Laboratory Hotplates/Stirrers. A Guide Jan. 23, 2015

Among the most required tools of the qualitative or quantitative laboratory are Hotplates, Stirrers and a combination of Hotplate/Stirrer. Usually these instruments are affordable, reliable and easy to use. A magnetic Hotplate/Stirrer uses a rotating magnetic field, that causes a stir bar to spin thereby mixing a solution. In the lab a container with the liquid to be stirred is placed on the ceramic or metal top (plate) of the Hotplate/Stirrer. The stirring speed and the desired temperature is selected either by a digital readout or with a manual knob on the instrument front panel and stirring and heating is begun. Temperatures from ambient (room) to 550 degrees C may be achieved and stirring speeds from 100 to 1500 rpm are possible.

Types of Laboratory Stirrers

When it comes to laboratory stirrers there are generally two types of devices you can use – you can either go with a mechanic stirrer, or a magnetic one. Both have their advantages and disadvantages, but when you’re using the instrument in a laboratory environment, the magnetic version typically proves to be more convenient.

The basic difference is that a mechanic stirrer uses mechanical systems to stir the liquid – i.e. a rod driven by a motor, in the simplest sense of the machine’s design. More complicated designs exist too, which rotate the liquid container separately in order to get additional stirring. Furthermore, heat may be applied to catalyze the process of binding the chemicals.

On the other hand, magnetic laboratory stirrers use a small rod driven by magnetic forces to stir the liquid. While this proves to be much more efficient in getting the binding right, it’s limited by its design to only handling small amounts of liquid and can’t cope with a larger container very efficiently. This is why most labs tend to have one of both – considering that you normally don’t need such precise stirring when mixing a large container of liquid or high viscosity liquid, using a mechanic stirrer isn’t so disadvantageous in this case.

1. An Overhead Stirrer

Overhead stirrers are powered by a drive motor affixed to an adjustable support stand.  Mixing tools, a.k.a. impellers, paddles and blades, are attached to a steel rod of sufficient length to immerse the tool at the required depth into the sample beaker. The upper end of the rod is inserted and tightened into the mixing motor chuck.

Overhead stirrer motors can be specified by torque power necessary to work with samples based on viscosity, and by stirring speed and duration. Self-test features will shut the equipment down in the event of overheating or sample viscosity exceeding the unit’s limits.

Overhead stirrer controls vary depending on the model. All have adjustable speed control (typically 500 to 2000 RPM) and basic LED displays. Higher end mixers display set and actual parameters along with other operational details. RS232 interfaces can be used to operate overhead stirrers by computers and collect operating data for record keeping. Possesses a big mixing power. This type of stirrers is presented to you by IKA Works and Scilogex manufacturers.

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2. Magnetic Stirrers

When you hear someone talking about magnetic stirrers and aren’t sure what exactly the device is, you should know that the device is actually known by several names, and the more commonly accepted one is a “hotplate stirrer.” The hotplate isn’t actually a part of the device at all times, but it’s a commonly used element and it’s actually rare to find a magnetic stirrer without one.

Anyway, the magnetic stirrer uses magnetic forces to provide you with a high-speed stirring that gets you the perfect mixture of your materials which you need to carry out those experiments flawlessly. Any scientist would tell you that some processes just require stirring which humans can’t provide, and buying a mechanic stirrer has quite the few disadvantages – it’s a bulky and noisy machine that requires a lot of maintenance.

The magnetic stirrer on the other hand, is elegant and simple – you drop a small magnetized rod into the glass of liquid, turn on the machine and watch the rod start spinning around wildly as the magnetic forces emitted by the machine control its motion within the liquid. In just a few minutes you’ll have your liquids mixed in a homogenous substance, ideal for your requirements!

Because they are driven by magnetic power rather than a direct connection to the motor, magnetic stirrers are not efficient for high-viscosity samples.

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3. Lab Homogenizers (for High Speed Processing)

Lab homogenizers take on tasks that other stirring methods can’t perform. They are selected when the objective is thorough mixing of samples or the processing of samples for further analysis. They are also used in medical applications such as cell disruption.

They consist of a drive motor typically mounted on a stand for stability. The equipment is offered in several power ratings in watts and has variable speed controls that depending on the model can reach 45.000 RPM.

Mixing is accomplished by a homogenizer’s generator consisting of a rotor-stator assembly. The rotor shaft is attached directly to the drive motor while the stator is at the base of a tube affixed to the drive motor housing. Sharp teeth machined at the base of the rotor shaft interact with sharply machined slots on the companion stator. The configuration of the homogenizer’s generator is selected based on the properties and volume of samples being processed. Volumes can range from 0.1 milliliter to 20 liters.

Lab homogenizers draw the sample up into the generator assembly where the rotor violently propels it against the stator subjecting it to mechanical shearing and sonic energy.

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Additional subdivision

The Microwave Stirrer converts the motion from the turntable impeller inside a microwave oven into a magnetic stirrer, allowing simultaneous heating and stirring for increased efficiency. Heated stirring can improve dissolving and mixing, and permits preparation of super-saturated solutions.

Ideal for melting agar and agarose, concentrating urea solutions, or other applications that benefit from heated stirring. It is also useful for chemical purification.

Lab Rotators could be in their turn divided into Rotatory stirrers and Vortexes. Rotate flasks, test tube racks, beakers, vials, Petri dishes, microwell plates, culture plates, plastic/glass trays and slides in microbiological, immunological and general clinical application. Vortex types stirrers are manufactured by such trademarks as Ohaus and Scientific Industries.

Extra tips

Many of these instruments are available in LCD or LED that display the temperature and stirring speeds until the desired targets are reached. It is often very important to maintain constant speed and control of temperature, regardless of the load of the liquid solution. Therefore, attention should be paid to the capacity limits of the Hotplate/Stirrer before making a purchase.

It is important when selecting Hotplates and Stirrers to make sure the top surface or plate is compatible with a wide variety of glass beakers, flasks and other vessels. Attention should also be paid to the amount of liquid to be mixed and the load the instrument will be required to carry and process.

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