Selecting an Overhead Stirrer

How to Specify an Overhead Stirrer

If your laboratory or research organization is planning the purchase of an overhead stirrer there are a number of specification issues that should be addressed in writing up a purchase order.  First of all, you’ve reached the conclusion that an overhead laboratory stirrer is required due to sample volumes and viscosities that magnetic stirrers can’t efficiently handle.  Other considerations leading to an overhead stirrer decision vs. a magnetic stirrer are in our article When to Specify an Overhead Stirrer.

We’ll discuss several specification issues related to these laboratory mixers because in fact they are very much interrelated.

Overhead Stirrer Sample Viscosity and Size

Two key specification issues for overhead mixers are sample viscosity in and sample size.  Sample viscosity presented as mPa.s (millipascal seconds) is usually an indication of the amount of torque energy measured in Ncm (Newton centimeter) that the overhead stirrer motor will have to apply for satisfactory mixing.  For water and solvents there is little concern about viscosity and volume, but when high viscosity samples are processed or where viscosities change during processing these conditions must be considered.  So too will the configuration of the stirring elements being used, a consideration discussed briefly below.

As an example, for samples to 200 liters with viscosities from 10,000 to 100,000 mPa.s you’ll probably want a pilot plant overhead stirrer such as CAT’s R100CT with its 240-volt motor delivering up to 600 Ncm torque energy.  At the other end there’s the CAT R20 mini-stirrer that can handle 35-lliter samples, provides 20 Ncm torque, and handles viscosities to 10,000 mPa.s.

Overhead Lab Stirrer Features to Look For

Once you’ve narrowed down your specification criteria regarding sample size and viscosity you should look for additional features when specifying your overhead lab stirrer.  Here some examples:

• Stirring speed.  This varies depending on models but a “for instance” is 10 to 500 or 40 to 2,000 RPM via stepless speed adjustment.  Higher stirring speeds are available.
• Timed running.  Certain models can be programmed to run from 1 minute to 99 days.
• Soft start and stop.   This avoids sample splash out of the container and strains on the overhead stirrer motor.
• Overload protection.  If during processing sample viscosities become too high this feature will reduce stirring speed or shut down the overhead mixer to avoid damaging the drive motor.
• PC Connectivity.  This is usually provided by an RS 232 interface and permits programming and automatic monitoring of stirrer operation as well as recording data for record keeping.
•  Dynamic speed control.  This is a special feature on certain models such as the CAT R80D-PC to use with samples where viscosities vary during stirring.
• Stirring rod chuck size.  This is important if you use several different stirring rod/stirring element configurations.  A good benchmark is a chuck accommodating up to 10.5mm diameter stirring rods.
• Digital readouts that selectively show via a liquid crystal display the operating parameters programmed into the overhead mixer and actual operating data.
• Quiet operation

Over Head Stirrer Stirring Elements

Stirring elements as the name suggests do the actual stirring or mixing.  These are attached to the over head stirrer motor via a stainless steel shaft, the length of which and diameter of which is governed by the element geometry, the sample viscosity and the sample volume.  The broad classification of stirring elements is blade, centrifugal and propeller as described in the stirring shaft applicaton sheet.

Contact us for specific recommendations on selecting an overhead stirrer and the selection of stirring paddle configurations.