Rotational Viscometer
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27May/10Off
Rotational Viscometer Advantages vs. Bostwick Consistometer
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The Bostwick Consistometer was, for quite a long time, the industry standard for food laboratory viscosity testing. It was an extremely user friendly and dependable instrument, however, it had several key limitations which have since been addressed with the introduction of the rotational viscometer.
Made from a stainless steel, level, two compartment trough, the Bostwick Consistometer is a rather simple piece of equipment. The substance which is to be tested is inserted into the first compartment. A gate separates the two compartments and is released during a measurement to allow the liquid substance to flow into the second compartment. The second compartment consists of a graduated parallel lines at 0.5 cm which are 24 cm long. The sample is allowed to flow freely under the force of gravity down the slight slope for 30 seconds and the distance it travels is measured to determine the viscosity.
While a simple process, this instrument is constrained in a number of ways. The first limitation is that the instrument's measurements can't be used to determine rheological information due to the wetting power, surface tension and other factors outside of the liquid's viscosity influencing the reading. The second limitation is the lack of temperature control when performing the test. Room temperature is used by default and is subject to whatever atmospheric temperature changes may occur. Lastly, it is limited in the type of samples which can be tested. Higher viscosity substances are not likely to flow far enough over the 30 second time to allow for substantial variance between samples.
With these constraints in mind, the rotational viscometer was developed. The accuracy and precision of viscosity measurements are greatly improved. The temperature can be controlled using a constant temperature bath. Also, only a very small amount of sample liquid (appoximately 11 mL) is needed when using a rotational viscometer.
When using a rotational viscometer, the resistance of the liquid is measured and the sample viscosity is thus obtained. A rotating spindle is submerged in the sample and the rotational viscometer takes measurements of the additional torque needed to maintain a constant rotational rate while overcoming viscous resistance. This measured data is then converted to a viscosity value and output to the user.
With these factors in mind, it is safe to say that a rotational viscometer is essential for all serious food laboratory research.
7May/10Off
Haake VT550 Rotational Viscometer
The Haake VT550 is a rotational viscometer designed
for fast and precise measurement of viscosity and flow behavior
for quality control and has established itself as a state of the art
instrument in quality control.
Product Details
The Haake VT550 is a Searle viscometer. A rotational
speed is preset and the flow resistance of the sample is measured
in other words, the torque maintaining the set speed is proportional
to the viscosity. All final information on the viscosity, shear stress
and the shear rate is calculated from the torque required, the set
speed and the geometry factors of the applied sensor. The results
are displayed digitally and can be printed simultaneously.
For viscosity measurements a total of 60 different rotational speed
steps are available. 50 out of these 60 are factory set; the
remaining 10 are user programmable. The 60 steps cover a wide
range reaching from 0.5 rpm to 800 rpm. The resulting torque is
measured via a non-contact, low displacement sensor which enables
the extended measuring range with perfect signal linearity over the
whole range. The operating temperature can be measured by PT100
probes located either in the sensor used or directly in the test material.
Haake VT550 Applications
Perfect for quality-control investigations as well as routine measurements.
Food : Sauces, Mayonnaise, Mustard, Ketchup, Thickening Agents, Stabilizers, Starch, Pectin
Cosmetic/Pharmaceutical : Creams, Lotions, Shampoos, Toothpastes, Liquid Soaps
Paint : Water-based and Latex Paints, Bonding Agents, Resins
Chemical : Liquid and Paste-like Raw Materials
Haake VT550 Specifications
| Number of Speeds | Continuous |
| Speed Range | 0.5–800rpm |
| Speed Range Stability | ±0.1% |
| Temperature Range | -20° to +100°C |
| Temperature Range Stability | ±0.5% fsd |
| Typical Measuring Time | 1 min. |
| Number of Sensors | 2 cups, 2 rotors |
| Reproducibility | ±1% |
| Voltage/Frequency | 100-230V 50/60Hz |
3May/10Off
Rotational Viscometer Defined
A rotational viscometer uses the concept that the torque it
takes to rotate an object submerged in fluid is functionally
related to the fluid's viscosity. A rotational viscometer takes
precise measurements of the required torque to turn a bob
or disk in fluids at a predetermined speed.
Cup and Bob Rotational Viscometer Diagram
One particular type of rotational viscometer, known as a 'cup
and bob' viscometer, utilizes a known volume of sample fluid
which is, in turn, sheared within a test cell (the cup). The
applied torque for the bob to reach a predefined rotational
speed is measured and plotted to gain the desired viscosity
value. There are two general system types of 'cup and bob'
rotational viscometers, the Couette and the Searle systems.
These two types are differentiated by whether the bob or the
cup is rotated. A rotating test cell (cup) is preferred in certain
situations due to the reduction of Taylor vortices, however, this
type of rotation tends to lose accuracy in the measurement.
Cone and Plate Rotational Viscometer Diagram
A second type of rotational viscometer, dubbed the 'cone and plate'
rotational viscometer utilizes a shallow angled cone which is in bare
contact with a flat plate inside. A small quantity of fluid is sandwiched
between the cone and the plate, the cone is rotated at a steady
angular rate, and the required torque is measured to determine the
viscosity value.