Viscometers are used to define the viscous properties of a fluid at ambient or defined temperatures. They commonly come in the form of a calibrated capillary tube through which a liquid is allowed to pass at a controlled temperature in a specified time period. Other methods include rotational viscometry and falling ball tests.
Viscometers can have a few different technologies by which they operate. For rotational viscometry, torque is required to rotate a spindle at constant speed while immersed into the sample fluid. The torque is proportional to the viscous drag on the immersed spindle, and thus to the viscosity of the fluid. For falling ball technology, the viscosity is proportional to the time required for a ball to fall through the test liquid contained in a precise and temperature controlled glass tube. Capillary viscometers measure the flow rate of a fixed volume of fluid through a small orifice at a controlled temperature. The rate of shear can be varied from near zero to 106 s-1 by changing capillary diameter and applied pressure. The time it takes for a specific volume of fluid to pass through the orifice is proportional to the fluid viscosity. However, it also depends on the density of the fluid since the denser the fluid, the faster it will flow through the orifice. The property being measured in this example is then the kinematic viscosity and not the dynamic viscosity.
Viscometers can handle both Newtonian and non-Newtonian fluids. A Newtonian fluid is one whose viscosity does not depend on the shear rate -- no matter what shear is applied, the viscosity stays the same. In many applications, however, this is not the case and as the fluid is sheared at greater rates, the viscosity will change. These types of liquids are known as non-Newtonian and there are many classifications.
Mounting options for viscometers are quite varied. They can be handheld or portable meters, benchtop or fixtured instruments or in-line. Handheld and portable meters are popular for laboratory use, where a scientist may be taking readings from many different samples. Benchtop and fixtured instruments are also good laboratory devices. In-line or insertion style instruments allow for insertion and retraction without disturbing line flow.
Displays for the instruments can be analog meters, a numeric or alphanumeric digital display or video, CRT or LCD. Another option is to have no local display at all and have the data gathered by another instrument. Likewise, the user controls can also be analog or digital or can be operated through a host computer. To simplify the instruments, preprogrammed devices without user controls are available.
Electrical Output Options
Electrical output options for viscometers are the standard voltage and current outputs as well as an analog pulse or frequency or a change in state of switches or an alarm. Serial and parallel interfaces can help connect these instruments to a host computer.
Some features available for viscometers include temperature compensation, temperature sensing and data storage such as a hard or floppy drives.
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