Convert centistokes [cSt] to millimeter²/second [mm²/s] • Kinematic Viscosity Converter • Hydraulics — Fluids • Compact Calculator • Online Unit Converters (2024)

Acceleration

Did you know that fighter pilots wear G-suits that apply pressure to the stomach and legs to prevent blood from rushing away from the brain during acceleration?

Overview

Viscosity is a measure of internal resistance of fluid to the force causing the fluid to flow. There are two types of viscosity: absolute viscosity that is used more commonly in medicine, cosmetics, and cooking, and kinematic viscosity. The latter is more often used in the automotive industry.

Absolute Viscosity and Kinematic Viscosity

Absolute viscosity, also known as dynamic viscosity, measures the resistance of a fluid to a force that is acting upon it to make it flow. Kinematic viscosity measures this resistance, as relative to the density of the substance. It is calculated as absolute viscosity divided by the density.

When measuring kinematic viscosity, it is important to specify which temperature it was measured for, because it differs with temperature for each substance. When measuring viscosity for machine lubrication purposes, it is commonly measured for 40° C (104° F) and for 100° C (212° F). Decreased viscosity of oil with an increase in temperature is a very useful property for auto mechanics who change the oil. To remove most of the oil from the car, they run the engine to warm it up, increasing the flow of the oil.

Newtonian and Non-Newtonian Fluids

Viscosity changes for different types of fluids. A distinction is usually made between two types: Newtonian and non-Newtonian fluids. Fluids that deform at the same rate regardless of the force that causes this deformation are Newtonian. The rest of the fluids are not. Non-Newtonian liquids deform at a different rate when the amount of shear stress changes — this rate can either increase or decrease with the increase in deformation, depending on the substance.

A good example of a non-Newtonian fluid is ketchup. When it is in a bottle, applying a small amount of force to get it out is often futile. However, applying a lot of force like shaking the bottle hard makes the ketchup come out. High levels of stress make ketchup less viscous than low levels of stress — this is one of the effects that variable stress has on the viscosity of non-Newtonian fluids.

Other non-Newtonian fluids, on the contrary, become more viscous when stress increases. A mixture of corn starch and water has this property. If a person runs across a pool with this liquid, she will not sink, because she applies a considerable amount of pressure on the liquid. If, however, she simply stands on the surface, she will sink, because the force with which her soles push the liquid is smaller. The viscosity of some other non-Newtonian liquids changes based on the stress over a given period of time. For example, vigorous stirring will make some non-Newtonian liquids less or more viscous than they would be with a different amount of force applied during stirring. Honey behaves in this manner, becoming less viscous with vigorous stirring.

Viscosity in Machinery Lubrication

Viscosity is a very important property and it is often used in daily life, especially for liquids. The study of the flow of matter, especially of liquids, is called rheology. Among other things, it studies viscosity, because viscosity determines in which manner the flow happens for different liquids. Rheology is often concerned with studying both Newtonian and non-Newtonian liquids.

Viscosity Indicators for Oil

The oil used to lubricate machines is made with strict viscosity requirements, depending on which situation it is to be used in. The manufacturers test the viscosity before they sell the oil, and the mechanics measure the viscosity of oil before using it, to ensure that it fits the requirements. Measures in these two situations happen differently, with the manufacturers measuring kinematic viscosity while the mechanics measuring the absolute one and then converting it to kinematic viscosity. Different devices are used for these measurements. It is important to know the difference and not to confuse the two, because their values are not the same.

Oil manufacturers prefer kinematic viscosity to absolute because it provides them with more accurate measurements. Meters that measure kinematic viscosity are also usually cheaper than those that measure absolute viscosity.

The importance of the correct viscosity value of oil is paramount because, on the one hand, the oil has to provide enough viscosity to coat the metal surfaces that touch and decrease friction between them, and on the other hand, it has to flow well through the pipes. Decreasing friction helps preserve the car parts for longer, but the oil coating has to be thick enough. At the same time, the flow has to be smooth, even in cold weather. This means that the oil has to have low enough viscosity in low temperatures. Besides, if the oil is too viscous, then the parts that are lubricated with oil will not move well, and more fuel will be needed to operate the car.

A good viscosity balance is also important because motor oil is a mix of several different types of oil and it also contains other additives of variable viscosity, such as anti-foaming and detergent substances. The manufacturers need to adjust the viscosity of the final mix.

Oil Changes

The oil that is used to lubricate the car engine has to be changed after a certain amount of driving. Oil gets dirty while the car is in operation, and its additives run low after extended use. Changing it helps prevent clogging of the pipes, among other problems. While some companies that make oil suggest that oil is changed every 3,000 miles or roughly every 5,000 kilometers, some mechanics and car manufacturers propose to do it every 5,000 to 15,000 miles (8,000 to 24,000 km) if the car is in good condition. The original 3,000-mile recommendation was based on the older engines and is now a marketing ploy to make people buy more oil.

