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 when it comes to hotplates and stirrers. 


Pros & Cons of Hotplates & Stirrers

Below are some pros and cons 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.

 

 

 

Generally not suitable for large volumes and highly viscous samples, an overhead stirrer would be more suitable if this is your application. 

There are lots of options out there which 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 to 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).

Can be difficult 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

 - Plate Material - 

- Sample Properties - volume & viscosity - 

 - Control type - analogue & digital -

- Stir bar selection -


1. Plate Material 

Hotplates and hotplate stirring can have a number of 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. The surface however has less good chemical resistance and can be quite difficult to clean. The plate colour is not white and this may be a factor in colour work. Aluminium plates do not 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.

Sample Volume

  • 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. 

Sample Viscosity

  • 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

Analogue Controls

Digital Controls

  • Analogue 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 and this make control much more accurate compared with 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 an accurate picture of the sample temperature. 

4. Stir Bar Selection

Once you have selected a stirrer, the next step is to select the most appropriate stir bar (also called flea). In the specification of the stirrer, it will usually detail the maximum size stir bar that can be used to with the stirrer (this maximum is dictated by the length of the internal drive magnet). Many stirrers will actually include a stir bar to get you started, and will be suitable for general purpose operation. 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.


FAQs

Why use an external temperature probe?

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.

What do the heating and stirring speed control dials mean?

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.

Why is the temperature higher than the settings?

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.

Why does my hotplate overshoot?

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.

How do I reduce 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.

Why does the plate surface not have an even temperature?

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.

Why is the temperature not stable?

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.

Question 9

Answer 9 here.

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

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

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. 

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

Copyright © Camlab Ltd rights reserved.
Powered by PushON