In the design of the dew point meter, it is necessary to consider various factors that directly affect the heat and mass exchange in the condensation process. This principle also applies to the selection of operating conditions of dew point instruments that are not too automated. Here we mainly discuss the problem of mirror cooling rate and sample gas flow rate.
1. The temperature of the gas to be measured is usually room temperature. Therefore, when the airflow passes through the dew point chamber, it must affect the heat transfer and mass transfer process of the system. When other conditions are fixed, increasing the flow rate will facilitate mass transfer between the gas stream and the mirror. Especially when measuring low frost point, the flow rate should be increased appropriately to speed up the formation of the exposed layer, but the flow rate should not be too large, otherwise it will cause overheating. This is especially true for thermoelectric refrigeration dewometers with relatively low cooling power. Too much flow rate will also cause the dew point chamber pressure to decrease and the flow rate change will affect the heat balance of the system. Therefore, it is necessary to select an appropriate flow rate in the dew point measurement. The flow rate should be selected depending on the cooling method and the structure of the dew point chamber. The general flow rate ranges from 0.4 to 0.7L. Between min-1. In order to reduce the influence of heat transfer, it is considered to perform pre-cooling treatment before the gas to be measured enters the dew point chamber.
2. The control of the mirror cooling rate in dew point measurement is an important issue. It is determined by the design of the automatic photoelectric dew point meter, and the operation of the dew point meter for hand control of the cooling amount is an operation problem. Because the heat transfer between the cooling point, the temperature measuring point and the mirror surface of the cold source has a process and a certain temperature gradient exists. Therefore, thermal inertia will affect the process and speed of condensation (frost), which will bring errors to the measurement results. This situation varies with the temperature measuring element used. For example, due to the structural relationship, 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, thereby making temperature measurement and condensation. Cannot be synchronized. Moreover, the thickness of the exposed layer is uncontrollable. This will produce a negative error for visual inspection.
3. Another problem is that cooling too fast can cause "too cold." 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 the water is below zero. This phenomenon is called supersaturation or "supercooling". For the condensation (or frost) process, this phenomenon is often caused by the very clean gas and mirror surface, or the lack of a sufficient number of condensation cores. Suomi found in the experiment that if a highly polished mirror is clean and chemically compatible, the temperature of the dew formation is a few degrees lower than the true dew point temperature. The phenomenon of supercooling is short-lived, and the length of time is related to the dew point or frost temperature. This phenomenon can be observed through a microscope. One solution is to repeat the operation of heating and cooling the mirror until this phenomenon is eliminated. Another solution is to directly use the vapor pressure data of the cold water. And this is exactly the same as the definition of relative humidity when the meteorological system is below zero.
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