Where does the moisture come from?
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When using a portable dew point analyzer to test carbon dioxide tank cars, excessive moisture levels are detected when rubber hoses are used as the gas supply tubes, while testing with metal corrugated hoses yields satisfactory results; it’s really puzzling! Can water penetrate? Even if there is penetration, the pressure inside the pipe is high – can water get in?
Correct! The moisture comes from pipeline infiltration. All materials have a certain solubility for water; coupled with their ability to migrate and permeate, as well as the negative pressure created by the drying gas’s demand for water, supercooled gases develop a dew point during measurement. Depending on the material and its molecular structure, different dew point values can be observed for the same gas source.
This is caused by penetration. The driving forces for penetration include two aspects: pressure and concentration. When rubber tubes are used, despite the high pressure inside the tube, moisture from the atmosphere or the environment still penetrates from areas of high concentration to those of low concentration, as rubber tubes are much less dense than metal tubes. Especially when the water content on one side is very low, this osmosis is quite noticeable. The factors that determine equilibrium include not only pressure, but also chemical potential (concentration or partial pressure), which are important factors affecting equilibrium as well.
This is the perspective of ordinary process engineers. Low pressure can definitely penetrate high pressure, which is caused by the partial pressures between molecules. Water-impermeable pipes still allow water molecules to pass through, and this is due to their molecular structure. Below the molecule, there are things like electron migration as well. Permeability varies depending on the material, temperature, and pressure.
Substances always permeate from areas of low concentration to those of high concentration. All materials have a certain degree of solubility; it is normal for leakage to increase. It is essential to conduct sampling in a sealed environment, and the lower the permeation rate of the sampling tubing, the better
You can look up Dalton’s law of partial pressures; what’s important here is the vapor pressure of water inside and outside the tube. Although the pressure inside the tube is high, the gas is very dry with a low water vapor partial pressure; whereas in air the water vapor partial pressure is high, allowing water to penetrate in. For measuring dew point, it is best to use stainless steel or PTFE tubes.
Was the problem found? Is it possible that there was already moisture at the interface? The first time a rubber tube was used to measure moisture, the level was found to be high; when a metal tube was used instead, the moisture had already been displaced by carbon dioxide, so no moisture could be detected. I’m not clear about your testing process