Magnetic and hotplate stirrers aren't usually the most expensive items in the lab, but they can be some of the most useful. However, there are many types to pick from, and not all will be perfect for your use. As a result, you should carefully analyze your options.

In an earlier post, we highlighted several broad deciding considerations when selecting a stirrer, such as longevity and maintenance. In this section, we look at variables to consider when selecting a magnetic or hotplate stirrer:

• Capacity
• Safety features
• Sample viscosity
• Temperature range
• Accuracy

Let’s take a look at each of these in turn:

1. Capability

Capacity is an important consideration when selecting the best magnetic stirrer with hot plate. Surprisingly, certain stirrers can handle relatively huge quantities. The AREX CerAlTopTM Hot Plate Stirrer (below left) can handle up to 20L, and Ohaus Advanced Large Capacity Stirrers (below right) have capacities of 100L or 200L. 

Please keep in mind that your stirrer may not be able to handle quantities up to the maximum specified for higher viscosity samples (more on this below).

2. Security Features

Safety is the most important consideration in any stirring application. This is particularly true when using a hotplate stirrer.

You may come across the following safety features:

• Speed ramping: This feature allows you to gradually increase the speed to avoid splashing.
• A hot top warning indicator indicates when a hotplate stirrer is hot (usually above 50°C). 
• Stir protection: If the stirrer motor stops or fails, the heating will turn off immediately.
• Probe protection: Similar to stir protection, if the probe disengages from the sample, the heater will turn off.
• Current limit protection: This safeguards against high currents that could cause a fire.
• High-temperature setting: This function allows you to program the maximum temperature the hotplate can achieve, which is especially useful if you have samples with sensitive flash points.
• Safety shields: Available as add-ons for some units, these shields protect against splashes, spills, and aerosols.

3. Sample Viscosity

When working with viscous materials, your stir bar is more likely to get uncoupled than when working with less viscous samples. Many variables are at play here, including sample size, stirring speed, and the type of stir bar used. When it comes to the stirrer, a stronger magnetic coupling is required to limit the possibility of the stir bar spinning out.

You may think that greater capacity stirrers can manage higher viscosities, however this is not necessarily the case. It all relies on the strength of the drive magnet. VELP Scientifica stirrers, for example, have powerful drive magnets. However, magnetic stirrers are often only suitable for liquids with a low viscosity.

4. Temperature Range

Of course, for hotplate stirrers, you must guarantee that the temperature is high enough for your application. The MS-H280-Pro Digital Hotplate Stirrer (below left) can reach temperatures of up to 250°C, whilst the AREC Digital Ceramic Hot Plate Stirrer (below right) can reach temperatures of up to 550°C.

Keep in mind that your sample will not reach the maximum temperature of your stirrer, so you'll need a hotplate stirrer that can give a temperature higher than what your sample requires.

When it comes to temperature, range isn't the only thing to consider; uniformity may be as well. In terms of plate material, this is where there may be a trade-off. Aluminum plates are more consistent, but ceramic plates are more chemical resistant and easier to clean. However, many plates are constructed of aluminum but have a ceramic coating, allowing you to have the best of both worlds.

5. Accuracy

When selecting a stirrer, accuracy may not be the first thing that comes to mind, but it can be an essential aspect for some applications, especially if you want to assure repeatability or plan to scale up your operation.

One of the most important considerations to make when it comes to precision is whether to use a digital or analog stirrer, with the latter generally delivering less accuracy.