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Dew point meter measurement methods 1. Weight method: It is a classic measurement method. Pass the sample gas under test through a desiccant; the moisture contained in it is absorbed by the desiccant. By accurately measuring the amount of moisture absorbed by the desiccant and dividing it by the volume of the sample gas, the humidity of the sample gas can be determined. The advantage of this method is its high precision, with a maximum allowable error of 0.1%; the disadvantage is that the actual operation is rather difficult, especially since a sufficient amount of water absorbed is required (usually not less than 0.6 grams), which is particularly challenging for gases with low humidity. This forces an increase in the flow rate of the sample gas, resulting in longer measurement times and greater errors (the measured humidity is not an instantaneous value). Therefore, this method is only suitable for measuring gases with a dew point above -32°C; it can be said that there are few instruments on the market that use this method solely for humidity measurement. 2. Electrolysis method: This involves using an electrolytic cell to split the water absorbed by the desiccant into hydrogen and oxygen, which are then expelled. The magnitude of the electrolytic current is proportional to the water content; by measuring this current, it is possible to determine the humidity of the sample gas. This method overcomes the shortcomings of the gravimetric method: it can measure temperatures below -80°C, offers good precision, and is inexpensive. The disadvantages are that the gas path in the electrolytic cell needs to be dried for a long time before use, and high requirements are placed on the corrosiveness and purity of the gases. The vibration frequency method involves replacing the desiccant used in the weight method with a hygroscopic quartz crystal. Utilizing the fact that this crystal exhibits different vibration frequencies depending on the amount of moisture it absorbs, the sample gas and a standard dry gas stream are passed through the crystal, resulting in different vibration frequency differences of △f1 and △f2. By calculating the difference between these two frequencies, the humidity of the sample gas can be determined. This method has the same advantages as the electrolytic method, and does not require drying before use. 3. The cold mirror method is also a classic measurement technique. The sample gas is passed through the condensing mirror in the dew point cold chamber; through isobaric cooling, the sample gas is brought to a saturated condensation state (liquid droplets form on the condensing mirror). The temperature of the condensing mirror at this point is the dew point of the sample gas. The main advantage of this method is its high precision; with the use of semiconductor cooling and photodetection techniques, the uncertainty can even reach 0.1℃. The disadvantages are its slow response speed, especially at dew points below -60℃, where the equilibrium time can take several hours. Additionally, this method requires high purity and low corrosivity of the sample gas, as otherwise it may affect the performance of the photodetection or cause \"pseudo-dew\" leading to measurement errors. 4. The resistance-capacitance method is a humidity measurement technique that is constantly being improved. A high-purity aluminum rod is used; its surface is oxidized to form an ultra-thin layer of alumina, which is then coated with a porous network-like gold film. A capacitor is formed between the gold film and the aluminum rod. Due to the water-absorbing properties of the alumina layer, the capacitance value changes depending on the amount of moisture in the sample gas. By measuring this capacitance value, it is possible to determine the humidity of the sample gas. The main advantage of this method is that its measurement range can be lower, even down to -100°C. Another notable advantage is its very fast response time – the response from dry to wet conditions can reach 90% within one minute – which makes it suitable for use in field and rapid measurement applications. The downside is its relatively poor accuracy, with an uncertainty of typically ±2~3°C. Severe aging and drift occur, requiring calibration after 3 to 6 months of use.