Vacuum drying oven is a box type drying equipment that dries materials under negative pressure conditions. Its working principle is to use a vacuum pump to extract air and moisture, creating a vacuum state in the working chamber, reducing the boiling point of water, and accelerating the drying speed. The principle of vacuum drying oven mainly includes the following points:

1. The reason for the difference between the instrument reading of the vacuum drying oven and the reading of the glass rod thermometer in the vacuum chamber is that most electric vacuum drying ovens use the method of heating the vacuum chamber wall first, and then radiating heating from the wall to the workpiece. In this way, the temperature sensor of the temperature control instrument can be arranged on the outer wall of the vacuum chamber. Sensors can simultaneously receive convection, conduction, and radiation heat. The glass rod thermometer in the vacuum chamber can only accept radiant heat, and because the blackness of the glass rod cannot reach 1, a considerable part of the radiant heat is refracted. Therefore, the temperature value reflected by the glass rod thermometer is definitely lower than the temperature reading of the instrument. Generally speaking, it is normal for the temperature reading of the instrument to differ from the reading of the glass rod thermometer by less than 30 ℃ under the working condition of 200 ℃. If the temperature sensor of the temperature control instrument is arranged in the vacuum chamber, the difference between the temperature value of the glass rod thermometer and the temperature reading of the instrument can be appropriately reduced, but it cannot be eliminated, and the sealing reliability of the vacuum chamber increases a potentially unreliable link. If you don't want to see this difference from a practical operational perspective, you can use the display correction function of the temperature control instrument Ttou to solve it.

2. The reason for vacuum drying oven to first evacuate and then heat up is that 1) the workpiece is placed in the vacuum oven to evacuate the gas components that can be extracted from the workpiece material. If the workpiece is heated first, the gas will expand when exposed to heat. Due to the excellent sealing of the vacuum box, the enormous pressure generated by the expanding gas may cause the tempered glass of the observation window to burst. This is a potential danger. By following the procedure of vacuuming first and then heating up, this danger can be avoided. 2) If the procedure of heating up first and then vacuuming is followed, when the heated air is pumped out by the vacuum pump, the heat will inevitably be carried to the vacuum pump, resulting in a high temperature rise of the vacuum pump and a possible decrease in its efficiency. 3) The heated gas is directed towards the vacuum pressure gauge, which will generate a temperature rise. If the temperature rise exceeds the specified operating temperature range of the vacuum pressure gauge, it may cause indication errors in the vacuum pressure gauge. The correct usage method should first evacuate and then heat up. When the rated temperature is reached and the vacuum degree is found to have decreased, add some more suction appropriately. This is beneficial for extending the service life of the equipment.

3. The reason why vacuum drying ovens do not have temperature uniformity parameters is that most electric (blast) drying ovens have temperature uniformity parameters: natural convection drying ovens have a working temperature upper limit multiplied by 3%, and forced convection drying ovens have a working temperature upper limit multiplied by 2.5%. Only the electric vacuum drying oven does not have a temperature uniformity parameter, because the possibility of achieving uniform temperature in the working chamber through the movement of gas molecules in the vacuum drying oven is almost non-existent. Therefore, conceptually, we can no longer apply the temperature uniformity defined by the usual electric (blast) drying oven to the vacuum drying oven. Setting this indicator under vacuum conditions is also meaningless. The amount of thermal radiation is inversely proportional to the square of the distance. The radiation heat received by the same object at a distance of 20cm from the heating wall is only 1/4 of that at a distance of 10cm from the heating wall. The difference is significant. This phenomenon is similar to the fact that in winter, when exposed to the sun, the side that is exposed to the sun is very warm, while the side that is not exposed to the sun is relatively cold. Due to the difficulty in achieving uniform and consistent radiation heat at various points in the three-dimensional space of the working chamber in the structure of the vacuum drying oven, as well as the lack of evaluation methods, this may be the reason why the temperature uniformity parameter is not included in the standard for electric vacuum drying ovens.