Types of viscometer

Capillary viscometerA capillary viscometer is an instrument used to measure the viscosity, or thickness, of a liquid by measuring how long

it takes the liquid to flow through a small-diameter tube, or capillary.

The flow, or efflux, time is directly proportional to the liquid's kinematic viscosity, and may be converted directly to viscosity by use of a conversion factor unique to each instrument. Viscosity is generally temperature dependent, so the capillary viscometer is usually used in a controlled-temperature water bath set to a specific temperature Viscosity may be thought of as the internal friction of a liquid, or its tendency to resist flowing. Viscosity is thus an important property of fluids. It is of critical concern in lubricants, paints, beverages, and in any case where a liquid must be transferred, stirred, or manipulated.A capillary viscometer may take any of several designs, but most common is the U-shaped or Ostwald viscometer, of which the Cannon-Fenske and Ubbelohde types are typical. A Cannon-Fenske capillary viscometer is a U-shaped piece of glass tubing bearing two glass bulbs or chambers on one arm, separated by a calibrated length of capillary tubing. Another bulb is low on the other arm, to which the sample is charged. The sample and the capillary viscometer are then suspended in a fixed-temperature water bath and allowed to come to thermal equilibrium.Once thermal equilibrium is reached, the sample is drawn up into the upper chamber, and the test begins. The test sample is allowed to flow from the upper chamber to the lower through the capillary, and the efflux time, or time it takes to traverse the length of the capillary, is measured. Capillary

viscometers come with a manufacturer-supplied conversion factor which allows calculation of the kinematic viscosity directly from the efflux time.

Ostwald ViscometerIt is a type of capillary viscometer. There is ‘U’ shape tube with two bulbs and two marks as shown in the following figure,It is used to determine the viscosity of Newtonian liquids. In one arm of the U is a vertical section of precise narrow bore (the capillary). Above this is a bulb, with it is another bulb lower down on the other arm. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate a known volume. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. Most commercial units are provided with a conversion factor, or can be calibrated by a fluid of known properties.

Ubbelohde suspended level viscometer

A Ubbelohde type viscometer or suspended-level viscometer is a measuring instrument which uses a capillary based method of measuring viscosity [1]. It is recommended for higher viscosity cellulosic polymer solutions. The advantage of this instrument is that the values obtained are independent of the concentration. The device was invented by the German chemist Leo Ubbelohde (1877-1964). The Ubbelohde viscometer is closely related to the Ostwald viscometer. Both are u-shaped pieces of glassware with a reservoir on one side and a measuring bulb with a capillary on the other. A liquid is introduced into the reservoir then sucked through the capillary and measuring bulb. The liquid is allowed to travel back through the measuring bulb and the time it takes for the liquid to pass through two calibrated marks is a measure for viscosity. The Ubbelohde device has a third arm extending from the end of the capillary and open to the atmosphere. In this way the pressure head only depends on a fixed height and no longer on the total volume of liquid.

Falling sphere viscometerFalling sphere viscometer consists of cylindrical transparent tube having graduated section near the middle of its length and generally a steel ball that is allowed to fall through the tube. Stokes' law is the basis of the falling sphere viscometer, in which the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. A series of steel ball bearings of different diameter is normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses

glycerine as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. It includes many different oils, and polymer liquids such as solutions.In 1851, George Gabriel Stokes derived an expression for the frictional force (also called drag force) exerted

on spherical objects with very small Reynolds numbers (e.g., very small particles) in a continuous viscous fluid by changing the small fluid-mass limit of the generally unsolvable Navier-Stokes eq

Rotational viscometers use the idea that the force required to turnan object in a fluid, can indicate the viscosity of that fluid. Theviscometer determines the required force for rotating a disk or bobin a fluid at known speed. 'Cup and bob' viscometers work by defining the exact volume of sample which is to be sheared within a test cell, the torque required to achieve a certain rotational speedis measured. TheExample digital viscometer with temperature controlre are two classical geometries in "cup and bob" viscometers, knownas either the "Couette" or "Searle" systems - distinguished bywhether the cup or bob rotates. 'Cone and Plate' viscometers usea cone of very shallow angle in theoretical contact with a flat plate.With this system the shear rate beneath the plate is constant toa modest degree of precision, a graph of shear stress (torque)against shear rate (angular velocity) yields the viscosity.

