How Does Mirror Contamination Affect Dew Point Measurements?
What is 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) are looming on the mirror, and the average temperature of the mirror 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.
Gases with different moisture content will condense on the mirror surface at different temperatures. Using photoelectric detection technology, the exposed layer is detected and the temperature when dew condensation is measured, and the dew point is directly displayed. The methods of mirror cooling include semiconductor refrigeration, liquid nitrogen refrigeration and high-pressure air refrigeration. The mirror type dew point meter adopts the direct measurement method. Under the premise of ensuring accurate dew detection, high mirror cooling efficiency and precise measurement of condensation temperature, this kind of dew point meter can be used as a standard dew point meter.
Effect of Mirror Pollution on Dew Point Measurement
In dew point measurement, mirror contamination is a prominent problem, and its influence is mainly manifested in two aspects: one is the Raoult effect, and the other is to change the background radiation level of the mirror. The Raoult effect is caused by water-soluble substances. If the measured gas carries this substance (usually soluble salts), the mirror will condense in advance, which will cause a positive deviation in the measurement result. If the pollutants are water-insoluble particles, such as dust, etc., it will increase the scattering level of the background, thus causing the zero-point drift of the photoelectric dew point meter. In addition, the vapor of some easily condensable substances (such as organic substances) with a lower boiling point than water will obviously 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 industrial process gas analysis pollution is more serious. But even in pure gas measurements the contamination of the mirror surface will accumulate over time.
How to choose measurement conditions?
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. Here we mainly discuss the cooling speed of the mirror surface and the flow rate of the sample gas.
The temperature of the gas to be measured 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 dew point measurement, and the choice of flow rate should be determined by 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 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 the cooling capacity controlled by hand, 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 (frost), which will bring errors to the measurement results. This situation varies with the temperature measuring elements 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 relatively slow, which makes the temperature measurement temperature and dew condensation. cannot be done synchronously. Moreover, the thickness of the exposed layer cannot be controlled. This will create a negative error for visual inspection.
Another problem is that cooling down too quickly can cause "overcooling". 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 it 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. Suomi has found in experiments that if a mirror is highly polished and chemically clean, the dew formation temperature is 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 supercooled water. And doing so is precisely in line with the definition of relative humidity when the weather system is below zero.