Viscosity is the most important physical constant characterizing operational properties boiler houses and diesel fuels, petroleum oils, and a number of other petroleum products. The viscosity value is used to judge the possibility of atomization and pumpability of oil and petroleum products.
There are dynamic, kinematic, conditional and effective (structural) viscosity.
Dynamic (absolute) viscosity [μ ], or internal friction, is the property of real fluids to resist shearing tangential forces. Obviously, this property manifests itself when the fluid moves. Dynamic viscosity in the SI system is measured in [N·s/m2]. This is the resistance that a liquid exhibits during the relative movement of its two layers with a surface of 1 m2, located at a distance of 1 m from each other and moving under the influence of an external force of 1 N at a speed of 1 m/s. Given that 1 N/m 2 = 1 Pa, dynamic viscosity is often expressed in [Pa s] or [mPa s]. In the CGS system, the dimension dynamic viscosity
- [din·s/m2]. This unit is called poise (1 P = 0.1 Pa s). μ Conversion factors for calculating dynamic [
| ] viscosity. | Units | Micropoise (mcP) | Centipoise (sp) | Poise ([g/cm s]) | Pa s ([kg/m s]) | kg/(m h) |
| Units | 1 | 10 -4 | 10 -6 | 10 7 | kg s/m 2 | 3.6·10 -4 |
| Micropoise (mcP) | 10 4 | 1 | 10 -2 | 10 -3 | 3,6 | 1.02·10 -8 |
| Centipoise (sp) | 10 6 | 10 2 | 1 | 10 3 | 1.02·10 -4 | 3.6 10 2 |
| Poise ([g/cm s]) | 10 7 | 10 3 | 10 | 1 3 | 1.02·10 -2 | 3.6 10 3 |
| Pa s ([kg/m s]) | 1.02·10 -1 | 2.78 10 3 | 2.78·10 -1 | 2.78·10 -3 | 1 | 2.78·10 -4 |
| kg/(m h) | 2.84·10 -3 | 9.81 10 7 | 9.81 10 3 | 9.81 10 2 | 9.81 10 1 | 1 |
3.53 10 4 [ν Kinematic viscosity μ ] is a quantity equal to the ratio of the dynamic viscosity of the liquid [ ρ ] to its density [ ] at the same temperature: ν = μ/ρ. Unit kinematic viscosity
is [m 2 /s] - the kinematic viscosity of such a liquid, the dynamic viscosity of which is equal to 1 N s / m 2 and the density is 1 kg / m 3 (N = kg m / s 2). In the CGS system, kinematic viscosity is expressed in [cm 2 /s]. This unit is called Stokes (1 Stokes = 10 -4 m 2 /s; 1 cSt = 1 mm 2 /s). ν Conversion factors for calculating dynamic [
| ] viscosity. | Conversion factors for calculating kinematic [ | mm 2 /s (cSt) | cm 2 /s (St) | m 2 /s |
| Conversion factors for calculating kinematic [ | 1 | 10 -2 | 10 -6 | m 2 /h |
| mm 2 /s (cSt) | 10 2 | 1 | 10 -4 | 0,36 |
| cm 2 /s (St) | 10 6 | 10 4 | 1 | 1.02·10 -2 |
| m 2 /s | 3.6·10 -3 | 2,78 | 2.78 10 2 | 1 |
2.78 10 4 Oils and petroleum products are often characterized, which is taken to be the ratio of the flow time of 200 ml of petroleum product through the calibrated hole of a standard viscometer at a certain temperature [ t] by the time 200 ml of distilled water has flowed at a temperature of 20°C. Conditional viscosity at temperature [ t] is designated VU sign, and is expressed by the number of conventional degrees.
Conditional viscosity is measured in degrees VU (°VU) (if the test is carried out in a standard viscometer according to GOST 6258-85), Saybolt seconds and Redwood seconds (if the test is carried out on Saybolt and Redwood viscometers).
You can convert viscosity from one system to another using a nomogram.
In petroleum dispersed systems under certain conditions, unlike Newtonian liquids, viscosity is a variable value depending on the shear rate gradient. In these cases, oils and petroleum products are characterized by effective or structural viscosity:
For hydrocarbons, viscosity depends significantly on their chemical composition: It increases with increasing molecular weight and boiling point. The presence of side branches in the molecules of alkanes and naphthenes and an increase in the number of cycles also increase viscosity. For various groups hydrocarbons, the viscosity increases in the series alkanes - arenes - cyclanes.
