Typically, we rely on the readings from a glass rod thermometer to determine the air temperature in an ambient environment. However, when placed inside a vacuum chamber, the thermometer still shows a reading. Can we then assume this reading reflects the actual temperature of the vacuum chamber? The answer is no. In a vacuum, there is no air, and therefore, no concept of "air temperature" exists. What the thermometer actually measures is the temperature caused by absorbed heat radiation. This process is influenced by the surface properties of the thermometer itself—such as its roughness and its ability to absorb, reflect, or transmit infrared radiation. In infrared engineering, this property is referred to as "emissivity." The higher the emissivity (closer to 1), the more radiant heat the object absorbs, leading to a higher measured temperature. Conversely, lower emissivity means less absorption and a lower reading. Therefore, the temperature shown on the thermometer only represents the temperature of the thermometer itself after absorbing infrared radiation, not the actual temperature of the objects inside the chamber, which may have different surface materials and emissivity values.
As a result, when using an electric vacuum drying oven under vacuum conditions, it's important to be cautious about interpreting the glass rod thermometer's readings. These readings should not be taken as an accurate reflection of the true temperature of the items being dried. Instead, users should rely on their experience, observe how different materials respond under vacuum, and adjust their procedures accordingly. By gaining hands-on knowledge over time, operators can optimize the drying process and achieve better results with greater confidence.
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