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Hot Plate/Stirrers

Consider Top Plate Material, Sample, Size, and Viscosity

by Lab Manager
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According to Anthony Cassiano, director, Torrey Pines Scientific, Inc., the biggest advance in hotplate stirrer design came with Corning’s introduction of the ceramic glass heater top. Heating hotter and faster than any prior tops made of metal, it resisted chemical spills much better and took heater-plate limits from 300° C and 400° C to 550° C, and quickly.

The story behind that breakthrough goes like this: Corning made a ceramic glass tray for the airlines to hold their meals, way back when. Some engineer at Corning turned that tray over and put a heater element under it, and thus the ceramic glass heater top was born.


Corning calls their glass-ceramic top Pyroceram® and it was introduced in 1964 for laboratory hot plate applications.



Teri Trefz, Product Manager, Henry Troemner LLC, says that a top plate material’s ability to deliver temperature uniformity should be the first thing to consider when buying a hotplate stirrer. “Ceramic top plates are more chemical resistant, heat up very quickly, and are easy to clean. The white reflective surface aids in viewing the sample. However, ceramic tops are subject to thermal shock. Heating of metallic vessels should be avoided. The edges of a ceramic top plate may not be as hot as the center where the heating element is located. Aluminum top plates offer a more uniform heating surface, will not crack or chip but are more susceptible to corrosion and more difficult to clean,” says Trefz.

The size or volume of the sample is another important factor to consider when selecting a hotplate stirrer. Trefz says to identify the largest sample you may be working with and look for a stirrer that can handle that capacity. But be aware that most listed capacities are based on water. A viscous sample will weigh more.
Sample viscosity also plays a role when selecting a stirrer. The magnetic coupling strength will determine which size stirrer to choose, as the right drive magnet and stir bar combination is needed to efficiently stir the sample. “Stir bar size and shape, the distance between the drive magnet and the stir bar, vessel shape and size, speed and viscosity also must be considered. The more viscous the sample, the greater magnetic coupling strength is needed,” adds Trefz.
As an alternative to drive magnets and stir bar combinations, Anthony Cassiano, Director, Torrey Pines Scientific Inc., tells us that some manufacturers have gone to motorless stirrers. “In the motorless versions, a series of coils are placed under the top surface and the fields are activated in a way to produce a magnetic field that rotates - simulating a rotating magnet. It's a bit more expensive to do, and the advantages are still in question,” cautions Cassiano.




The Corning® PC-420D stirring hot plate has a 5x 7Pyroceram ® top and digital temperature and stirring speed displays. It is designed to operate on 120V/60Hz with a standard US three-prong plug. The unit’s microprocessor maintains consistent and repeatable temperature settings from 5° C up to 550° C. The digital LED temperature display is adjustable in 5° C increments and blinks until set temperature is reached. An optional external temperature controller

eliminates the need to closely monitor liquid temperature.




Thermo Scientific

The Cimarec® Series digital stirrers are available in three sizes, offering flexibility from microscale chemistries to larger vessels. The microprocessor feedback control ensures a constant speed despite changes in viscosity. A hot-surface alert system protects users from burns, while a rugged cast aluminum body provides stability and durability to prevent tipping and spilling. A seamless reflective white ceramic top enhances sample visibility and provides an easy-to-clean and alkali/acid-resistant surface. Proprietary StirTrac™ technology offers improved slow-speed stirring, consistent speed control, and stronger magnetic coupling. Speed is adjustable from 60 rpm to 1200 rpm and an integral ring-stand holder accommodates a 0.5(1.3 cm) diameter support rod.





Torrey Pines Scientific

A new line of multi-position analog stirring hot plates and stirrers features 5 or 9 stirrers. The large 12 sq. in. (30.48cm) ceramic heater tops have a temperature range to 450° C. The 5-position stirring units can stir 5-800 ml beakers and the 9-position units can stir 9-500 ml beakers of aqueous solutions with a stirring range from 100 rpm to 1500 rpm. The units can support more than 50 pounds (22.6kg) on the plate surface and are designed to keep spills out of the chassis. All controls are mounted well in front of the heater surface to protect against accidental burns. The units are available in 100VAC/50Hz, 115VAC/60Hz, 220VAC/60Hz, and 230VAC/50Hz.




Henry Troemner LLC

Talboys hotplates, stirrers, and hotplate/ stirrers come with a variety of safety features that will protect the operator and the sample, such as a hot top indicator light, maximum temperature limits, temperature overshoot protection, and a ramping feature for stirring. Other safety features include probe and stir protection, where the heater will shut down if the stirrer motor fails or the probe disengages from the sample. Also, for critical temperature control of samples, the digital units are supplied with an external temperature probe and a flexible-arm probe kit.





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