To determine viscosity, special standard instruments are used - viscometers, which differ in their operating principle.
Kinematic viscosity is determined for relatively low-viscosity light petroleum products and oils using capillary viscometers, the action of which is based on the fluidity of the liquid through the capillary in accordance with GOST 33-2000 and GOST 1929-87 (viscometer type VPZh, Pinkevich, etc.).
For viscous petroleum products, the relative viscosity is measured in viscometers such as VU, Engler, etc. The liquid flows out of these viscometers through a calibrated hole in accordance with GOST 6258-85.
There is an empirical relationship between the values of conditional °VV and kinematic viscosity:
The viscosity of the most viscous, structured petroleum products is determined on a rotational viscometer according to GOST 1929-87. The method is based on measuring the force required to rotate the inner cylinder relative to the outer one when filling the space between them with the test liquid at a temperature t.
In addition to standard methods for determining viscosity, sometimes research work non-standard methods are used, based on measuring viscosity by the time of falling of a calibration ball between marks or by the time of damping of vibrations of a solid body in the test liquid (viscometers of Heppler, Gurvich, etc.).
In all described standard methods viscosity is determined at a strictly constant temperature, since with its change the viscosity changes significantly.
Dependence of viscosity on temperature
The dependence of the viscosity of petroleum products on temperature is very important characteristic both in oil refining technology (pumping, heat exchange, sludge, etc.) and in the use of commercial petroleum products (draining, pumping, filtering, lubrication of rubbing surfaces, etc.).
As the temperature decreases, their viscosity increases. The figure shows curves of changes in viscosity depending on temperature for various lubricating oils.
Common to all oil samples is the presence of temperature regions in which a sharp increase in viscosity occurs.
There are many different formulas for calculating viscosity depending on temperature, but the most commonly used is Walther's empirical formula:
Taking the logarithm of this expression twice, we get:
Using this equation, E. G. Semenido compiled a nomogram on the abscissa axis of which, for ease of use, temperature is plotted, and viscosity is plotted on the ordinate axis.
Using the nomogram, you can find the viscosity of a petroleum product at any given temperature if its viscosity at two other temperatures is known. In this case, the value of the known viscosities is connected by a straight line and continued until it intersects with the temperature line. The point of intersection with it corresponds to the desired viscosity. The nomogram is suitable for determining the viscosity of all types of liquid petroleum products.
For petroleum lubricating oils, it is very important during operation that the viscosity depends as little as possible on temperature, since this ensures good lubricating properties of the oil over a wide temperature range, i.e., in accordance with the Walther formula, this means that for lubricating oils, the lower the coefficient B, the higher the quality of the oil. This property of oils is called viscosity index, which is a function of the chemical composition of the oil. For different hydrocarbons, viscosity changes differently with temperature. The steepest dependence (large value of B) is for aromatic hydrocarbons, and the smallest for alkanes. Naphthenic hydrocarbons in this respect are close to alkanes.
Exist various methods determination of viscosity index (VI).
In Russia, IV is determined by two values of kinematic viscosity at 50 and 100°C (or at 40 and 100°C - according to a special table of the State Committee of Standards).
When certifying oils, IV is calculated according to GOST 25371-97, which provides for determining this value by viscosity at 40 and 100°C. According to this method, according to GOST (for oils with VI less than 100), the viscosity index is determined by the formula:
For all oils with ν 100 ν, ν 1 And ν 3) are determined according to the GOST 25371-97 table based on ν 40 And ν 100 of this oil. If the oil is more viscous ( ν 100> 70 mm 2 /s), then the values included in the formula are determined using special formulas given in the standard.
It is much easier to determine the viscosity index using nomograms.
An even more convenient nomogram for finding the viscosity index was developed by G.V. Vinogradov. Determining IV is reduced to connecting known viscosity values at two temperatures with straight lines. The intersection point of these lines corresponds to the desired viscosity index.
Viscosity index is a generally accepted value included in oil standards in all countries of the world. The disadvantage of the viscosity index is that it characterizes the behavior of the oil only in the temperature range from 37.8 to 98.8 ° C.
Many researchers have noted that the density and viscosity of lubricating oils to some extent reflect their hydrocarbon composition. A corresponding indicator was proposed linking the density and viscosity of oils and called the viscosity-mass constant (VMC). The viscosity-mass constant can be calculated using the formula of Yu. A. Pinkevich:
Depending on the chemical composition of the VMC oil, it can be from 0.75 to 0.90, and the higher the VMC of the oil, the lower its viscosity index.
