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.