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A viscometer is a device made to measure fluids for their viscosity, and scientists, manufacturing plants, chefs and anyone else that needs properly viscous fluids can use this. Viscometers can only be used on fluids that have a stable viscosity, despite how they are flowing. To properly measure a fluid, either the fluid or the viscometer has to remain stationary. Each meter has a different measurement range, and the user will have to choose a proper meter based on the approximate viscosity of the fluid. Measuring viscosity is important for some tasks, because fluids may need to be at a certain viscosity for chemical or structural changes to occur.
Some fluids change their viscosity depending on how they flow, and these fluids require a rheometer for measurement, because a viscometer will not work on these fluids. Viscometers can only be used on fluids that, regardless of the force applied, will not change viscosity. Most fluids, such as oil, sauces and paints, do not alter their viscosity because of force.
When a viscometer is used to measure viscosity, it must be used in one of two ways, regardless of what type of viscometer is used. Either the device is still while the fluid is forced through it, or the device goes through the fluid while the fluid is still. It tends to be easier — and requires less parts or hardware — to use the latter method, making it popular. To have the fluid go through the meter instead, a motor or similar device is generally required to make the fluid move adequately.
Different fluids require a differently calibrated viscometer, depending on the fluids' approximate viscosity. For example, a meter made for low-viscosity measurements cannot be used on thick-viscosity liquids, because it will not have the proper measurement capabilities. Using the wrong meter may break the machine, especially if it is made for low-viscosity fluids and goes through thick fluids. Low-viscosity fluids are generally oils and paints; medium-viscosity fluids are things such as creams and glue; high-viscosity fluids are generally gels and honey.
Measuring the viscosity of fluid can be important for some people, because exact viscosities are needed for some reactions. For example, a manufacturing plant may need a fluid to be at a certain viscosity to complete a necessary chemical change. A fluid also may have to be the correct viscosity before it can be sold, making the viscosity test also function as a quality check. If fluids need to be pumped, then using a viscometer will show workers how powerful the pumps need to be to adequately move the fluids.