In the low temperature range lubricating oils acquire a structure that is characterized by yield strength, plasticity, thixotropy or viscosity anomaly characteristic of dispersed systems.
Oil with an intact structure has a significantly higher viscosity than after its destruction. If you reduce the viscosity of such an oil by destroying the structure, then in a calm state this structure will be restored and the viscosity will return to its original value. The ability of a system to spontaneously restore its structure is called thixotropy. With an increase in the flow speed, or more precisely the speed gradient (section of curve 1), the structure is destroyed, and therefore the viscosity of the substance decreases and reaches a certain minimum. This minimum viscosity remains at the same level with a subsequent increase in the velocity gradient (section 2) until a turbulent flow appears, after which the viscosity increases again (section 3).
Dependence of viscosity on pressure
The viscosity of liquids, including petroleum products, depends on external pressure. The change in oil viscosity with increasing pressure is of great practical importance, since high pressures can arise in some friction units.
The dependence of viscosity on pressure for some oils is illustrated by curves; the viscosity of oils changes parabolically with increasing pressure. Under pressure R it can be expressed by the formula:
In petroleum oils, the viscosity of paraffin hydrocarbons changes least with increasing pressure, and naphthenic and aromatic hydrocarbons change slightly more. The viscosity of high-viscosity petroleum products increases with increasing pressure more than the viscosity of low-viscosity petroleum products. The higher the temperature, the less the viscosity changes with increasing pressure.
At pressures of the order of 500 - 1000 MPa, the viscosity of oils increases so much that they lose the properties of a liquid and turn into a plastic mass.
To determine the viscosity of petroleum products at high pressure, D.E. Mapston proposed the formula:
Based on this equation, D.E. Mapston developed a nomogram, using which known values, for example ν 0 And R, are connected by a straight line and the reading is obtained on the third scale.
Viscosity of mixtures
When compounding oils, it is often necessary to determine the viscosity of mixtures. As experiments have shown, additivity of properties manifests itself only in mixtures of two components that are very close in viscosity. When there is a large difference in the viscosities of the petroleum products being mixed, the viscosity is usually less than that calculated by the mixing rule. The viscosity of an oil mixture can be approximately calculated by replacing the viscosities of the components with their reciprocal values - mobility (fluidity) ψ cm:
To determine the viscosity of mixtures, you can also use various nomograms. Most Applications found the ASTM nomogram and the Molina-Gurvich viscosigram. The ASTM nomogram is based on the Walther formula. The Molina-Gurevich nomogram was compiled on the basis of the experimentally found viscosities of a mixture of oils A and B, of which A has a viscosity °ВУ 20 = 1.5, and B has a viscosity °ВУ 20 = 60. Both oils were mixed in different ratios from 0 to 100% (vol.), and the viscosity of the mixtures was established experimentally. The nomogram shows the viscosity values in el. units and in mm 2 /s.
Viscosity of gases and oil vapors
The viscosity of hydrocarbon gases and oil vapors is subject to different laws than for liquids. With increasing temperature, the viscosity of gases increases. This pattern is satisfactorily described by the Sutherland formula:
Viscosity determines internal resistance liquid force, which is aimed at making this liquid flow. There are two types of viscosity - absolute and kinematic. The first is usually used in cosmetics, medicine and cooking, and the second is more often used in the automotive industry.
Absolute viscosity and kinematic viscosity
Absolute viscosity fluid, also called dynamic, measures the resistance to the force causing it to flow. It is measured regardless of the properties of the substance. Kinematic viscosity, on the contrary, depends on the density of the substance. To determine kinematic viscosity, the absolute viscosity is divided by the density of the liquid.
Kinematic viscosity depends on the temperature of the liquid, therefore, in addition to the viscosity itself, it is necessary to indicate at what temperature the liquid acquires such viscosity. Engine oil viscosity is typically measured at temperatures of 40°C (104°F) and 100°C (212°F). When changing oil in cars, auto mechanics often take advantage of the property of oils to become less viscous as the temperature rises. For example, to delete maximum amount oil from the engine, it is preheated, as a result the oil flows out easier and faster.