Rotational viscometers fall into two main types:

1. Synchronous (Stepper) Motor / Spring2. Servo Motor / Digital encoder

The first type uses a stepper motor to drive the main shaft. A spring & pivot assembly rotates on the shaft. The spindle or rotor hangs from this assembly. As the spindle rotates the spring is deflected by the viscosity of the sample under test.The second type uses a precision servo motor to drive the shaft. The Spindle or rotor is attached directly to the shaft. High speed microprocessors measure the speed from a digital encoder and calculate the current required to drive the rotor at the test speed. The current required is proportional to the viscosity of the sample under test.

Stabinger viscometer

By modifying the classic Couette rotational viscometer, an accuracy comparable to that of kinematic viscosity determination is achieved. The internal cylinder in the Stabinger Viscometer is hollow and specifically lighter than the sample, thus floats freely in the sample, centered by centrifugal forces. The formerly inevitable bearing friction is thus fully avoided. The speed and torque measurement is implemented without direct contact by a rotating magnetic field and an eddy current brake. This allows for a previously unprecedented torque resolution of 50 pN·m and an exceedingly large measuring range from 0.2 to 20,000 mPa·s with a single measuring system. A built-in density measurement based on the oscillating U-tube principle allows the determination of kinematic viscosity from the measured dynamic viscosity employing the relation

The Stabinger Viscometer was presented for the first time by Anton Paar GmbH at the ACHEMA in the year 2000. The measuring principle is named after its inventor Dr. Hans Stabinger.

Stormer viscometer

The Stormer viscometer is a rotation instrument used to determine the viscosity of paints, commonly used in paint industries. It consists of a paddle-type rotor that is spun by an internal motor, submerged into a cylinder of viscous substance. The rotor speed can be adjusted by changing the amount of load supplied onto the rotor. For example, in one brand of viscometers, pushing the level upwards decreases the load and speed, downwards increases the load and speed.The viscosity can be found by adjusting the load until the rotation velocity is 200 rotations per minute. By examining the load applied and comparing tables found on ASTM D 562, one can find the viscosity in Krebs units (KU), unique only to the Stormer type viscometer.

Oscillating Piston Viscometer

Sometimes referred to as Electromagnetic Viscometer or EMV viscometer, was invented at Cambridge Viscosity in 1986. The sensor (see figure below) comprises a measurement chamber and magnetically influenced piston. Measurements are taken whereby a sample is first introduced into the thermally controlled measurement chamber where the piston resides. Electronics drive the piston into oscillatory motion within the measurement chamber with a controlled magnetic field. A shear stress is imposed on the liquid (or gas) due to the piston travel and the viscosity is determined by measuring the travel time of the piston. The construction parameters for the annular spacing between the piston and measurement chamber, the strength of the electromagnetic field, and the travel distance of the piston are used to calculate the viscosity according to Newton’s Law of Viscosity.Oscillating Piston Viscometer Schematic View

The Oscillating Piston Viscometer technology has been adapted for small sample viscosity and micro-sample viscosity testing in laboratory applications. It has also been adapted to measure high pressure viscosity and high temperature viscosity measurements in both laboratory and process environments. The viscosity sensors have been scaled for a wide range of industrial applications such as small size viscometers for use in compressors and engines, flow-through viscometers for dip coating processes, in-line viscometers for use in refineries, and hundreds of other applications. Improvements in sensitivity from modern electronics, is stimulating a growth in Oscillating Piston Viscometer popularity with academic laboratories exploring gas viscosity.

Bubble viscometerBubble viscometers are used to quickly determine kinematic viscosity of known liquids such as resins and varnishes. The time required for an air bubble to rise is directly proportional to the visosity of the liquid, so the faster the bubble rises, the lower the viscosity. The Alphabetical Comparison Method uses 4 sets of lettered reference tubes, A5 through Z10, of known viscosity to cover a viscosity range from 0.005 to 1,000 stokes. The Direct Time Method uses a single 3-line times tube for determining the "bubble seconds", which may then be converted to stokes