What is motor oil viscosity?
Simply put viscosity is a physical property of a fluid or gas that reflects it’s tendency to flow. We commonly refer to high
viscosity fluids as being “thick” and low viscosity fluids as being “thin”. It’s an important property of a motor oil because
changes in viscosity affect the ability of the oil to lubricate and protect the moving parts of an internal combustion engine. If
the oil is too thin the oil pump cannot maintain enough pressure to circulate it and the oil will not withstand the forces that form
between moving parts. The metal parts will rub against each other and wear out or fail prematurely from lack of proper
lubrication. Conversely, if the oil is too thick the oil pump will again have problems circulating the oil and it will be too thick
to penetrate into the tiny openings between moving parts. The result is the same – premature wear and failure. So it’s important
that the viscosity of a motor oil be a proper balance between too thin and too thick.
How is viscosity measured?
There are two units of measure commonly used for viscosity of fluids. The basic unit is the centipoise (cP or
mPa·s). This unit describes the movement of the different layers of a fluid when subjected to a horizontal force. It
is commonly known as dynamic or simple viscosity. Another unit of viscosity is the centistoke (cSt or mm2
/s).
This unit describes the ease with which a fluid moves under the force of gravity and is the form of viscosity with
which we are most commonly familiar. This form of viscosity is known as kinematic viscosity.
One of the oldest and most common methods for measuring kinematic viscosity is an apparatus called the
capillary viscometer (see picture). This apparatus is a precisely graduated glass tube. A small quantity of the fluid
to be tested is placed into the top of the tube and held there with air pressure. The pressure is then released and the
length of time it takes for the fluid to move to a graduated mark on the other end of the tube is measured. Higher
viscosity fluids move slower, lower viscosity fluids move faster. The viscometer is usually immersed in a
temperature controlled bath during the test so that the measurement is calibrated to a specific fluid temperature.
Simply put viscosity is a physical property of a fluid or gas that reflects it’s tendency to flow. We commonly refer to high
viscosity fluids as being “thick” and low viscosity fluids as being “thin”. It’s an important property of a motor oil because
changes in viscosity affect the ability of the oil to lubricate and protect the moving parts of an internal combustion engine. If
the oil is too thin the oil pump cannot maintain enough pressure to circulate it and the oil will not withstand the forces that form
between moving parts. The metal parts will rub against each other and wear out or fail prematurely from lack of proper
lubrication. Conversely, if the oil is too thick the oil pump will again have problems circulating the oil and it will be too thick
to penetrate into the tiny openings between moving parts. The result is the same – premature wear and failure. So it’s important
that the viscosity of a motor oil be a proper balance between too thin and too thick.
How is viscosity measured?
There are two units of measure commonly used for viscosity of fluids. The basic unit is the centipoise (cP or
mPa·s). This unit describes the movement of the different layers of a fluid when subjected to a horizontal force. It
is commonly known as dynamic or simple viscosity. Another unit of viscosity is the centistoke (cSt or mm2
/s).
This unit describes the ease with which a fluid moves under the force of gravity and is the form of viscosity with
which we are most commonly familiar. This form of viscosity is known as kinematic viscosity.
One of the oldest and most common methods for measuring kinematic viscosity is an apparatus called the
capillary viscometer (see picture). This apparatus is a precisely graduated glass tube. A small quantity of the fluid
to be tested is placed into the top of the tube and held there with air pressure. The pressure is then released and the
length of time it takes for the fluid to move to a graduated mark on the other end of the tube is measured. Higher
viscosity fluids move slower, lower viscosity fluids move faster. The viscometer is usually immersed in a
temperature controlled bath during the test so that the measurement is calibrated to a specific fluid temperature.
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