Magnetic and hotplate stirrers aren’t typically the biggest budget items you’ll find in the lab, but they can often be some of the most useful. That said, there are a great number of models to choose from and not all will be suitable for your application. As such, it’s important to consider your options carefully.
We discussed some general deciding factors when choosing a stirrer in an earlier post, including durability and maintenance. Here, we examine factors to consider when choosing a magnetic or hotplate stirrer:
- Safety features
- Sample viscosity
- Temperature range
Let’s take a look at each of these in turn:
One of the biggest determining factors when choosing the most suitable magnetic or hotplate stirrer is capacity. Perhaps surprisingly, some stirrers can deal with some fairly large volumes. For example, the AREX CerAlTop™ Hot Plate Stirrer (below left) can handle up to 20L, and you can purchase Ohaus Advanced Large Capacity Stirrers (below right) with a maximum volume of 100L or 200L.
Note that for higher viscosity samples (more on this below), your stirrer may not be able to handle volumes up to the maximum specified.
2. Safety Features
With any stirring application, safety is a primary consideration. This is especially the case when using a hotplate stirrer.
Some safety features you may come across include:
- Speed ramping: This feature enables you to slowly ramp up the speed to avoid splashing.
- Hot top warning indicator: A warning light signals when a hotplate stirrer is hot (typically above 50°C).
- Stir protection: If the stirrer motor stops or fails, the unit’s heater will automatically shut down.
- Probe protection: Similar to stir protection, the heater will shut off if the probe disengages from the sample.
- Current limit protection: This protects against excessive currents which could pose a fire risk.
- High-temperature setting: Especially helpful if you have samples with sensitive flash points, this feature allows you to program the maximum temperature the hotplate may reach.
- Safety shield: These can be purchased as add-ons for some units and protect against splashes, spills, and aerosols.
Advanced Round Top Hotplate Stirrers (left) come with a hot top indicator, stir protection, and speed ramping. Professional Round Top Hotplate Stirrers (right) also come with a maximum temperature limiting feature and probe protection.
3. Sample Viscosity
If you’re working with a viscous samples, there’s more of a concern that your stir bar could become uncoupled than when you’re stirring less viscous samples. Many factors are involved here, such as sample size, stirring speed, and the type of stir bar you use. When it comes to the stirrer itself, to reduce the chance of the stir bar spinning out, you need a unit with greater magnetic coupling strength.
You may assume that larger capacity stirrers can handle higher viscosities but this is not always the case. It really depends on the power of the drive magnet. For example, VELP Scientifica stirrers come with high-powered drive magnets. Ultimately, however, magnetic stirrers are usually only suitable for relatively low-viscosity liquids.
4. Temperature Range
For hotplate stirrers, of course, you need to ensure that the temperature will be high enough for your application. For example, the MS-H280-Pro Digital Hotplate Stirrer (below left) goes up to 250°C whereas the AREC Digital Ceramic Hot Plate Stirrer (below right) heats up to 550°C.
Bear in mind that your sample won’t achieve the maximum temperature of your stirrer, so you need a hotplate stirrer that can provide a temperature above that you need your sample to reach.
Range isn't the only important factor when it comes to temperature, and uniformity may be important to you too. This is where there may be a trade-off as far as plate material is concerned. Aluminum plates give you better uniformity, but ceramic plates offer improved chemical resistance and are easier to clean. That said, there are many plates that are made from aluminum but have a ceramic coating, giving you the best of both worlds.
Accuracy might not be the first thing that springs to mind when choosing a stirrer, but this can be an important factor for certain applications, especially if you want to ensure repeatability or plan to scale up your process.
When it comes to accuracy, one of the main decisions you need to make is whether to go with a digital or analog stirrer, with the latter generally providing less accuracy.