Camlab's Guide To : Benchtop Heating and Stirring
So you want to buy a hotplate or hotplate/magnetic stirrer? Hotplates, hotplate stirrers and magnetic stirrers are our most popular pieces of benchtop kit because of their versatility and are a great option for many labs. However they may not be best option for your application especially if you are dealing with high sample volumes or high viscous samples.
We have compiled our guide to the features that they have and how this fits in with different applications including plate material and analogue or digital controls. We also explain the different types of stir bars available and which to select for your application. Finally we have our FAQs which are a list of real questions we have been asked - we hope these answer any queries you may have regarding hotplates and stirrers.
Pros & Cons of Hotplates & Stirrers
Below are some pros and cons including application types to help you decide if a hotplate/hotplate stirrer really is the best choice for you:
Good for heating / stirring small sample volumes.
Lots of options across all budgets including analogue models for when only rough control is needed, digital models for more accurate control and choice of plate materials.
Small footprints - hotplate stirrers combine heating and stirring providing two functions in a small footprint.
High temperature ranges, the maximum temperatures of some plates can reach as high as around 550°C.
Submerisble magnetic stirrers exist meaning you can couple them with a water bath.
They can be used with accessories such as Drysyn to essentially turn your hotplate into a heating mantle for round bottomed flasks. Drysyn has also been shown to deliver heating performance that is equal to or better than silicon oil making it a safer alternative to oil baths.
Generally not suitable for large volumes and highly viscous samples, an overhead stirrer would be more suitable if this is your application.
There are many options out there that can be overwhelming when deciding - including different plate materials, digital or analogue controls and different capacities. They each have pros and cons but if you continuing reading this guide, we hope it helps explain the choices.
Will overshoot the temperature when heating up - not suitable for applications where your sample is very temperature-sensitive (consider a water bath).
It can be challenging to maintain your sample temperature within tight bounds, a water bath would be more suitable if tight temperature control is required.
Selecting the right hotplate stirrer
When purchasing a new hotplate / hotplate stirrer, you should want to consider the following features:
- Plate Material -
- Sample Properties - volume & viscosity -
- Control type - analogue & digital -
- Stir bar selection -
- Use of accessories such as heat blocks -
1. Plate Material
Hotplates and hotplate stirring can have several different plate materials, the most suitable of which depends on your application and the features you require of the plate. The plate material will influence the temperature range and uniformity of the hotplate.
Ceramic plates will heat to very high temperatures and have excellent chemical resistance. However, they have poor temperature uniformity across the plate due to cool edges and cannot be used with any kind of reflective vessel or a sandbath.
- Aluminium plates are very robust and strong against knocks in the lab and have much better temperature uniformity as they offer all over heating of the plate. Some other key factors include:
Their surface however has is less good chemically resistant and can be quite challenging to clean.
The plate colour is not white, and this may be a factor in colour work.
They cannot heat to as high a temperature as ceramic plates.
Aluminium plates are not very common; you will mostly find them as ceramic coated aluminium plates.
Ceramic coated plates combine the best of both worlds – they offer good temperature uniformity across the plate along with a high level of chemical resistance given by the ceramic coating. As the plate itself is composed of aluminium, they do not heat to as high a temperature as strictly ceramic plates do.
You can read more information on the comparison of the plate materials to help decide which is the most appropriate for you here.
Plate Material Feature Comparision Table:
2. Sample Properties
Magnetic stirrers and hotplate stirrers work using an internal drive magnet. The magnetic stir bar that’s placed into your sample to be stirred will couple with the internal drive magnet and spin as driven by this. It is the strength of this coupling (along with that of the motor itself) that dictates the amount and type of sample that the stirrer is capable of working with. Most magnetic stirrers are only designed to work with small volume, low viscosity samples.
- The specifications will tell you what the maximum volume is. Usually as the size of the top plate increases, the maximum volume compatible will also increase.
- The maximum volume of most stirrers is between 10-20L.
- Generally, stirrers are only validated to stir a set volume of water, or liquid with the viscosity around that of water. In practise, most stirrers will work with no issues when a sample viscosity is no higher than 100mPas. If your sample is too viscous, the stir bar will become uncoupled and break away or the motor could burn out.
- For samples with a high viscosity, we advise that you consider using an overhead stirrer which has a much stronger stirring action.
Heavy Duty Magnetic Stirrer Suitable For Large Volume Viscous Samples
We recommend the use of an overhead stirrer for large volumes and viscous samples. However if you wish to use magnetic stirrer, the max drive system by 2mag has a maximum stirring volume of up to 250L and can cope with viscous liquids.
3. Analogue or Digital Controls
- makAnalogue hotplates and stirrers use a dial, usually with an arbitrary numbered scale for you to set your temperature and stirring speeds. These are designed for users who have applications that don’t need to be performed to exact specifications. You may wish to immerse a thermometer into your sample to get a measure of the temperature. Some analogue hotplate stirrers are compatible with external temperature controllers allowing accurate control of your sample temperature.
- Digital hotplates and stirrers feature an LCD or LED display where temperature and speed values are displayed making control much more accurate than analogue counterparts. Temperature values displayed will be that of the plate surface so consider using an external temperature probe immersed into your sample to get a precise picture of the sample temperature. Read more about the benefits of an external temperature probe here.
