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Whitewater rafts that are subject to the sun while off the water, and cold in the water, are subject to significant changes in pressure. In the water this can mean reduced performance due to lower than optimal pressure, while on the bank it can mean damage due to overpressure. My question is: Would one be better served if the raft were filled with dry air (say from a compressed breathing air source) or would there be no significant difference?

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While in the vapor phase, the water will act pretty much like the rest of the air. What is different, or course, is if the temperature drops so that some of the vapor condenses because it now exceeds the saturation value (from, say, steam tables).

So, how bad can it be? Well, Googling around for a quick saturation table I chose The Engineering Toolbox. There, one can see that if you filled the rafts with fully saturated air (i.e. 100% relative humidity) at about 100F (about 40C), about 1 psi of the total would be water, or a little over 6% by volume. Taking the raft down to freezing means that almost all of the water vapor condenses out (saturated pressure is 0.09 psi), so you would have the pressure in the raft drop to 14/15's (0.933) of what it was to begin with. I specifically chose a pretty extreme variation in temperature here. What you actually see will most likely be less. I might suggest that small leaks will be far more important than condensation of water vapor with respect to inflation variation.

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