API Gravity

1. Faculty of Chemical & Process
Engineering Technology
Formation & Reservoir Technology
Laboratory Manual
BTO3114
Module 2:
API Gravity
(Crude Oil Classification)
Prepared by:
Mohd Shahrul Azuar Mat Roseh
(BTO’s Vocational Training Officer)

2. MSAMR BTO 3114 – Formation & Reservoir Technology Lab
2
INTRODUCTION
Measurement of Density
The most commonly used methods for determining density or specific gravity of a liquid are:
1. Westphal balance
2. Specific gravity balance (chain-o-matic)
3. API hydrometer
4. Pycnometer
5. Bicapillary pycnometer.
The first two methods are based on the principle of Archimedes: A body immersed in a liquid is
buoyed up by a force equal to the weight of the liquid it displaces. A known volume of the liquid to
be tested is weighted by these methods. The balances are so constructed that they should exactly
balance in air.
The API hydrometer is usually used for determining oil gravity in the oil field. When a hydrometer
is placed in oil, it will float with its axis vertical after it has displaced a mass of oil equal to the mass
of hydrometer (Fig. 2.0a). The hydrometer can be used at atmospheric pressure or at any other
pressure in a pressure cylinder.
The pycnometer (Fig. 2.0b) is an accurately made flask, which can be filled with a known volume of
liquid. The specific gravity of liquid is defined as the ratio of the weight of a volume of the liquid to
the weight of an equal volume of water at the same temperature.
Both weights should be corrected for buoyancy (due to air) if a high degree of accuracy is required.
The ratio of the differences between the weights of the flask filled with liquid and empty weight, to
the weight of the flask filled with distilled water and empty weight, is the specific gravity of the
unknown fluid. The water and the liquid must both be at the same temperature.
The bicapillary pycnometer (Fig. 2.0c) is another tool for accurate determination of density. The
density of the liquid sample drawn into the pycnometer is determined from its volume and weight.
Figure 2.0: Schematic diagram of (a) hydrometer, (b) Pycnometer, and (c) Bicapillary pycnometer

3. MSAMR BTO 3114 – Formation & Reservoir Technology Lab
3
EXPERIMENT 2: API GRAVITY (CRUDE OIL CLASSIFICATION)
Objective
The objectives of this API Gravity (Crude Oil Classification) are to study the procedures in order to
obtain fluids’ (crude oil samples) density, specific gravity and API Gravity. By performing lab
procedures and calculation, crude oil eventually, can be classified based on its API Gravity values.
Learning Outcomes
At the end of the experiment, the students will be able to:
(1) Understand the procedure to measure the Density and Specific Gravity (SG) of crude oil
samples by using Pycnometer.
(2) Capable to calculate the density, SG and API Gravity of crude oil samples by using standard
equation.
(3) Capable to classify type of crude oil by referring API Gravity value.
Theory
Density (ρ) is defined as the mass of the fluid per unit volume. In general, it varies with pressure and
temperature. The dimension of density is kg/m3
in SI or lb/ft3
in the English system.
Density (ρ) = Mass/Volume …………… Eq. (2.0)
Where;
Mass = kg @ g
Volume = m3 @ cm3
Specific Gravity (γ) is defined as the ratio of the density of liquid to the density of water at the same
temperature;
Density of fluid or substance, ρ (kg/m3 @ g/cm3) …… Eq. (2.1)
Density of water, ρ H2O (kg/m3 @ g/cm3)
The specific gravity is an important property used to identify substances. It is important in sense that
it shows how heavy a substance is compared to water, and it will show if it will sink or float if is
mixed with water. Crude oils from different places have different specific gravity due to the different
in compositions. Specific gravity can be expressed in degrees using API gravity. Water has API
Gravity of 10, if the substance has the API greater than ten is less dense than water while if its density
is lower than 10, it is denser than water.
The specific gravity of liquid in the oil industry is often measured by some form of hydrometer
that has its special scale.
Specific Gravity (γ) =

4. MSAMR BTO 3114 – Formation & Reservoir Technology Lab
4
The American Petroleum Institute (API) has adopted a hydrometer for oil lighter than water for
which the scale, referred to as the API scale, is
Crude oil is a complex mixture of various components. Depending on its geographic origin, its
chemical composition and consistency vary. The different types of crude oil need to be classified for
further treatment. Both a crude oil's viscosity and its API (American Petroleum Institute) degree are
important parameters for classification. Crude oil's viscosity can vary from a low-viscosity liquid to
up to tar-like, nearly solid form.
*Classification according to the API degree:
Apparatus
(1) Pycnometer
(2) Weight Balance
(3) Thermometer
Consumable Materials
(1) Crude oil samples
(2) Distilled water
(3) Acetone
API° Gravity =
Where;
γ = Specific Gravity
…..……….. Eq. (2.2)
Table 2.0: Comparison of different types of crude oil according to API degree and specific density.

5. MSAMR BTO 3114 – Formation & Reservoir Technology Lab
5
Procedures
(1) Wash the 50 ml pycnometer and stopper with soap solution using a brush to remove dirt. After
that, rinse the pycnometer with tap water.
(2) Pipette 10 ml Sulfuric Acid into the pycnometer. Attach the stopper and shake the pycnometer
to clean the remaining dirt.
(3) Empty pycnometer, washing with tap water carefully.
(4) Finally rinse the pycnometer with distilled water and dry.
(5) Take weight of the empty dry pycnometer. Note the weight as Wdry
(6) Now, fill the pycnometer with the crude oil, Sample A.
(7) Place the pycnometer in a water bath at 24°C for 30 minutes.
*Temperature of crude oil sample can be assumed at 24°C if water bath not available in the
lab.
(8) Take out Sample A from water bath and attach stopper tightly.
(9) Dry and clean the outer surface of the pycnometer with dry tissue paper.
(10) Take weight of the Sample A filled pycnometer. Note the weight as Wsa.
(11) Repeat step (6) to (10) for all remaining crude oil samples.
(12) Calculate the crude oil density and specific gravity (SG) for each sample and key in the values
in the Table 2.2 below.
(13) Also, calculate the API Gravity value for each crude oil sample. Based on that API Gravity
values, please identify and classify all the crude oil samples on which type?
Result & Discussion
*Temperature: °C
Fluid
Pycnometer
Mass (g)
Pycnometer
+ Liquid (g)
Liquid (g)
Pycnometer
Volume (cm3)
Density P,
(g/cm3)
Specific
Gravity, γ
(1) Calculate the API Gravity value for each crude oil sample. Based on that API Gravity values,
please identify and classify all the crude oil samples on which type? Give a brief explanation
Table 2.2
Table 2.1: Reference; density of water, kg/m3 at different temperatures

6. MSAMR BTO 3114 – Formation & Reservoir Technology Lab
6
on each type of crude oil.
(2) Also, based on API Gravity values, plot graph API Gravity vs Specific Gravity and discuss the
relationship between them.
Question
(1) What are the differences between density and specific gravity (SG)?
(2) What is effect of temperature on specific gravity?
(3) What is API Gravity? Explain.
(4) What is the significance of density and SG test in oil industry field?
*Experiment data
• Pycnometer Mass (Wdry) = 35 g
• Pycnometer + Liquid;
- Sample A = 62.5 g
- Sample B = 79.5 g
- Sample C = 80.5 g
- Sample D = 83.5 g
- Sample E = 86.0 g