Dew point dew point measurement conditions

Selection of dew point meter dew point measurement conditions: In the design of the dew point meter, it is important to consider various factors that directly affect the heat exchange in the dew condensation process. This principle also applies to the selection of dew point meter operating conditions that are not very automatic. Here mainly discuss the mirror cooling speed and sample gas flow problems.

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 airflow and the mirror. Especially when measuring the low frost point, the flow rate should be appropriately increased to speed up the formation of the exposed layer, but the flow rate should not be too large, otherwise it will cause overheating problems. This is especially true for thermoelectric refrigeration dewpoint meters with relatively low cooling power. Too much flow rate will also result in dew point chamber pressure drop and the change of flow rate will affect the thermal balance of the system. Therefore, it is necessary to select the appropriate flow rate in the dew point measurement. The choice of flow rate depends on the cooling method and the structure of the dew point chamber. The general flow rate range is 0.4~0.7L. Between min-1. In order to reduce the effect of heat transfer, pre-cooling may be considered before the measured gas enters the dew point chamber.

2. The control of the mirror cooling rate in the dew point measurement is an important issue. The dew point meter for the automatic dew point meter is determined by the design, and the dew point meter for hand control of the cooling amount is an operation problem. Because there is a process of heat conduction between the cooling point, temperature measurement point and mirror surface of the cold source, there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of condensation (frost) and introduce errors into the measurement results. This situation differs depending on the temperature measuring element used. For example, due to the structural relationship, the temperature gradient between the measuring point and the mirror surface of the platinum resistance temperature sensing element is relatively large, and the heat conduction speed is also relatively slow, so that the temperature measurement and the condensation occur. Cannot be synchronized. Moreover, the thickness of the exposed layer cannot be controlled. This will produce negative errors for visual inspection.

3. Another problem is that cooling too fast may cause "too cold." We know that under certain conditions, when water vapor reaches saturation, the liquid phase still does not appear, or water still freezes below freezing, this phenomenon is called supersaturation or “too cold”. For the condensation (or frost) process, this phenomenon is often caused by the fact that the gas to be measured and the mirror surface are very clean, or even lack a sufficient number of condensation cores. Suomi found in experiments that if a highly polished mirror is clean and chemically satisfactory, the formation temperature of the dew is a few degrees lower than the true dew point temperature. The phenomenon of overcooling is short-lived, and the total length of time is related to the dew point or frost point temperature. This phenomenon can be observed through a microscope. One of the solutions is to repeat the heating and cooling of the mirror until the phenomenon is eliminated. Another solution is to directly use the vapor pressure data of supercooled water. And this coincides with the definition of relative humidity when the meteorological system is below zero.

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