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- The control of the cooling rate of the mirror surface in dew point measurement is an important issue. For automatic photoelectric dew point meters, it is determined by design, and for dew point meters with manual control of cooling capacity, it is a problem in operation. Because there is a process of heat conduction between the cooling point, temperature measurement point and mirror surface of the cold source and there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of dew condensation (frost), and will bring errors to the measurement results. This situation varies with the temperature measuring element used. For example, due to the structural relationship of the dew point meter, the temperature gradient between the measuring point of the platinum resistance temperature sensing element and the mirror surface is relatively large, and the heat conduction speed is also relatively slow, which makes the temperature measurement and Condensation cannot be synchronized. Moreover, the thickness of the dew layer cannot be controlled. This will produce a negative error for visual inspection.
- Another problem is that cooling too fast may cause “over-cooling”. We know that under certain conditions, when the water vapor reaches saturation, the liquid phase still does not appear, or the water does not freeze when it is below zero. This phenomenon is called supersaturation or “supercooling.” For the condensation (or frost) process, this phenomenon is often caused by the measured gas and the mirror surface being very clean, and the dew point instrument or even lacking a sufficient number of condensation cores. In the experiment, it was found that if a highly polished mirror surface and its cleanliness meet the chemical requirements, the dew formation temperature is a few degrees lower than the real dew point temperature. The supercooling phenomenon is short-lived, and the total time is related to the dew point or frost point temperature. This phenomenon can be observed through a microscope. One of the solutions of the dew point meter is to repeat the operation of heating and cooling the mirror surface until this phenomenon is eliminated. Another solution is to directly use the water vapor pressure data of supercooled water. And this just coincides with the definition of relative humidity when the meteorological system is below zero.