How Dynamic Viscosity Overhead Stirrers Work

Dynamic viscosity overhead stirrers are designed, in slang terms, to “go with the flow.”  What we mean here is that their power systems can accommodate an increase (or decrease as the case may be) of sample viscosity. In this post we’ll take a look at what goes into these precision lab devices and how they “react” if viscosity gets too high.

But first it might be worthwhile to explain

What is Viscosity?

Viscosity relates to the thickness of a fluid.  It may be high such as beauty creams or low such as water.

Overhead stirring and mixing applications in the research lab or on the production line must take into account the viscosity of the product(s) in question and how they impact on ability of the stirrer motor to accommodate viscosity resistance on the stirring paddle.

The scientific term here is “torque” or the twisting force that requires rotation.  You yourself apply torque when you twist open a new jar of pickles. Or when using a wrench tighten a nut on a bolt.

Torque is measured in units called Ncm or Newton centimeters (among other measurements).  We won’t get into the hairy details except to add that auto mechanics use what is called a torque wrench to tighten bolts to manufacturers’ specifications.

So, What is Dynamic Viscosity?

The term dynamic viscosity refers to a change in the viscosity value during processing.  It can go either way, becoming higher or lower with continued stirring and processing.

An example of increased viscosity can also occur when different constituents are added to the mix during the operation.  In either case the overhead stirrer motor must be powerful enough to process the highest viscosity material without overheating and possible burnout.

In other words, a dynamic viscosity overhead stirrer must “go with the flow.”

A Dynamic Viscosity Overhead Stirrer Example

CAT Scientific offers a dynamic viscosity overhead stirrer – the R80D-PC overhead stirrer – specifically designed for such applications.

It is powered by a 100-watt motor and has the capacity to process samples up to 150 liters at 20 to 700 RPM and delivers a maximum of 80 Ncm torque.

This makes it suitable for viscosities ranging, for example, from water to wallpaper paste.

But there is more involved.  This is because the configuration of the stirrer impellers has a direct affect on the equipment’s ability to accommodate viscosity.  Paddle stirrers, for example, rotate relatively larger surfaces (the paddles) against the sample compared to dissolver or propeller configurations.

How Dynamic Stirrer Overload Protection Works

When operating CAT Scientific R80D-PC  overhead stirrers, if the maximum allowable torque is exceeded a red warning LED will illuminate and the electronic system automatically reduces the power of the device.

When and if the overload is removed or overcome the light goes off and mixing resumes.  Should the overload be continuous and reduce the motor to less than 20 rpm the motor switches off.

Protection also extends to overheating, which causes the LED to illuminate and reduce the power.  Continuous overheating will shut down the overhead mixer.  Once the system has cooled switching the unit off and on will restore normal operation.

How to Operate the Dynamic Viscosity Overhead Stirrer

The control panel for this device is a study in simplicity.

The on-off switch illuminates a green LED when on (adjacent to the red LED warning light).  The speed control knob does just that, with the speed shown on the LCD panel.  The unit should be started at the low speed and slowly ramped to the desired speed.

Operation can be automated and run via a PC using the RS-232 interface, which also can be used for record keeping operations.  The unit’s microprocessor control system supports procedures requiring reproducible stirring processes – either one after the other or when simultaneous experiments are being conducted with several stirrers.

In fact, if your lab has several overhead stirrers in operation you might want to consider CAT Scientific’s option for automating overhead stirrer operations.

Taking Care of the Equipment

Overhead stirrers of any type require proper care.  For details on maintaining the R80D-PC or any other of the CAT products please visit our article on stirrer maintenance.

We at CAT Scientific are ready to help you in selecting the right equipment for your research and manufacturing needs.  To get started all you have to do is contact us and provide some preliminary information.  We’ll get back to you pronto.

 

Bob Wilcox

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid handling and related laboratory equipment since 2002 when Staufen, Germany-based CAT Ingenieurbüro M. Zipperer GMbH established operations in North America. Bob oversees CAT Scientific laboratory apparatus sales and service organization from the company’s headquarters in Paso Robles, CA. He also is in charge of the parent company’s line of JetCat jet turbines, turboprop, and helicopter power plants for hobbyists’ radio controlled fixed wing and helicopter model aircraft. -- Earlier in Bob’s career he was involved in visual and special effects as well as camera and electronics supervisory responsibilities for the motion picture and television industry.

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