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Several issues to note in dew point measurement (I) The impact of mirror contamination on dew point measurement In dew point measurement, mirror contamination is a significant problem, and its effects are manifested in two main ways: one is the Raoult effect, and the other is the alteration of the background radiation level of the mirror. The Raoult effect is caused by water-soluble substances. If the gas being tested contains such substances (usually soluble salts), condensation occurs on the mirror surface ahead of time, causing a positive deviation in the measurement results. If the pollutants are water-insoluble particles such as dust, they will increase the background scattering level, thereby causing a zero drift in the photometric dew point meter. Furthermore, the vapors of some substances with a boiling point lower than that of water (such as organic compounds) that are prone to condensation will, needless to say, interfere with the measurement of the dew point. Therefore, any type of dew point meter should prevent contamination of the mirror surface. Generally speaking, the impact of pollution from industrial process gas analysis is quite severe. But even in the measurement of pure gases, mirror contamination accumulates over time. (II) Selection of measurement conditions for dew point meters—In the design of dew point meters, it is necessary to take into account various factors that directly affect the heat and mass exchange during the dew formation process; this principle also applies to the selection of operating conditions for dew point meters with a lower degree of automation. Here, the main topics discussed are the mirror cooling rate and the sample gas flow rate. ―The temperature of the gas being measured is usually room temperature. Therefore, when air flow passes through the dew point chamber, it inevitably affects the heat and mass transfer processes in the system. With other conditions held constant, increasing the flow rate facilitates mass transfer between the airflow and the mirror surface. Especially when performing low frost point measurements, the flow rate should be increased appropriately to speed up the formation of the dew layer; however, the flow rate must not be too high, as this could lead to overheating problems. This is particularly evident in thermoelectric dew point meters with relatively low cooling power. Too high a flow rate can also cause the pressure in the dew point chamber to decrease, and this change in flow rate in turn affects the thermal equilibrium of the system. Therefore, it is necessary to select an appropriate flow rate in dew point measurement; the choice of flow rate should depend on the refrigeration method and the structure of the dew point chamber. The typical flow rate range is between 0.4 and 0.7 L.min-1. To reduce the impact of heat transfer, pre-cooling the gas to be measured before it enters the dew point chamber can be considered. In dew point measurement, controlling the cooling rate of the mirror is an important issue; for automatic photoelectric dew point meters, this is determined by the design, whereas for dew point meters with manual cooling control, it is a matter related to operation. This is because there is a process involved in the heat conduction between the cooling point of the cold source, the temperature measurement point, and the mirror surface, as well as a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of dew formation (frost), introducing errors into the measurement results. This situation varies depending on the temperature sensing element used; for example, due to structural reasons, the temperature gradient between the measurement point of the platinum resistance temperature sensor and the mirror surface is relatively large, and the heat conduction rate is slow as well, which prevents simultaneous temperature measurement and dew detection. And it results in the thickness of the exposed layer being uncontrollable. This will result in a negative error for visual inspection. Another issue is that too rapid a cooling rate can cause `supercooling`. We know that under certain conditions, when water vapor reaches a saturated state, no liquid phase appears, or water does not freeze even at temperatures below zero; this phenomenon is known as supersaturation or supercooling. In the case of dew (or frost) formation, this phenomenon is often caused by the fact that the gas being measured and the mirror surface are extremely clean, resulting in a lack of sufficient condensation nuclei. In experiments, Suomi found that if a surface is highly polished and its cleanliness meets chemical requirements, the temperature at which dew forms is several degrees lower than the actual dew point temperature. ―The supercooling phenomenon is brief, and its duration is related to the dew point or frost point temperature. This phenomenon can be observed under a microscope. One solution is to repeat the process of heating and cooling the mirror until this phenomenon is eliminated. Another solution is to directly use the vapor pressure data of supercooled water. And doing so precisely corresponds to the definition of relative humidity in meteorological systems when the temperature is below zero degrees. ―As can be seen from the above, whether considering thermal inertia or the phenomenon of supercooling, the cooling rate should not be too fast. If it exceeds a reasonable range, the faster the cooling rate, the greater the thermal inertia, which in turn leads to larger errors in dew point measurement and an increased likelihood of supercooling. The optimal cooling rate is generally determined through experiments.