Do you need to measure viscosity?
Viscosity is a measure of a fluid’s resistance to flow, or more precisely, it is the ratio of the force required to overcome internal friction between layers of fluid (shearing stress) to the change in speed between layers of fluid (velocity gradient).
Knowing the viscosity of a fluid can be quite important when you need to measure viscosity. Many quality control measures are based on viscosity. For example, to maintain consistent quality a ketchup producer needs to maintain the proper viscosity, so the consumer ends up with the product as intended. Paint needs to be able to spread properly, but should not be dripping off of the brush. Ink must come out of a nozzle in a precise manner. In other cases, the design and engineering of equipment and systems must take viscosity into consideration to ensure that they will function as required. The sizing of pumps and mixers depends on the design and power of the equipment to handle a given viscosity.
Many different types of viscometers and other viscosity measurement devices are used to measure viscosity for various types of fluids and units of measure.
Here are six types of viscosity measurement devices to consider:
Falling ball viscometers
A falling ball viscometer measures the viscosity of fluids and some units can also measure the viscosity of gases.
The various forms of viscosity cups use gravity to allow a fluid to flow through an orifice located at the bottom in a precise amount, which can be measured over time to calculate a viscosity value. The most common viscosity cups are Ford, Ford Dip, and Zahn cups.
The consistometer is a metal trough with graduations which measures viscous materials as they flow at an incline under their own weight. These are primarily used to measure paint viscosity to ensure conformity to military specifications. Consistometers also work well for many food applications such as syrups, jellies, and sauces, as well as cosmetics. Consistency, viscosity, and flow rates can all be verified against established standards. The consistometer doesn’t actually measure viscosity values directly: Its measurement is based on how far a fluid will flow down an incline in a certain time frame. This can be correlated to viscosity by using established standards. Users can develop their own standards and procedures specific to the product tested. While consistometers can’t be used with all samples, the low maintenance and ease of use make them very popular.
Glass capillary viscometers
A glass capillary viscometer is used in conjunction with test methods which conform to a particular ASTM. A wide range of glass capillary viscometers are available, including Ubbelhode, Cannon-Fenske, and Zeitfuchs.
Tuning fork vibration viscometers
Featuring a level of 1% of reading, a tuning fork vibration viscometer delivers a high level of accuracy. This measures viscosity by detecting the driving electric current needed to resonate two sensor plates at a constant frequency.
A rotational viscometer accommodates a wide range well into millions of centipoise and is considered the most versatile type of viscometer.
Read the complete article, In the Thick of Things: Measuring Viscosity.
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