indew point meterIn the design of dew point instruments, it is important to consider various factors that directly affect the heat and mass exchange during the condensation process. This principle also applies to the selection of operating conditions for dew point instruments with low automation levels. This mainly discusses the issues of mirror cooling rate and sample gas flow rate.

The temperature of the gas being tested is usually room temperature. Therefore, when the airflow passes through the dew point chamber, it inevitably affects the heat and mass transfer processes of the system. When other conditions are fixed, increasing the flow rate will facilitate mass transfer between the airflow and the mirror surface. Especially when measuring low frost points, the flow rate should be appropriately increased to accelerate the formation of the dew layer, but the flow rate should not be too high, otherwise it will cause overheating problems. This is particularly evident for thermoelectric dew point meters with relatively low cooling power. Excessive flow rate can also lead to a decrease in dew point chamber pressure, and changes in flow rate will affect the thermal equilibrium of the system. Therefore, it is necessary to choose an appropriate flow rate in dew point measurement, and the choice of flow rate should depend on the refrigeration method and the structure of the dew point chamber. The general flow rate range is between 0.4 and 0.7 L · min-1. To reduce the impact of heat transfer, pre cooling treatment can be considered before the measured gas enters the dew point chamber.

2. The control of mirror cooling rate is an important issue in dew point measurement. For automatic photoelectric dew point meters, it is determined by design, while for manually controlled dew point meters, 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 exists a certain temperature gradient. So thermal inertia will affect the process and speed of condensation (frost), causing errors in the measurement results. This situation also varies depending on the temperature measuring element used. For example, due to structural relationships, 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 velocity is also relatively slow, which makes it difficult to synchronize temperature measurement and condensation. And it also leads to uncontrollable thickness of the exposed layer. This will result in negative errors for visual inspection.

Another issue is that too fast a cooling rate may cause "overcooling". We know that under certain conditions, when water vapor reaches saturation, the liquid phase still does not appear, or water does not freeze below zero degrees. This phenomenon is called supersaturation or "supercooling". For the condensation (or frost) process, this phenomenon is often caused by the measured gas and mirror surface being very clean, or even lacking a sufficient number of condensation cores. Suomi found in her experiment that if a highly polished mirror surface meets chemical requirements for cleanliness, the dew formation temperature is several degrees lower than the actual dew point temperature. The phenomenon of supercooling is short-lived, and the duration is related to the dew point or frost point temperature. This phenomenon can be observed through a microscope. One solution is to repeat the process of heating and cooling the mirror surface until this phenomenon is eliminated. Another solution is to directly utilize the vapor pressure data of supercooled water. And this is precisely in line with the definition of relative humidity when the meteorological system is below zero degrees.