Newtonian and non-Newtonian fluids
Viscosity varies differently depending on the type of liquid. There are two types - Newtonian and non-Newtonian fluids. Newtonian fluids are those whose viscosity changes regardless of the force deforming it. All other liquids are non-Newtonian. They are interesting because they are deformed with at different speeds depending on the shear stress, that is, deformation occurs at a higher or, conversely, lower speed depending on the substance and the force that presses on the liquid. Viscosity also depends on this deformation.
Ketchup is a classic example of a non-Newtonian fluid. While it is in the bottle, it is almost impossible to force it out with a little force. If, on the contrary, we apply great force, for example, we start shaking the bottle vigorously, then the ketchup will easily flow out of it. Thus, a large voltage makes ketchup fluid, while a small voltage has almost no effect on its fluidity. This property is inherent only in non-Newtonian liquids.
Other non-Newtonian fluids, on the contrary, become more viscous with increasing voltage. An example of such a liquid is a mixture of starch and water. A person can calmly run through a pool filled with it, but will begin to sink if he stops. This happens because in the first case the force acting on the fluid is much greater than in the second. There are non-Newtonian fluids with other properties - for example, in them the viscosity changes not only depending on the total amount of stress, but also on the time during which the force is applied to the fluid. For example, if the overall stress is caused by a larger force and is applied to the body for a short period of time, rather than being distributed over a longer period with less force, then a liquid, such as honey, becomes less viscous. That is, if you stir honey vigorously, it will become less viscous compared to stirring it with less force but for a longer time.
Viscosity and lubrication in technology
Viscosity - important property liquids that are used in everyday life. The science that studies the flow of liquids is called rheology and deals with a number of topics related to this phenomenon, including viscosity, since viscosity directly affects the flow of different substances. Rheology typically studies both Newtonian and non-Newtonian fluids.
Engine oil viscosity indicators
The production of machine oil occurs in strict compliance with the rules and recipes, so that the viscosity of this oil is exactly what is needed in a given situation. Before sale, manufacturers control the quality of the oil, and mechanics at car dealerships check its viscosity before pouring it into the engine. In both cases, measurements are taken differently. When producing oil, its kinematic viscosity is usually measured, while mechanics, on the contrary, measure absolute viscosity and then convert it to kinematic viscosity. In this case they use different devices for measuring. It is important to know the difference between these measurements and not to confuse kinematic viscosity with absolute viscosity, since they are not the same.
To obtain more accurate measurements, manufacturers machine oils prefer to use kinematic viscosity. Kinematic viscosity meters are also much cheaper than absolute viscosity meters.
For cars, it is very important that the viscosity of the engine oil meets the standard. In order for car parts to last as long as possible, it is necessary to reduce friction as much as possible. To do this, they are covered with a thick layer motor oil. The oil must be viscous enough to remain on the rubbing surfaces for as long as possible. On the other hand, it must be sufficiently liquid to pass through the oil passages without a noticeable decrease in the flow rate even in cold weather. That is, even with low temperatures The oil should remain not very viscous. In addition, if the oil is too viscous, then the friction between moving parts will be high, which will lead to increased fuel consumption.
Motor oil is a mixture of different oils and additives, such as antifoaming and detergent additives. Therefore, knowing the viscosity of the oil itself is not enough. It is also necessary to know the final viscosity of the product, and, if necessary, change it if it does not meet accepted standards.
Oil change
With use, the percentage of additives in motor oil decreases and the oil itself becomes dirty. When the contamination is too high and the additives added to it have burned out, the oil becomes unusable and must be changed regularly. If this is not done, dirt may clog oil channels. The viscosity of the oil will change and will not meet standards, causing various problems such as clogged oil passages. Some repair shops and oil manufacturers advise changing the oil every 5 000 kilometers (3 000 miles), but car manufacturers and some auto mechanics say that changing the oil after every 8 000 to 24 000 kilometers (5 000 to 15 000 miles) is sufficient if the car is in good condition and in good condition. good condition. Replacement every 5 000 kilometers is suitable for older engines, and now advice on such frequent replacement oils - publicity stunt, forcing car enthusiasts to buy more oil and use the services service centers more often than is actually necessary.
As engine designs improve, so does the distance a vehicle can travel without changing the oil. Therefore, to decide when to fill your car with new oil, follow the information in the operating instructions or the car manufacturer’s website. In some Vehicle There are also sensors installed that monitor the condition of the oil - they are also convenient to use.
