1

Laboratory of Petroleum and Gas Properties

2. Density and Specific Gravity (Hydrometer)

The aim of this experimentThis test method covers the laboratory determination of the density, relative density (specific gravity), or API gravity of crude petroleum, petroleum products, or mixtures of petroleum using a glass hydrometer,

TheoryThe density (ρ) of a substance is its weight per unit volume.

Specific gravity (SG) For liquid systems, the SG is defined as the ratio of density of a liquid to that of water, and therefore, it is a dimensionless quantity.

2

Laboratory of Petroleum and Gas Properties

Hydrometer is considered the simplest and the fastest method in determination of specific gravity of a liquid.

The operation of the hydrometer is based on the Archimedes principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced.

The American Petroleum Institute Gravity (API gravity), is a measure of how heavy or light a petroleum liquid is compared to water.

Petroleum is lighter than water and floats: API gravity > 10Petroleum is heavier than water and sinks: API gravity < 10

Hydrometer

3

Laboratory of Petroleum and Gas Properties

Thus, the lighter the liquid (that is, the less its specific gravity), the deeper the body sinks because a greater amount of liquid is required to equal the body's weight.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright.

Hydrometers usually contain a scale inside the stem, so that the specific gravity can be read directly.

4

Laboratory of Petroleum and Gas Properties

It is important to know that most hydrometers are calibrated on15.5 0C.

Density of water at 15 0C is 0.999099 g/cm3

Materials and apparatus1. Hydrometer , 2. Thermometer, 3. Graduated cylinder 4. Funnel 5. Tested liquid (Sample) 6. Stirring road

𝑆𝑝 .𝑔𝑟𝑟=𝑆𝑝 .𝑔𝑟 𝑡

1−[23∗10− 6 (𝑡−𝑟 )−2∗10−8 (𝑡−𝑟 )2]

Where:Sp.grr= Hydrometer reading at the reference temperature, r oCSp.grt = Hydrometer reading on hydrometer scale whose reference temperature is t oC

5

Laboratory of Petroleum and Gas Properties

The procedure:

a) Fill the cylinder with the sample.b) Insert the thermometer in the samplec) Stir the sample slowly with a stirring road to ensure uniform

temperatured) Record the temperature of the liquid by the thermometer (t)

and remove the thermometere) Lower the hydrometer into the liquid and release it carefully,

taking care to avoid wetting the stem above the level at which it floats freely.

f) When the hydrometer has come to rest floating freely away from the walls of the cylinder and away from the bottom of cylinder , read the hydrometer scale

6

Laboratory of Petroleum and Gas Properties

Hydrometer Scale Reading

7

Laboratory of Petroleum and Gas Properties

Discussion and Conclusion

Calculations

- Calculate the specific gravity of the sample at 20 oC- Calculate the density of the sample at 20 oC

1. Compare your result (specific gravity and the density of the sample) with any references.

2. At which time of the day, tanking your car is the cheapest? Why?