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Viscosity (from the Latin viscum, gui, glu) can be defined as the totality of the phenomena of resistance to the movement of a fluid for a flow with or without turbulence. Viscosity reduces the fluid’s freedom of flow and dissipates its energy.

Laboratory viscometers and process viscometers

Measure viscosity in laboratory viscometers:

In the laboratory, a wide range of measuring devices, from the simplest to the most sophisticated, allows to measure the dynamic viscosity or the kinematic viscosity, or even the rheological behavior of fluids. Nevertheless, laboratory measurements do not allow an optimal control of an industrial process because of the sample collection, its transport to the laboratory and the measurement time itself.

Measure viscosity in process viscometers:

The so-called “process viscometers” are measuring

instruments directly installed in industrial processes. They are referred to in the technical literature as “inline viscometers” when they are installed directly on the main pipeline, and as “online viscometers” when they are installed on a bypass.

The process viscometers must meet specific constraints:

• to be installed directly on pipes, tanks or reactors
• be resistant to process conditions and industrial environment (temperature, pressure, corrosion, fouling, rain, humidity, salinity, …)
• comply with current regulations (equipment operating in explosive areas, equipment operating under pressure, etc.)
• operate automatically
• provide an instant and continuous measurement
• require minimum maintenance
• be repeatable

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Calculation of reference temperature for viscosity

The sensor calculate the viscosity at the reference temperature through the simultaneous measurements of viscosity and temperature in the process:

ηReference temperature = f(ηProcess temperature, Τprocess )

This calculation, called temperature compensation, is possible when data about the influence of temperature on a reference product; with a behavior close to the measured product are available. This data can be in the form of a table, a curve or an equation.

The accuracy of the temperature-compensated viscosity extrapolation naturally depends on the accuracy of the data, the difference in behavior between the reference product and the actual product, and the difference between the process and reference temperatures. For petroleum products, whose behavior can be described using the mathematical relationships of ASTM D 341-93, temperature compensation gives very good results.

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Measurement of the viscosity of the kinematic of fluids

If Newtonian liquids are tested with capillary viscometers, the viscosity is expressed in units of kinematic viscosity. Gravity is used as the force driving the liquid through the capillary. The density of the sample is an additional parameter.

Kinematic viscosity and dynamic viscosity are related by the following formula:

ν=η/ρ (m²/s) with

ν = viscosité cinématique en m²/s,   

η = viscosité dynamique en Pa.s 

and

ρ = masse volumique en kg/m³

Kinematic viscosity was previously expressed in “stokes” [St] or “centistokes” [cSt].

1 cSt = 1 mm²/s = 10-6 m²/s knowing that 1St = 100 cSt

For a liquid of density close to 1000kg/m3 (density = 1), we can make the following approximation: 1cSt = 1cP (see dynamic viscosity).

Note: The values “Ford Cup seconds”, “Engler Degrees”, “Saybolt or Redwood seconds” are only relative viscosity values which, for non-Newtonian liquids, cannot be converted into absolute viscosity values η or ν.

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Continuous control of the dynamic viscosity, a factor of optimal quality of the final product

Viscosity (from Latin viscum) n. : State of being viscous. PHYS Property that any fluid has of resisting the forces that tend to move the particles that make it up relative to one another.  Source: © Hachette Livre, 1997

Isaac Newton was the first to establish the fundamental law of viscosity describing the flow behavior of an ideal liquid.

τ = η.D where τ = tangential stress (N/m²) and D or g = velocity gradient (n-1)

The solution of this equation for the dynamic viscosity gives: η = τ/D in Pa.s

Note: in the SI system, the unit is the Pascal.second (Pa.s) or milliPascal.second (mPa.s). In the industrial environment we find other units of which the most used are the centipoise (cP) and the poise (P):

Viscosity versus temperature curve

• 1 Pa.s = 1 000 mPa.s
• 1 mPa.s = 1 cP
• 1 P = 100 cP

Typical viscosity values at 20 °C (mPa.s):

• Acetone: 0.32
• Water : 1
• Mercury: 1.5
• Grapefruit juice: 2~5
• Blood (at 37°C): 4~15
• Coffee cream: 10
• Olive oil: 10²
• Honey: 10⁴
• Tar: 10⁶
• Bitumen : 10⁸

The viscosity of liquids decreases when the temperature increases.

In industry, viscosity has a very great practical importance, because it conditions the flow of fluids in pipes and along the walls. It allows to measure directly or indirectly certain characteristics of products (texture, polymer size…). It conditions the correct operation of processes (combustion, printing, coating…).

During industrial manufacturing processes, the vibration viscometers certified explosion-proof (ATEX) and/or sanitary 3A provide all the guarantees of an excellent continuous control of the dynamic viscosity, factor of an optimal quality of the final product.

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Or come and discover our solutions

Viscosity fluid is an important physical characteristic of the majority of liquids manufactured and used in industry. It allows :

• Directly characterize the viscosity fluid quality of the final product (lubricating oils, fuels, inks, paints, …)
• Indirectly characterize a property of use of the final product or in the course of manufacture (dry matter content, Brix, texture of a cheese speciality, polymer size, …)
• Control the progress of physico-chemical reactions during the manufacturing process,
• Ensure the proper functioning of an equipment and guarantee its performance or the quality of the final product (burner, motor, printing machine, …),
• Sizing the equipment (pumps, agitators, …)

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This index is used to characterise the quality of lubricants and hydraulic fluids. It takes into account the influence of temperature on the kinematic viscosity of the fluid; which decreases as the temperature increases and vice versa. The viscosity of lubricants and hydraulic fluids is a very important factor.

The viscosity index is particularly used in the oil industry.

If the lubricant is too viscous, the process will have to provide much more energy to set the parts in motion, generating excessive energy consumption with reduced yields. If the oil is too fluid, the quality of the lubrication is reduced with a risk of wear of the moving parts. Precise control of the viscosity index ensures the quality and stability of the oil for a given use.

We can cite the following example concerning engine lubrication: lubricating oils for motor vehicles must reduce friction between the components when the engine is cold when starting (ambient temperature) and during operation (200° to 300°C). The best oils corresponding to the highest viscosity index will have a “constant” viscosity according to their temperature range. The viscosity index scale uses the following reference temperatures:

• 100° Fahrenheit (40° C)
• 210° Fahrenheit (100° C)

Originally this scale was graduated from 0 to 100 VI (high quality oil). But nowadays, with the improvement of manufacturing processes and additives, we can find oils with a much higher index. Nowadays, the index of some synthetic oils can exceed 400 VI.

During the manufacturing process of lubricating oils, the MIVI 9731 Thermoset, certified explosion-proof (ATEX), provides all the guarantees of an excellent continuous control of the viscosity index, a factor of optimal quality of the final product.

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