Based on the law of floatation, it makes use of the characteristics of a floating body.

Volume of body × Density of body = Volume of liquid displaced × Density of liquid

Working of a Hydrometer

A simple hydrometer is a glass test tube with lead shots at the bottom. This test tube, along with a scale, is immersed partially and then set free into the liquid whose density has to be measured. The length of the immersed portion of the tube, as indicated by the scale, gives the density of the liquid.

Let the two liquids be water and alcohol. Immerse the test tube along with the scale into water first. Let the length of the test tube under the water be l_l. Therefore, this length gives the density of water. As the density of water is 1 g cm⁻³, this length can be considered as standard.

Now, immerse the tube into the alcohol. Measure the length of the immersed portion. Let the length be l₂ this time.

Therefore,

Commercial Hydrometers

1. Hydrometer for lighter liquids

Construction:

• Long narrow stem

• Big floatation bulb (so that the hydrometer does not sink in the lighter liquid)

• Small gravity bulb

2. Hydrometer for heavier liquids

• Longer narrow stem

• Small floatation bulb

• Small gravity bulb

3. Lactometer

Very small range of densities can be measured using a lactometer. It is used for measuring the amount of water added, if any, to milk.

The relative density of pure milk is 1.045 g cm⁻³. Thus, this hydrometer is used for measuring densities in the range 1 to 1.045 g cm⁻³.

4. Acid-battery Hydrometer

Batteries contain concentrated sulphuric acid. The density of this sulphuric acid is required to be maintained. It has been found that if sulphuric acid density falls below 1.16, then the battery may get damaged permanently.

­­­­­­­­­

Acid-battery hydrometer is specially designed to be put over a battery. This hydrometer consists of an acid sucker, which takes samples from the batter…