4. Stir Bar Selection
Once you have selected a stirrer, the next step is to choose the most appropriate stir bar (also called flea). The stirrer's specification will usually detail the maximum size stir bar that can be used with the stirrer (this maximum is dictated by the length of the internal drive magnet). Many stirrers will include a general-purpose stir bar to get you started. Choosing a stir bar which is the right size and shape for your task can make the stirring action much more effective. You can read our blog post here.
|Stir Bar Type||Application|
|Cylindrical Stir Bar||General purpose for smooth stirring|
|Plain Stir Bar||Similar to cylindrical but with more turbulence at lower speeds|
|Pivot Ring Stir Bar||for use when the container base is slightly curved or uneven. The pivot lets the stir bar adopt the optimum position for stirring|
|Octahedral Stir Bar||Similar to pivot ring type but with more turnelence at lower speeds|
|Spherical Stir Bar||For stirring in test tubes or eccentric stirrring|
|Oval Stir Bar||for use in round bottom flasks - boiling flasks|
|Cross Stir Bar||Suitable for general purpose stirring but very stable to reduce 'jumping'|
|Crosshead Stir Bar||For use in tube shaped containers or other containers - ideal when working in both test tubes and beakers or flasks|
|Triangular Stir Bar||For dissolving solids or mixing of sediments. The flat base gives a scraping action for distrubing solids.|
5. Use of heating blocks or a sand bath
Heating blocks such as Drysyn or a sand bath can be used with plate materials apart from ceramic hotplates (the heat reflecting back to the ceramic surface may cause it to crack and discolour). They work by conducting heat through the base and side walls of your heating vessel as opposed to simply through the base alone like with your typical hotplate set up - similarly to how waterbaths or oilbaths work. This helps to prevent hotspots and allows for a more gentle and even heating throughout the vessel.
Drysyn blocks can be used with round bottomed flasks and will effectively turn the hotplate into a heating mantle. This setup retains its ability to stir the samples. Drysyn has also been shown to deliver heating performance that is equal to or better than silicon oil making it a safer alternative to oil baths.
We would recommend that you use an external temperature probe with this type of set up to enable you to have better control of your sample temperature.
Check out our most asked questions when it comes to hotplates and stirrers:
Some hotplate stirrers have the ability for you to connect an external temperature probe. The sensing end of the probe is placed within your sample and works to tell you what your actual sample temperature is. Without using a probe, the unit will report the plate temperature, but this may be different to your sample temperature. Your sample will absorb and retain heat according to its physical properties. The thermal conductivity of the plate or heating vessel and room conditions may also affect the temperature of your sample.
Analogue hotplates and stirrers use a dial with arbitrary numbers on scale for you to set your temperature and stirring speeds. These are designed for users who have applications that don’t need to be performed to exact specifications. Analogue units are cheaper than their digital counterparts. Digital units feature buttons or dials that correlate to an exact figure that is displayed on the unit.
When heating up, a hotplate will typically overshoot before slowly returning to the set temperature. Depending on factors such as your sample and vessel type, your sample may be hotter than the settings as it retains the heat. It is advised to allow your hotplate to come up to temperature and then stabilise by waiting 20-30 mins before using if temperature is especially important.
If you have just reduced the temperature, your set up may take some time to dissipate the heat
If the ambient conditions are very warm and you are working at temperatures only just above this, as your hotplate naturally fluctuates within bounds,it will find it quite difficult to maintain the set temperature. Any overshoots and increase in heat will take longer for the heat to dissipate (transfer) out of your sample. Cooling rate is proportional to the temperature difference between the room and the sample.
Even with hotplates that have a feedback loop PID control, overshoot should be expected. Once the plate reaches its set temperature, the heating is cut off. Your sample however will continue to absorb heat from the plate causing the temperature to rise. A few factors can influence this including the thermal properties of your sample, conductivity of the heating plate and vessel and the settings in the PID control itself (which also influence how easy it is for the hotplate to maintain the sample temperature at a constant).
Not all PID controls are made equal and some have more intuitively designed settings thereby reducing the overshoot.
- Using a metal container compared with glass with minimise the over shoot
- Set your target temperature as lower than required then increase once this is stable.
If temperature stability is important then it is recommended to use a waterbath.
The plate surfaces (both aluminium and ceramic) do not have completely uniform plate temperatures. Ceramic plates have a heated area in the middle of the plate and cool edges. You should consider this when selecting the hotplate/vessel combination. Aluminium plates have a lot higher temperature uniformity of around ±10°C depending on model, size and temperature used.
Hotplates and hotplate stirrers will work within specified bounds of temperature stability rather than maintain a 100% stable temperature. The stability quoted in the technical specifications can often be an overestimate of stability as they assume that the hotplate is working in perfect operating conditions and usually this is not the case. There are many factors that can affect how stable your sample temperature is. Even if your hotplate itself is pretty stable, this may not directly translate to how stable your sample temperature is.
Got a question that is not answered above? Please email our technical support team at Support@camlab.co.uk who will endeavour to give you an answer.
Other benchtop heating and stirring options
Overhead stirrers are ideal for samples where a magnetic stirrer is not powerful enough such as large volume or highly viscous samples. If you require heating, you may want to consider using in combination with a hotplate.
Water baths are ideal for samples that require a delicate and uniform temperature control. Water is a very good conductor of heat, so the sample can heat very evenly when submersed in water. If you also require stirring, some magnetic stirrers are submersible.
Ideal for biological (e.g. enzymatic) experiments.
Bringing a waterbath up to temperature can take a long time. Also if the waterbath is used at hotter temperatures it is advisable to use a lid.
Dry Block Heaters
Dry block heaters are ideal for heating lots of small volume samples. Heaters are typically supplied with a fixed block or interchangeable block. With the latter, there can be dozens of options, compatible with many tube types.
Some models have the option of using a temperature probe. The heat is conducted from the metal block through the walls of your vessel.