Lab Equipment: How to Choose a Mixer

With some types of lab equipment, such as mixers, you have a wide range of models to choose from. How do you choose? Here is a list of characteristics to consider when making your choice:

1) Container capacity: tank diameter and batch height.

2) Liquid Viscosity: Different types of liquids display unique characteristics when force is applied. Four most common types of liquid behavior are:

Dilatant Liquids—viscosity increases as shear rate increases. Mixers can bog down and stall after initially mixing such liquids. Dilatant liquids include slurries, clay, and candy compounds.

Newtonian Liquids—viscosity remains constant regardless of shear rate or agitation. As mixer speed increases, flow increases proportionately. Newtonian liquids include water, mineral oils, and hydrocarbons.

Pseudoplastic Liquids—viscosity decreases as shear rate increases, but initial viscosity may be sufficiently great to prevent mixing. Typical pseudoplastic liquids are gels, latex paints, and lotions.

Thixotropic Liquids—as with pseudoplastic liquids, viscosity decreases as shear rate or agitation increases. When agitation is stopped or reduced, hysteresis occurs and viscosity increases. Often the viscosity will not return to its initial value. Thixotropic liquids include soaps, tars, shortening, glue, inks, and peanut butter.

3) Torque requirements: the rotational force required of the mixer motor—measured in in-oz or in-lb.

4) Horsepower (hp) requirements: the efficiency required of the mixer motor with regard to torque (in-oz) and to the rotation speed (in revolutions per minute—rpm) as defined by the following equation:

hp = (in-oz x rpm)/1,008,400

5) Rotational speed (rpm) and diameter of mixing propeller: A small increase in the rpm or diameter greatly increases the power required for mixing. This relationship can be expressed as follows:

Power ∝ rpm3 x dia5

6) Duty cycle: the time interval devoted to starting, running, stopping, and idling when a device is used intermittently.

7) Power supply: electric (115 or 230 VAC) or air-drive mixers for areas where electricity is unsafe or impractical.

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