1. Reasons for the difference between the instrument readings and the glass rod thermometer readings in the vacuum drying oven:
Typical electric vacuum drying ovens use a method of first heating the vacuum chamber walls, then radiating heat to the workpiece from the walls. In this method, the temperature sensor of the temperature controller can be placed on the outer wall of the vacuum chamber. The sensor can simultaneously receive convection, conduction, and radiant heat. However, the glass rod thermometer inside the vacuum chamber can only receive radiant heat. Furthermore, because the emissivity of the glass rod cannot reach 1, a significant portion of the radiant heat is refracted. Therefore, the temperature value reflected by the glass rod thermometer will definitely be lower than the instrument reading. Generally speaking, a difference of less than 30°C between the instrument reading and the glass rod thermometer reading is normal at 200°C. If the temperature sensor of the temperature controller is placed inside the vacuum chamber, the difference between the glass rod thermometer reading and the instrument reading can be reduced to some extent, but it cannot be eliminated. This adds a potentially unreliable element to the sealing reliability of the vacuum chamber. From an operational perspective, if this difference is undesirable, the display correction function specific to the temperature controller can be used to resolve it.
2. Reasons for Vacuuming Before Heating in a Vacuum Drying Oven:
1) Vacuuming the workpiece in the vacuum chamber removes removable gaseous components from the workpiece material. If the workpiece is heated first, the gas will expand. Due to the excellent sealing of the vacuum chamber, the enormous pressure generated by the expanding gas could potentially shatter the tempered glass observation window. This is a potential hazard. Operating according to the procedure of vacuuming before heating avoids this danger.
2) If the procedure is to heat before vacuuming, the heat from the heated air being drawn out by the vacuum pump will inevitably be carried to the pump, causing excessive temperature rise and potentially reducing its efficiency.
3) The heated gas is directed to the vacuum pressure gauge, causing it to heat up. If the temperature rise exceeds the vacuum pressure gauge's specified operating temperature range, it may cause reading errors.
The correct method is to vacuum first and then heat. If the vacuum level drops after reaching the rated temperature, evacuate again as needed. This practice is beneficial for extending the equipment's lifespan.
3. Reasons for Not Setting Temperature Uniformity Parameters in Vacuum Drying Ovens
Generally, electric heating (blown air) drying ovens have temperature uniformity parameters: 3% of the upper limit of the working temperature for natural convection drying ovens, and 2.5% for forced convection drying ovens. However, electric vacuum drying ovens do not have temperature uniformity parameters because the possibility of achieving uniform temperature within the working chamber through gas molecule movement is almost nonexistent. Therefore, conceptually, we cannot apply the temperature uniformity definition specified for ordinary electric heating (blown air) drying ovens to vacuum drying ovens. Setting this parameter under vacuum conditions is meaningless. The amount of thermal radiation is inversely proportional to the square of the distance. For the same object, the radiant heat received at a distance of 20cm from the heating wall is only 1/4 of that received at a distance of 10cm. The difference is significant. This phenomenon is similar to sunbathing in winter, where the side exposed to the sun is warmer and the side not exposed to the sun is colder. Because it is structurally difficult to ensure uniform radiant heat distribution at all points within the three-dimensional space of a vacuum drying oven, and because there is a lack of authoritative evaluation methods, this may be the reason why temperature uniformity parameters are not included in the standards for electrically heated vacuum drying ovens.