1. The reason why the vacuum constant temperature drying oven is evacuated first and then heated up
1) If you operate according to the procedure of first heating and then vacuuming, when the heated air is drawn by the vacuum pump, the heat will inevitably be brought to the vacuum pump, which will cause the temperature of the vacuum pump to rise too high, which may reduce the efficiency of the vacuum pump.
2) The heated gas is directed to the vacuum pressure gauge, which will generate a temperature rise. If the temperature rise exceeds the operating temperature range specified by the vacuum pressure gauge, it may cause the value error of the vacuum pressure gauge.
The correct method of use should be evacuated before heating up. After the rated temperature is reached, if it is found that the degree of vacuum has dropped, then add more pumping. This is beneficial to prolong the service life of the equipment.
3) The work piece is placed in a vacuum box to evacuate to remove the gas component that can be removed from the material of the work piece. If the workpiece is heated first, the gas will expand when heated. Due to the very good tightness of the vacuum box, the huge pressure generated by the expanding gas may burst the observation window tempered glass. This is a potential danger. You can avoid this danger by following the procedure of evacuating first and then heating up.
2. The cause of the difference between the reading of the vacuum constant temperature drying oven and the reading of the glass rod thermometer in the vacuum chamber
The general vacuum constant temperature drying oven adopts the method of heating the wall surface of the vacuum chamber first, and then performing radiant heating from the wall surface to the workpiece. In this way, the temperature sensor of the temperature control instrument can be arranged on the outside wall of the vacuum chamber. The sensor can simultaneously receive convection, conduction, and radiant heat. The glass rod thermometer in the vacuum chamber can only receive radiant heat, and because the blackness of the glass rod cannot reach 1, a considerable part of the radiant heat is refracted, so the temperature value reflected by the glass rod thermometer is definitely lower than the temperature reading of the instrument . Generally speaking, the difference between the temperature reading of the meter and the reading of the glass rod thermometer within 200 ° C within 30 ° C is normal. 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 is impossible to eliminate, and the sealing reliability of the vacuum chamber adds a possibly unreliable link . If you do not want to see this difference from a practical perspective, you can use the unique display correction function of the temperature control instrument to solve it.
3. The reason why the electric heating vacuum drying oven does not set the temperature uniformity parameter
General electric heating (blast) drying ovens are equipped with temperature uniformity parameters: natural convection drying ovens have an upper limit of working temperature multiplied by 3%, and forced convection drying ovens have an upper limit of working temperature multiplied by 2.5%. However, there is no temperature uniformity parameter for the electric vacuum drying oven. Why is this? There is almost no possibility that the temperature of the working chamber will be uniform by relying on the movement of gas molecules in the vacuum drying oven. Therefore, conceptually, we can no longer apply the definition of temperature uniformity prescribed by the usual electric heating (blast) drying oven to the vacuum drying oven. It is meaningless to set this index under vacuum. The amount of heat radiation is inversely proportional to the square of the distance. For the same object, the radiant heat received at 20 cm from the heating wall is only 1/4 of that at 10 cm from the heating wall. big difference. This phenomenon is the same as when the sun is shining in the winter, the sun is very warm on the side, and the cold side is cold. Due to the structural difficulty of the vacuum drying oven, it is difficult to make the radiant heat uniform at all points in the three-dimensional space of the studio, and it also lacks an authoritative evaluation method. This may be because the temperature uniformity parameter is not set in the standard of the electric heating vacuum drying oven. s reason.
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