skacur

Does anyone happen to know if there are any coatings that can be used to improve the gas permeability of Tygon? I've been looking into CVD(chemical vapor deposition), PVD(physical vapor deposition) and related technologies. There are plastic beverage bottle manufacturers that are using coatings, but I haven't found any contract manufacturers that do this work. Any ideas?

J.C. MacSwell: I looked into the water absorption of Tygon and it is about 0.2% after 24 hours. I haven't found what the saturation amount would be but this looks to typically be about 2X the 24 hour amount for most plastics. I think if the tubing were expanding, increasing the volume of the tube, this would explain the water slug climbing from the bottom since there would be the same amount of air in a larger tube. However, if the tube were to grow in length, the apparent depth would be greater and the pressure reading would increase. This is not the case, the pressure reading drops consistently over time.

Captain Panic: Thank you for your response. Your explanation makes perfect sense. I am struggling with one caveat however. We've submerged an inverted glass test tube to create a hermertically sealed chamber. This setup has shown no evidence of air escaping over several weeks (i.e. the water slug at the bottom is not climbing). I took this as evidence that we are experienceing a leak somewhere in our plastic setups. Let me know what you think.

Unfortunately, we can't rely on our customers removing/resubmerging the tube on a regular basis. Likewise with the purge method. I took a quick look at Henry's law. It looks like solubility is dependent on air pressure and liquid temperature. Does anybody know how the surface area of the air/water interface is accounted for?

Thank you for the responses. Regarding atmospheric pressure, there are fluctuations, but not a steady decrease (or increase). Also, we are using a differential pressure sensor which is referencing the atmospheric pressure, so the sensor output should not be affected by these atmospheric fluctuations. Likewise with temperature, there are small fluctuations (68°F to 75°F) but no steady increase or decrease. We've tried to pressurize the setup in a bath of water and have found no visible bubbles. We've done this to the 3 psi limit of the sensor. In operation, the sensor will see no more air pressure than 12" of water. We have actually tried a latex diaphragm/bladder, but have found that the response and accuracy are greatly diminished. Maybe there are better materials/geometries. Externet: The air dissolving in water is an interesting point. Do have any formulas to predict the extent of this? Thanks again.

we are experiencing what appears to be a slow air leak in the following setup: low pressure sensor connected to a tygon tube via 1/8" hose barb; the tygon tube is submerged in a liquid; the hydrostatic pressure of the liquid compresses the air in the tube; the sensor measures the air pressure; the linear relationship between hydrostatic pressure and liquid level enables liquid level determination problem: over the course of several weeks we've noticed a slow, steady drop in pressure while a constant liquid level is maintained. of note, the small "slug" of water at the bottom of the tube climbs slowly and steadily indicating an air leak. two questions: 1 - if this is an air leak, is it through the sensor, the hose barb seam or the tubing itself (permeation)? we have tested several variations of this with no conclusive answer 2 - if this is not an air leak, what is causing this phenomenon? thank you in advance