The mileage recommendations gradually increase as the engines of cars are improved, therefore before changing the oil it is a good idea to consult the user’s manual and the manufacturer’s website for this information. Some cars are equipped with a system that monitors the condition of the oil; using this system is also a good idea.

Choosing Oil

When choosing which oil to use for the vehicle it is important to select the correct grade, which corresponds to how thick the oil is and which temperatures it is best suited for. Some oils are designed to work better in colder climates, some — in warmer ones, and some work well in a range of temperatures. Oils can also be synthetic, conventional, and a blend of both, often called a synthetic blend. Synthetic oils are the most expensive, and the conventional ones are the cheapest. This is due to the cost of manufacturing synthetic oils. They are becoming popular because they last longer and do not change their viscosity for a large range of temperatures, as discussed below. It is important to note though, that if asking for synthetic oil, it is necessary to ensure that the filter will last as long as this oil, until the next oil change.

The rate of change of the oil’s viscosity due to temperature is not uniform for all oils. Viscosity Index (VI) represents how much the viscosity changes. 0 is for the oil, the viscosity of which changes the most with temperature. It is preferable for vehicles that the oil does not change its viscosity significantly, even with low or high temperatures, thus a high VI is desirable. This is especially important for cold climates, where the temperature difference between the cold and the warm engine is significant. At the moment synthetic oils rate higher on the VI scale than do conventional ones, with the blend being in the middle between the two.

To preserve the viscosity of oils at a larger range of temperatures manufacturers sometimes add special substances to the oil. These often burn off before the recommended oil change, and the oil loses its properties quickly. As a result, the driver has to check the oil somehow for the amount of these substances still present and change the oil more frequently than intended, or suffer the consequences of poor lubrication in the engine. This makes such oil more high-maintenance compared to the oils with a lower VI.

Oil for Other Machines and Vehicles

Other machinery has similar concerns, but sometimes these needs vary by the machine or vehicle. Bike owners also have to choose between oils with higher or lower viscosity. The highly viscous oil reduces friction well, but gathers dust and other external pollutants since the chain of the bicycle is exposed. On the other hand, oil low in viscosity washes off easily under the elements and does not provide thick enough coating, requiring frequent reapplication or causing the gears to wear away.

Measuring Viscosity

Viscosity can be measured by rheometers if the viscosity of the liquid is different depending on the conditions, or by viscometers for all other fluids. Some rheometers have a cylinder that rotates inside another one. They measure the force, with which the liquid is trying to rotate the internal cylinder. Others place the liquid on a plate, then put a cone inside the liquid, and measure the force with which the liquid acts on a cone. There are other types of rheometers available, but many of them measure the force that the liquid applies to the device.

Viscometers (viscosimeters) measure the drag of the fluid that moves against the walls of the measuring device. This drag is generally measured by forcing a substance through a thin pipe (capillary). The resistance of the substance to flow through, shown by the time it takes to travel a given area within the capillary, reflects the viscosity of the substance. The time can be converted to viscosity by using the conversion table, available for each measuring device.

References

This article was written by Kateryna Yuri

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Hydraulics — Fluids

Hydraulics is a field of applied science and engineering dealing with the mechanical properties of liquids. Hydraulics focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control, and transmission of power by the use of pressurized liquids. Fluid mechanics is the branch of physics that studies fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion.

Kinematic Viscosity Converter

Kinematic viscosity is defined as the ratio of absolute or dynamic viscosity to density. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with its mass density.

In the SI the unit is m²/s. However, the commonly used measure is the stokes (or stoke) where 1 St = 10⁻⁴ m²/s. Its symbol is St. In engineering, centistokes (cSt) are usually used. 1 St = 100 cSt or 1 cSt = 10⁻⁶ m²/s. Water at 20 °C has a kinematic viscosity of about 1 cSt.

Using the Kinematic Viscosity Converter Converter

This online unit converter allows quick and accurate conversion between many units of measure, from one system to another. The Unit Conversion page provides a solution for engineers, translators, and for anyone whose activities require working with quantities measured in different units.

You can use this online converter to convert between several hundred units (including metric, British and American) in 76 categories, or several thousand pairs including acceleration, area, electrical, energy, force, length, light, mass, mass flow, density, specific volume, power, pressure, stress, temperature, time, torque, velocity, viscosity, volume and capacity, volume flow, and more.
Note: Integers (numbers without a decimal period or exponent notation) are considered accurate up to 15 digits and the maximum number of digits after the decimal point is 10.

In this calculator, E notation is used to represent numbers that are too small or too large. E notation is an alternative format of the scientific notation a · 10^x. For example: 1,103,000 = 1.103 · 10⁶ = 1.103E+6. Here E (from exponent) represents “· 10^”, that is “times ten raised to the power of”. E-notation is commonly used in calculators and by scientists, mathematicians and engineers.

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