Five Characteristics for Choosing Stirrers and Hot Plates

Product selection is easier with more knowledge.

When you do your homework, it’s easy to find the stirrers, hot plates or stirring hot plates to meet your needs for years to come. Familiarize yourself with the following factors and then choose your ideal product. Going into any buying process without researching can set you up with an inferior product. Here are a few factors to consider before you add one of these products to your lab.

Accuracy and Stability

Basic analog heating and stirring units do not provide exact temperature or stirring speed control. However, they do offer economy, reliability, and ease of use when precise control is not required. Units with demand-type bimetallic thermostats that sense changes in top plate temperature through a mechanical connection offer somewhat better heat control. These units simulate closed-loop control and minimize temperature overshoot.

Digital heating and stirring equipment with electronic feedback controls offer the greatest degree of accuracy and stability and are recommended for applications in which the control of temperature and stirring speed is crucial—with heat control stability better than ±8°C and stirring speed and control better than ±20 rpm. A closed-loop PID microprocessor control monitors top plate temperatures and/or stirring speeds and automatically compensates for changes in the system relative to a selected set point. Although more costly, these precise controls can hold a specific temperature or stirring speed, minimize temperature overshoot and undershoot, accurately monitor top-plate and solution temperatures.

Range and Uniformity

Top plate composition—ceramic, porcelain, or aluminum—and the type of temperature control determine the range of temperatures achievable by your hot plate and the uniformity of temperature across its surface.

The ceramic top plate heats quickly and is highly resistant to corrosion. Yet, it does not offer the same uniformity of temperature across the surface as other top plate materials. Ceramic tops are also susceptible to thermal shock and should not be used when heating metal vessels or sand baths.

Porcelain top plates offer improved temperature uniformity and good resistance to corrosion. Their surface, however, may flex somewhat near maximum temperatures. Both ceramic and porcelain top plates offer better sample visibility than their aluminum counterparts.

Superior temperature uniformity and stability are the primary advantages of aluminum top plates. Typically, aluminum top plates offer temperature uniformity of ±10°C, depending on the top plate size and operation temperature. They are ideal for larger hot plate surfaces and for applications involving multiple vessels. Although they are generally resistant to physical forces, they are vulnerable to corrosive environments and generally are more difficult to clean.

Volume

Stirrers, hot plates and stirring hot plates come in many different sizes and configurations—from small, single-vessel units to large-capacity, multi-unit ones.

Viscosity

The ability of a drive-magnet and stir-bar combination to effectively stir a solution is a function of several variables such as drive-magnet shape and size, stir-bar shape and size, distance between the stir bar and drive magnet, vessel shape and size, desired stir speed, and your solution’s viscosity.

Given normal conditions, most stirrers will perform adequately at stirring speeds between 100 rpm and 1000 rpm. Stirring more viscous solutions, however, requires a unit with greater magnetic coupling strength: Select a stirrer with a larger drive magnet (>12 cm in length), heavy-duty motor, and the capacity to accommodate longer stir bars.

Hazardous Environment

Organic solvents and chemical mixtures often pose a hazard in your lab because standard heating and stirring equipment can ignite fumes at medium to high temperatures. Reduce your risk of damage, injury, and increased liability by using only explosion-proof equipment.

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