15 May

Overhead Stirrer

You need a stirrer for your lab but most of your samples have a higher viscosity than aqueous samples, or your sample is high volume; what do you use?  You use an overhead stirrer from Talboys or Heidolph.   For blending of many ingredients or creating emulsions and gels an overhead stirrer is a must.  In the product development laboratory, or the pharmaceutical testing lab, or in food studies over head stirrers have a distinct advantage over magnetic stirrers.  Overhead stirrers have the option of using many different types of paddles, propellers, or other shaped stirring element.  Collapsible paddles enable stirring in narrow necked vessels such as round bottom flasks.  Overhead stirrers have higher torque for handing higher volumes and higher viscosities.  Digital stirrers which indicate the paddle speed are available; this makes consistent sample processing easier.  Many manufacturers of overhead stirrers have application support available.

When choosing a mixer the factors to be considered are:

The volume of fluid to be mixed;

the viscosity of the fluid;

the speeds at which you must mix;

the amount of agitation desired;

the type of paddle needed to produce the mixing profile desired.

The power of the mixer is important as the motor is designed to turn off if the load is too great.  The load is dependent on the speed, the viscosity, the volume, and the size of the mixing element or paddle.  It is important to choose a mixer with enough power to handle your sample.  Remember, during formulation development or sample processing the viscosity can change; a mixer which will not change speed with increasing or decreasing viscosity would be helpful in this situation.

When choosing a stirring element you must consider:

The volume of your sample;

the viscosity of the fluid;

the mixing profile desired;

the shape of the mixing vessel.

Remember that slow speed mixing with an element chosen with the above considerations will be more efficient than high speed mixing.  High speed mixing can result in a hole in your sample with much agitation near the stirrer and very little everywhere else (especially when stirring non-Newtonian fluids).  Catalogs and manufacturers websites are helpful in guiding your choice of stirrer and element.  Experimentation is essential as you may not make the right choice initially.  Some manufacturers offer trial periods where you can try out the mixers before making a final choice.

Robin Prymula