How to choose the right engine oil
In order not to make a mistake with the choice of viscosity, when choosing an oil you need to take into account what weather and for what conditions it is intended. Some oils are designed to work in cold or hot conditions, and some are good in any weather. Oils are also divided into synthetic, mineral and mixed. The latter consist of a mixture of mineral and synthetic components. The most expensive oils- synthetic, and the cheapest are mineral, since their production is cheaper. Synthetic oils are becoming increasingly popular due to the fact that they last longer and their viscosity remains unchanged over a wide temperature range. When purchasing synthetic motor oil, it is important to check whether your filter will last as long as the oil.
Changes in engine oil viscosity due to temperature changes occur in different oils differently, and this dependence is expressed by the viscosity index, which is usually indicated on the packaging. An index equal to zero is for oils whose viscosity is most dependent on temperature. The lower the viscosity depends on temperature, the better, which is why motorists prefer oils with high index viscosity, especially in cold climates where the temperature difference between the hot engine and cold air is very large. On this moment viscosity index synthetic oils higher than mineral ones. Mixed oils are in the middle.
In order for the viscosity of the oil to remain unchanged longer, that is, to increase the viscosity index, various additives are often added to the oil. Often these additives burn out before the recommended oil change period, meaning the oil becomes less usable. Drivers using oils with such additives are forced to either regularly check whether the concentration of these additives in the oil is sufficient, or change the oil frequently, or be content with oil with reduced qualities. That is, oil with a high viscosity index is not only expensive, but also requires constant monitoring.
Oil for other vehicles and mechanisms
Oil viscosity requirements for other vehicles often coincide with those for automobile oils, but sometimes they are different. For example, the requirements for the oil used for a bicycle chain are different. Bicycle owners usually have to choose between a non-viscous oil that is easy to apply to the chain, such as from an aerosol spray, and a viscous oil that sticks to the chain well and for a long time. Viscous oil effectively reduces friction and does not wash off the chain during rain, but quickly becomes dirty as dust, dry grass and other dirt get into the open chain. There are no such problems with thin oil, but it must be reapplied often, and inattentive or inexperienced cyclists sometimes do not know this and damage the chain and gears.
Viscosity measurement
To measure viscosity, devices called rheometers or viscometers are used. The former are used for liquids whose viscosity changes depending on environmental conditions, while the latter work with any liquid. Some rheometers consist of a cylinder that rotates inside another cylinder. They measure the force with which the fluid in the outer cylinder rotates inner cylinder. In other rheometers, liquid is poured onto a plate, a cylinder is placed in it, and the force that the liquid exerts on the cylinder is measured. There are other types of rheometers, but the principle of their operation is similar - they measure the force with which the liquid acts on the moving element of this device.
Viscometers measure the resistance of the fluid that moves inside measuring instrument. To do this, the liquid is pushed through a thin tube (capillary) and the resistance of the liquid to movement through the tube is measured. This resistance can be found by measuring the time it takes for the liquid to move a certain distance in the tube. Time is converted to viscosity using calculations or tables provided in the documentation for each device.
Use a convenient converter for converting kinematic viscosity to dynamic viscosity online. Since the ratio of kinematic and dynamic viscosity depends on density, it must also be indicated when calculating in the calculators below.
Density and viscosity should be specified at the same temperature.
If you set the density at a temperature different from the viscosity temperature, it will entail some error, the degree of which will depend on the influence of temperature on the change in density for a given substance.
Calculator for converting kinematic viscosity to dynamic viscosity
The converter allows you to convert viscosity with dimension in centistokes [cSt] in centipoise [cP]. Please note that the numerical values of quantities with dimensions [mm2/s] and [cSt] for kinematic viscosity and [cP] and [mPa*s] for dynamic - they are equal to each other and do not require additional translation. For other dimensions, use the tables below.
To determine the kinematic viscosity, the viscometer is selected so that the flow time of the oil product is at least 200 s. It is then thoroughly washed and dried. A sample of the test product is filtered through paper filter. Viscous products are heated to 50–100°C before filtering. If there is water in the product, it is dried with sodium sulfate or coarse table salt, followed by filtration. The required temperature is set in the thermostatic device. The accuracy of maintaining the selected temperature is of great importance, therefore the thermostat thermometer must be installed so that its reservoir is approximately at the level of the middle of the viscometer capillary with simultaneous immersion of the entire scale. Otherwise, a correction is introduced for the protruding column of mercury using the formula:
^T = Bh(T1 – T2)
- B – thermal expansion coefficient working fluid thermometer:
- for a mercury thermometer – 0.00016
- for alcohol – 0.001
- h – the height of the protruding column of the working fluid of the thermometer, expressed in divisions of the thermometer scale
- T1 – set temperature in thermostat, оС
- T2 – ambient air temperature near the middle of the protruding column, °C.
