Rank Repeatability Time Volume Usability Saftey
1 Capillary EMS EMS EMS EMS
2. EMS Brookfield Rheometer Rheometer Rheometer
3. Rheometer Rheometer Capillary Capillary Capillary
4. Brookfield Capillary Brookfield Brookfield Brookfield
Classical Rotational Viscometers (Rheometers)
The measuring principle of Rotational Viscometers is based on the fact that the torque required to turn an object in a fluid depends on the rotational speed of the object and the viscosity of the fluid.
In Rotational Viscometers the liquid (1) whose viscosity is to be measured fills the space between two vertical coaxial cylinders (cup and bob ) or between a cone and a plate.
There are two classical geometries in cup and bob viscometers:
Couette systems (A): The outer cylinder (cup, 2) rotates at a constant rate, the resulting torque on the inner cylinder (bob, 3) is measured (e.g. with a torsion wire).
Searle systems (B): A rotational speed for the inner cylinder (bob, 3) is preset and the torque required to maintain this speed measured.
In Cone and Plate Viscometers (C) the liquid (1) is fills the gap between the cone (4) and plate (5). The torque required to maintain a preset rotational speed of the cone is measured.
Rotational Viscometers are suitable to examine the rheological behaviour of liquids by determining the share rate dependency of the viscosity.
Classical Rotational Viscometers cannot accurately measure very low viscosities. This is due to friction in the mechanical support of the rotor.
Simple Rotational Viscometers ("Brookfield Type")
The so called Brookfield Type Viscometer is the least expensive commercial rotational viscometer. The measuring principle is simple: The measuring tool – a so called spindle (2) – is immersed in the sample to be measured (1). The spindle rotates at a fixed speed and the resulting torque is measured with a calibrated torsion spring (3) – the higher the viscosity of the sample the higher the torque. Conversion factors are required to calculate the viscosity from the measured torque. These factors are normally pre-calibrated for specific spindle and container geometries. The type of spindle must be selected according to the viscosity range of the sample.
Brookfield Type Viscometers are cheap, robust and fairly simple to use. For this reason they are widely used in routine quality control.
For many applications Brookfield Type Viscometers are not suitable as they offer limited capabilities and precision:
An accurate temperature control of the sample is difficult to achieve.
The measurements are performed in an open container. When performing measurements over a longer period of time or/at elevated temperatures, volatile components of the sample can evaporate.
Due to friction in the mechanical support of the spindle, very low viscosities cannot be measured with a good repeatability of the results.
Capillary Viscometers are often referred to as U-tube viscometers because of their shape. There are three common types of Capillary Viscometers: Ubbelohde , Ostwald and Cannon-Fenske. Capillary Viscometers are widely used for measuring the viscosity of Newtonian fluids mainly in the oil industry.
The measuring procedure is as follows: Suction is applied to tube B to draw the sample through the capillary (3) to a level above the line (1). Then tube B is left open and the sample flows freely down. The time it takes for the meniscus to pass from mark 1 to mark 2 is measured. The kinematic viscosity of the sample is calculated by multiplying the efflux time in seconds by the viscometer constant.
Capillary Viscometers allow accurate measurements with an excellent repeatability. They have, however, two considerable drawbacks:
Capillary Viscometers cannot be used to measure samples with a non-Newtonian behaviour such as emulsions, polymer suspensions, etc.
Viscosity determinations with Capillary Viscometers are very time consuming. One measurement can take more than 1 hour. The most time-consuming part of the measurement with non automated Capillary Viscometers is the cleaning and drying of the capillary between samples.