How Programmable Hotplate Magnetic Stirrers Work

Hotplate magnetic stirrers are benchtop units designed to process samples up to very high temperatures.  These temperatures, if not monitored, can pose hazards if the stirrers are left unattended in labs where researchers often find themselves attending to several other tasks.  This post will describe how programmable hotplate magnetic stirrers work and how they contribute to both efficiency and safety in the lab.

First – What is a Hotplate Magnetic Stirrer?

Or, what is a magnetic stirrer?  Unlike overhead stirrers and mixers, mixing and stirring action is provided by a specially coated rotating bar magnet resting on the bottom of beakers and flasks.  The flasks are placed on a plate beneath which is a motor powered magnet.

As the motor  powered magnet rotates (it speed is controlled by the operator) the beaker magnet does likewise, thus performing the stirring and mixing action.  Mixing time is also set by the operator.

This is an extremely simple and efficient way to mix samples up to 10 liters at speeds (in some models) to 1600 rpm.   Their one drawback is that due to how they stir, sample viscosity can be a limit.

Hotplate magnetic stirrers allow samples to be heated – in some models to 440⁰C (842⁰F).

The combination of sample size, stirring speed and high temperature capability make it imperative that close attention be paid to the process.  The answer is found in programmable hotplate magnetic stirrers.

A Closer Look at Programmable Hotplate Magnetic Stirrers

The following paragraphs will give you some insight on how this equipment works and what it does.  Our example today is the microprocessor controlled MCS 77 programmable hotplate magnetic stirrer offered by CAT Scientific.

It weighs approximately 3.9 kg (8.5 pounds), has a 140 mm (5.5-inch) diameter anodized aluminum hotplate and a maximum hotplate temperature that can be precisely controlled up to 330⁰C (626⁰F).  From a soft start (to avoid sample splashing) speed is controllable up to 1600 rpm.

Liquid and hotplate temperatures are fuzzy logic* controlled to provide fast heating time to but not exceeding the temperature you set on the intuitive solvent-proof membrane-protected control panel.  Among the operating parameters you can establish are:

  • Stirring profiles
  • Temperature and temperature ramp up or down over time
  • Hotplate and sensor temperature
  • Motor speed

Operating data are shown on the bright, backlit LCD.

Best when Paired with the Pt100 Temperature Probe

Important features of any hotplate magnetic stirrer include protection against over-heating, hotplate failure and other malfunctions that could cause damage to the stirrer and danger to operating personnel and nearby equipment.   Further reliability and safety are provided by the optional Pt100 platinum temperature probe which, when engaged, takes charge of operating the CAT Scientific MCS 77 hotplate magnetic stirrer.

Briefly explained the Pt100 temperature probe is a device that monitors the sample temperature vs. the hotplate temperature set by the operator.

Affixed to a support stand and connected electronically to the MCS 77 the probe is inserted into the sample, closely monitors the heating of a solution and confirms temperature ranges remain on track throughout the stirring cycle.

As a safety function, should the Pt100 detect an unsafe increase in solution temperatures or should the liquid amount drop to unsafe levels (typically due to evaporation or a crack in the beaker) the probe automatically terminates stirrer operation.

With the probe engaged temperature deviation is ±0.3⁰C.

MCS 77 safety features include:

  • Self test of all safety functions
    • Hotplate failure
    • Pt100 probe failure
    • Out-of-liquid condition, e.g. stirring vessel breaks
    •Differential safety cut-out (an extreme fall in liquid temperature)

Record-keeping is an essential part of experimentation, especially in food and drug processing where thorough documentation is crucial to back up inspections by regulatory bodies.  An RS485 interface allows the MCS 77 magnetic stirrer to be controlled by a PC that also records operating parameters for record keeping.

Please feel free to contact us if you have questions on selecting and operating a magnetic stirrer

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*Fuzzy logic deals with reasoning that is approximate rather than fixed and exact.

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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