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How to Safely Operate Hotplate Magnetic Stirrers

A common laboratory instrument  for mixing samples at high temperatures is the hotplate magnetic stirrer.  These benchtop units can handle samples up to 10 liters (2.5 gallons) allowing researchers to develop processing techniques at high temperatures.  Depending on models, hotplate magnetic stirrers can be set up to 450⁰C or 842⁰F.

Quite obviously these high temperatures call for safety precautions to avoid painful burns as well as controlling temperatures to avoid overheating and damage to adjacent equipment due to spillage.

This post describes safety features and operating procedures to employ when using CAT Scientific’s line of  hotplate magnetic stirrers offered by CAT Scientific.

But first:

A Quick Intro to Hotplate Magnetic Stirrers

Hotplate stirrers, also called magnetic hotplate stirrers, are versatile laboratory benchtop mixing tools capable of operating unattended for several hours.  Their only moving part is a rotating magnet in the base of the unit, the speed of which can be controlled by the operator.

This causes a coated bar magnet (called a flea) in the mixing beaker or flask to rotate and perform the mixing action  Stirring speeds as with the temperature, depend on the model but can reach 1600 RMP in equipment offered by CAT Scientific.

The simplicity of a rotating magnet accomplishing the stirring differs from laboratory homogenizers and overhead mixers.  In these units mixing is accomplished by a motor driven generator assemblies and shaft impellers.

The limitation is that they do not work well high viscosity samples because there is no physical connection between the flea and the rotating magnet in the magnetic stirrer’s base.

Magnetic Hotplate Stirrer Safety Tips

Researchers working with hotplate stirrers should take steps to reduce accidental spillage through two simple steps prior to filling flasks.

  1. Make certain that the flask is completely clean
  2. Examine the flask for hairline cracks or scratches.  Note that scratches may develop at the interface between the rotating bar magnet and the interior base of the flask or beaker.  If either are observed the flask should be discarded.

Standard and Optional Stirrer Safety Features

CAT Scientific’s line of hotplate magnetic stirrers are designed and built to anticipate and respond to situations that could cause an accident in the lab.

The features are especially important when experiments are conducted at high temperatures over a long period of time and are often left unattended by lab personnel.

Researchers and operators are encouraged to start with a system-wide check of all safety function once the unit is switched on.  As stirring proceeds response mechanisms include:

  • A shut down if the hotplate exceeds the programmed temperature
  • Failure of the hotplate temperature sensor
  • Overheated equipment housing
  • Hotplate triac failure

Safety Enhancements  with the Pt100 Temperature Probe

Perhaps the most important contribution to hotplate stirrer safety is the optional Pt100 temperature probe which when used literally takes over the operation of the equipment.  Immersed at least 2 cm in the sample the probe detects and initiates a shutdown in instances including:

  • Failure of the probe itself due to a disconnect or breakage
  • A dramatic drop in probe-detected temperature caused by a flask leakage or sample evaporation below the probe tip
  • The hotplate temperature increases but the probe-detected temperature does not
  • Container failure – such as a crack allowing the sample to escape
  • Failure of the mixer electronics

As another safety feature there is a programmable switch-off time delay that will continue the stirring operation in the event of a hotplate failure.  This will allow the sample to cool more quickly.

It should be obvious that labs using magnetic mixers especially at high temperatures are wise to invest in the Pt100 platinum resistance temperature probe.  For more information on this device see our article on why you need a temperature probe.

As always we’re ready to answer your questions on magnetic mixers or any other equipment in the CAT Scientific family of laboratory equipment.


Bob Wilcox

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid handling and related laboratory equipment since 2002 when Staufen, Germany-based CAT Ingenieurbüro M. Zipperer GMbH established operations in North America. Bob oversees CAT Scientific laboratory apparatus sales and service organization from the company’s headquarters in Paso Robles, CA. He also is in charge of the parent company’s line of JetCat jet turbines, turboprop, and helicopter power plants for hobbyists’ radio controlled fixed wing and helicopter model aircraft. -- Earlier in Bob’s career he was involved in visual and special effects as well as camera and electronics supervisory responsibilities for the motion picture and television industry.

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