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  • Discussion about Heating Problems with Heating Magnetic Stir

    Hot Plate Magnetic Stirrer is mainly used in the fields of chemical synthesis, analytical chemistry and life sciences. The major concern of heating function is the heating speed, overheating protection and temperature stability. But the heating speed invariably conflicts with increasing temperature, especially in an open environment where this contradiction appears more difficult to balance for application of different mediums and volumes. To meet the application needs of scientists, other customers of such products, we employ a similar comprised control method-allowing a certain degree of increase in temperature. As soon as the hotplate magnetic stirrer temperature reaches the set temperature, the increase in temperature will not be significant, only minor changes are observed in temperature, which will inevitably mean that this is only nearby the target temperature as soon as practical (as shown in Figure 1). This kind of products cannot reach the perfect standard temperature while heating and stirring are done together.

    At DCL Laboratory Products, we use water and silicone oil as medium for heating experiments under the same temperature range.
    Below are the results of such experiments (as shown in Figures 2-4)




    It can be seen from the experimental results that:
    1.At 20℃, under same experimental conditions, heating of silicone oil takes relatively short time in comparison to water and results in relatively large temperature increase, because the specific heat of water(4.2 x 103J/Kg) is larger than the specific heat of silicone oil(2.49 x 103J/Kg). A substance with less specific heat capacity is slow to dissipate heat and faster to increase the temperature with a large temperature rise when the same heat is applied.
    2.When the target temperature is set at 40℃ and 80℃ respectively, it takes less time to reach these target temperatures respectively with silicone oil is used as medium and it takes more time in case water is used as medium. The specific heat of silicone oil will decrease to 1.63 x 103J/Kg as it reaches 100℃ whereas the specific heat of water remains unchanged. Heating medium with smaller specific heat capacity heats faster under same heating conditions.
    3.Under the same experimental conditions, if the heating is performed at different volumes with silicone oil and water as medium, it is observed that the smaller the volume, the shorter the time required reaching the target temperature and larger is the increase in temperature.
    In general, the ambient temperature, surface area / volume of container, specific heat of heating medium, the difference between the set temperature and external ambient temperature will all affect the heating speed, ability to control increase in temperature and temperature stability. Therefore in certain applications, overheating protection is important.

    In applications where overheating protection is important, it is recommended to consider the below suggestions:
    (1) Ensure that the medium temperature is stable before you place the sample.
    (2) Initially, set the heating temperature lower than the desired temperature and gradually increase it to the required target temperature after the temperature attains stability.

    In applications where the time taken to reach target temperature, it is recommended to consider the below suggestions:
    (1) Using thermal insulation measures in the heating process, such as protective cover
    (2) Initially, set the heating temperature higher than the desired temperature and gradually decrease it to the required target temperature while the heating medium approaches the target temperature.

    In addition, to achieve the best possible heat transfer, it is advisable to use containers with bottom which fits exactly on the instrument workplate for efficient heat transfer.
     
    DCL Laboratory Products Ltd.