After K is calculated using a specimen fluid for which the viscosity and density are already known, µρ can be derived by comparing Ea with Ex. The measured amplitude varies according to the fluid’s viscosity. C : constant derived from vibration parts.K： constant on the driving force and the displacement.The following formula determines the vibration amplitude when the measured fluid surrounds the vibrating shaft, and vibrational restriction occurs. The vibrating viscometer determines viscosity by allowing a vibrating blade to vibrate in a liquid and measure the resultant viscous drag. Here is a quick demonstration of this test.Ī falling-sphere viscometer is a device used to measure the viscosity of a fluid by measuring the time required for a spherical ball to fall a certain distance under gravity through a tube filled with the fluid whose viscosity is to be determined. They are manufactured in accordance with ASTM D 4212, ASTM D1084 and ASTM D816 lubricants testing standards. Large number cup sizes are used when viscosity is high, while low number cup sizes are used when viscosity is low. There are five cup specifications, labelled Zahn cup #x, where x is the number from one through five (see table below). There is also a long handle attached to the sides. It is commonly a stainless steel cup with a tiny hole drilled in the centre of the bottom of the cup.
Here is a short video on the capillary viscometer operation:Ī Zahn cup is a viscosity measurement device widely used in the paint industry. Here is a list of the most common ways to measure viscosities together with videos of such operations.Ī capillary viscometer is an instrument used to measure a liquid’s viscosity or thickness by measuring how long it takes the liquid to flow through a small-diameter tube or capillary. There are many ways to measure viscosity of liquids. These are liquids like water and vegetable oil. Liquids that are not resistant to flow or flow fast have low viscosity. These are liquids like honey, sunscreen and dish soap. Liquids that are resistant to flow or flow slowly have high viscosity. Here is a simple calculator for calculating the viscosity of an oil according to the Walther equation: The lubricant manufacturers typically specify kinematic viscosities at 40C and 100C, which are then used in the equation above (but don’t forget to translate to kelvins). The constants and are calculated from the kinematic viscosities at 2 temperatures ( and ): For lubrication oils, it can be chosen as 0.7 (see the reference). Here is the kinematic viscosity given in, is temperature in kelvins, is a constant for a particular fluid. This equation is the standard used it ASTM, ISO and DIN charts. The known equations are due to Reynolds, Vogel, but the most commonly used one is known as Walther equation. These models are usually fits obtained using experimental data. There are a number of models exist to describe the relation between viscosity and temperature. Viscosity-Temperature Relations for Lubricating Oils: Walther equationįor lubricating oils, viscosity drops with temperature rapidly. Kinematic viscosity is typically used and reported in industry.ĭynamic Viscosity Measured at Atmospheric Pressures, mPas However, due to practicality reasons typically it is expressed in terms of centistokes (cS or cSt) 1 stokes = 100 centistokes = 1 cm2 s−1 = 0.0001 m2 s−1. The physical unit for kinematic viscosity is the stokes (St), named after George Stokes. Kinematic viscosity: Kinematic viscosity is the ratio of dynamic viscosity and the density, so. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP).Ģ. The physical unit for dynamic viscosity in the centimetre gram second system of units (cgs) is the poise (P). Dynamic viscosity: Dynamic viscosity is also known as absolute viscosity. The SI physical unit of dynamic viscosity ( μ) is the Pascal-second (Pa s), identical to 1 kg m−1 s−1. ĭynamic vs Kinematic Viscosity: What is the difference?ġ.
In lubrication practice, the earlier mentioned cP unit is widely used. The unit of viscosity is newton-second per square meter ( ), which is usually expressed as pascal-second ( ) in SI units. Viscosity can also help you determine the structure of molecules such as proteins in a solution or the force required to inject a new drug formulation into a patient. Viscosity will also determine how much energy is required to pump out process fluid. It will determine, for example – How long a reservoir will take to fill. Viscosity is key for any process involving fluid flow.
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