Selecting Stirrers and Hot Plates:
Five Important Factors
Basic analog heating and stirring units are not designed to provide exact control over temperature or stirring speed. However, such units do offer economy, reliability, and ease of use when precise control is not required.
Offering somewhat better heat control are units with demand-type bimetallic thermostats that sense changes in top plate temperature through a mechanical connection to the top plate. Such units simulate closed-loop control and minimize temperature overshoot.
For applications in which the control of temperature and stirring speed is crucial (heat control stability better than ±8°C and stirring speed and control better than ±20 rpm), digital heating and stirring equipment with electronic feedback controls offer the greatest degree of accuracy and stability. 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, and in some advanced models, even program heat ramp settings for reproducible results.
The range of temperatures achievable by your hot plate and the uniformity of temperature across its surface are determined by two factorstop plate composition (ceramic, porcelain, or aluminum) and the type of temperature control.
Primary advantages of the ceramic top plate are that it heats quickly and is highly resistant to corrosion. One of the drawbacks is that it does not offer the same uniformity of temperature across the top plate surface that other top plate materials do. 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.
Hot plates, stirrers, and stirring hot plates come in many different sizes and configurationsfrom small, single-vessel units to large-capacity, multi-unit ones. Units designed for synchronous stirring and heating of multiple vessels are available with individual stir controls for as many as nine vessels.
When it comes to magnetic coupling strength, all stirrers are not created equal. The ability of a drive-magnet and stir-bar combination to effectively stir a given 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 seeds 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.
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.