The determination of the expiration time is repeated several times. In accordance with GOST 33-82, the number of measurements is set depending on the expiration time: five measurements - with an expiration time from 200 to 300 s; four - from 300 to 600 s and three - with an expiration time of over 600 s. When carrying out readings, it is necessary to ensure that the temperature is constant and there are no air bubbles.
To calculate the viscosity, determine the arithmetic mean value of the flow time. In this case, only those readings are taken into account that differ by no more than ± 0.3% for accurate and ± 0.5% for technical measurements from the arithmetic mean.
Viscosity of liquids
Dynamic viscosity, or the coefficient of dynamic viscosity ƞ (Newtonian), is determined by the formula:
η = r / (dv/dr),
where r is the force of viscous resistance (per unit area) between two adjacent layers of liquid, directed along their surface, and dv/dr is their gradient relative speed, taken in a direction perpendicular to the direction of movement. The dimension of dynamic viscosity is ML -1 T -1, its unit in the CGS system is poise (pz) = 1g/cm*sec=1din*sec/cm2 =100 centipoise (cps)
Kinematic viscosity is determined by the ratio of dynamic viscosity ƞ to liquid density p. The dimension of kinematic viscosity is L 2 T -1, its unit in the CGS system is stokes (st) = 1 cm 2 /sec = 100 centistokes (cst).
Fluidity φ is the reciprocal of dynamic viscosity. The latter for liquids decreases with decreasing temperature approximately according to the law φ = A + B / T, where A and B are characteristic constants, and T denotes the absolute temperature. The values of A and B for a large number of liquids were given by Barrer.
Water viscosity table
Bingham and Jackson data, verified to the national standard in the USA and Great Britain on July 1, 1953, ƞ at 20 0 C = 1.0019 centipoise.
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Temperature, 0 C |
Temperature, 0 C |
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Table of viscosity of various liquids Ƞ, spz
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Liquid |
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Bromobenzene |
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Formic acid |
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Sulfuric acid |
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Acetic acid |
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Castor oil |
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Provencal oil |
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Carbon disulfide |
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Methyl alcohol |
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Ethanol |
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Carbon dioxide (liquid) |
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Carbon tetrachloride |
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Chloroform |
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Ethyl acetate |
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Ethyl formate |
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Ethyl ether |
Relative viscosity of some aqueous solutions (table)
The concentration of solutions is assumed to be normal, which contains one gram equivalent of a dissolved substance in 1 liter. Viscosity are given in relation to the viscosity of water at the same temperature.
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Substance |
Temperature, °C |
Relative viscosity |
Substance |
Temperature, °C |
Relative viscosity |
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Calcium chloride |
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Ammonium chloride |
Sulfuric acid |
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Potassium iodide |
Hydrochloric acid |
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Potassium chloride |
Caustic soda |
Table of viscosity of aqueous solutions of glycerin
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Specific gravity 25°/25°С |
Weight percentage of glycerin |
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Viscosity of liquids at high pressures according to Bridgman
Table of relative viscosity of water at high pressures
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Pressure kgf/cm 3 |
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Relative viscosity table various liquids at high pressures
Ƞ=1 at 30 ° C and pressure 1 kgf/cm 2
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Liquid |
Temperature, °C |
Pressure kgf/cm 2 |
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Carbon disulfide |
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Methyl alcohol |
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Ethanol |
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Ethyl ether |
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Viscosity of solids (VS)
Table of viscosity of gases and vapors
Dynamic gas viscosity usually expressed in micropoises (mpoise). According to kinetic theory, the viscosity of gases should be independent of pressure and vary in proportion to the square root of the absolute temperature. The first conclusion turns out to be generally correct, with the exception of very low and very high pressures; the second conclusion requires some corrections. To change ƞ depending on the absolute temperature T, the formula most often used is:
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Gas or steam |
Sutherland constant, C |
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Nitrous oxide |
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Oxygen |
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Water vapor |
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Sulphur dioxide |
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Ethanol |
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Carbon dioxide |
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Carbon monoxide |
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Chloroform |
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Table of viscosity of some gases at high pressures (μpz)
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Temperature, 0 C |
Pressure in atmospheres |
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Carbon dioxide |
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