Thread topic is interesting to me even though I don't own a PCP gun. I do know from having lived at 7450' and frequently driving both below and above that by several thousands of feet elevation that flexible bottles, aluminum cans, plastic bags, and closed sea kayak hatches visibly showed large differences in expansion and contraction. The only time that didn't happen was if there was a hole--even a tiny one--that equalized the atmospheric pressure inside. A rigid water bottle such as Nalgene with tightly closed lid didn't show any change.
To avoid damaging fiberglass kayaks, I learned to remove the hatch covers before driving anywhere since it was last closed. Differences in pressure that resulted from sudden *temperature* change were obvious any time that I put a kayak with closed hatch covers into much colder water at the same elevation. If it was bulging slightly from the very hot sun, it would immediately flatten. BTW, that difference was minor compared to what happened when finishing an outing on a 10,000' elev lake, forgetting to remove the hatch covers, and then stopping a couple hours later at 6500' to buy gas. The rear deck actually got sucked inward the same way a water bottle would. (The front compartment's bulkhead had had a tiny hole drilled in it, which all the bulkheads should have had.) Fortunately, the fiberglass and gelcoat did have some give and were robust enough to withstand my slowwwwwly lifting part of the rubber hatch covers to allow shape recovery.
I now live at 6440' but don't routinely drive such big elevation changes anymore. It's funny how I heard more people in Denver (5280') and surrounding areas use the word flatlanders than I do in this SW corner of the state, even though the elevation is higher here.
To avoid damaging fiberglass kayaks, I learned to remove the hatch covers before driving anywhere since it was last closed. Differences in pressure that resulted from sudden *temperature* change were obvious any time that I put a kayak with closed hatch covers into much colder water at the same elevation. If it was bulging slightly from the very hot sun, it would immediately flatten. BTW, that difference was minor compared to what happened when finishing an outing on a 10,000' elev lake, forgetting to remove the hatch covers, and then stopping a couple hours later at 6500' to buy gas. The rear deck actually got sucked inward the same way a water bottle would. (The front compartment's bulkhead had had a tiny hole drilled in it, which all the bulkheads should have had.) Fortunately, the fiberglass and gelcoat did have some give and were robust enough to withstand my slowwwwwly lifting part of the rubber hatch covers to allow shape recovery.
I now live at 6440' but don't routinely drive such big elevation changes anymore. It's funny how I heard more people in Denver (5280') and surrounding areas use the word flatlanders than I do in this SW corner of the state, even though the elevation is higher here.
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