What Should Pay Attention to When Using a Dew Point Meter?
A dew point meter is an instrument that can directly measure the dew point temperature. A mirror is cooled in the humid air of the sample until the moment when dew droplets (or ice crystals) appear on the mirror surface, and the average temperature of the mirror surface is measured, which is the dew point (frost) temperature. It has high humidity measurement accuracy, but requires a mirror surface with high smoothness, a temperature control system with high precision, and an optical detection system for dew droplets (ice crystals) with high sensitivity. During use, the pipe that sucks in the sample air must be kept clean, otherwise the impurities in the pipe will absorb or release moisture and cause measurement errors.
In the design of the dew point meter, various factors that directly affect the heat and mass exchange in the dew condensation process should be considered.
- The temperature of the measured gas is usually room temperature. Therefore, when the airflow passes through the dew point chamber, it will inevitably affect the heat and mass transfer process of the system. When other conditions are fixed, increasing the flow rate will be beneficial to the mass transfer between the airflow and the mirror. Especially in the low frost point measurement, the flow rate should be appropriately increased to speed up the formation of the dew layer, but the flow rate should not be too large, otherwise it will cause overheating problems. This is especially obvious for thermoelectric cooling dew point meters with relatively small cooling power. If the flow rate is too large, the pressure of the dew point chamber will also decrease, and the change of the flow rate will affect the thermal balance of the system. Therefore, it is necessary to select an appropriate flow rate in the dew point measurement, and the choice of the flow rate should depend on the cooling method and the structure of the dew point chamber. The general flow rate range is 0.4~0.7L·min. In order to reduce the influence of heat transfer, pre-cooling treatment can be considered before the measured gas enters the dew point chamber.
- In the dew point measurement, the control of the cooling speed of the mirror surface is an important issue. For the automatic photoelectric dew point meter, it is determined by the design, and for the dew point meter with manual control of the cooling capacity, it is a problem in operation. Because there is a process of heat conduction between the cooling point of the cold source, the temperature measurement point and the mirror surface, and there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of condensation, 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 relatively slow, which makes the temperature measurement temperature and dew condensation. Moreover, the thickness of the exposed layer cannot be controlled. This will create a negative error for visual inspection.
- Another problem is that cooling too fast may cause overcooling. We know that under certain conditions, when the water vapor reaches a saturated state, the liquid phase still does not appear, or the water still does not freeze when the temperature is below zero. This phenomenon is called supersaturation or supercooling. For dew condensation (or frost) processes, this phenomenon is often caused by the gas being measured and the mirror surface being very clean and even lacking a sufficient number of condensation cores. If a mirror is highly polished and chemically clean, the dew formation temperature will be several degrees lower than the true 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 with a microscope. One solution is to repeat the operation of heating and cooling the mirror until the phenomenon disappears. Another solution is to directly use the vapor pressure data of the supercooled water. And doing so is precisely in line with the definition of relative humidity when the weather system is below zero.