[huron]

Exped Synmat UL deflates in 20 minutes while attempting to sleep at 7,500 feet on snow. When back home, a bathtub test and valve inspection show no leaks or damage. Inflated and left some heavy boxes on top. No deflation overnight. Figured I had some sand or something in the valve while on first trip. Next trip, same thing. Now at home, have had two heavy boxes on this pad for 48 hours and no leaks. Could the valves be shrinking in the cold or something?

[Voxxjin]

To test your theory, try placing some ice packs on the valve.

[DarkHelmet]

Frost/ice build up on the valve?

[Ranger Smith]

cold air is less dense.

[Tomlike]

cold air compresses, so if you blow it up with warm air from your lungs, as it cools at altitude the pad deflates. if you re-inflate a second time, does it stay that way through the night?

[dennyt]

[Conrad]

I'm pretty sure cold air is more dense, by the gas temperature/volume principle plus the fact that cold air sinks. When it deflates on snow, does it deflate totally and repeatedly, i.e. you pump in some more air and it all leaks out again in 20 minutes? I imagine you're saying it does, but just checking. If it's emptying at the valve in :20, that's fast enough that you should be able to see bubbling by spreading spit on the valve. But you'd have to take it camping again to test again. Perhaps 2 boxes != a human, e.g. different shape to the weight, although I admit that's a long shot. But you could lay on it for 20 minutes to be sure.

[DIYSteve]

I suppose in theory the the male valve plug might contract in the cold. We had a leaky Exped UL7 valve that would not fully seal, leaked air very slowly, took a few hours to deflate. That pad also had a twist in it. REI took it back but advised that next time I should deal directly with Exped. Have you carefully cleaned both valves? Tomlike's explanation is sound and verified by experience. I surmise that RS intended to say "more dense." Conrad asks the right questions.

[texasbb]

Do you hike with practical jokers?

[DIYSteve]

One more idea: Do you use a Schnozzel to pump up your pad? If not, maybe water vapor from your breath is freezing and fouling your valve. I give a hardy thumbs up to the Schnozzel

[iron]

our synmats would deflate too. enough to warrant reblowing up. no such problems with the much better neoair xtherm. it's a game changer for warmth!

[huron]

Thanks for all the good theories. Dennyt, your guess on a funky flapper is a good one. I just checked it and it was a bit wrinkled. If that got caught in the valve, it could explain the problem. By the way, I asked Exped for help and they got back to me in a hurry with a response and offer to help. That's a company I'd buy from again.

[Malachai Constant]

We have noticed that our neo airs lose volume when they cool. I could not say they deflate as the loss is not total like a leak and stops when the air inside reaches equilibrium. This is an example of Charles Law which holds that the volume of a perfect gas decided by the temperature is a constant I.e. V/T=k. Note the T must be in degrees Kelvin the most I get is if you fill the pad with hot air of 40 C = 313 K then cools to freezing 273 K would decrease the volume about 87% of original volume. Note the pressure is constant so can be ignored in an inflatable pad. This complies,with experience. In practice the loss is smaller as we normally use an inflator bag.

[christensent]

I've definitely been sleeping on the ground from the gas cooling down in my neoair xtherm in extreme conditions. I'd bet it's that, especially if you're sleeping on snow! Get in the habit of filling it, setting it on the snow for 5 minutes, then filling it again. Never had it not stay super firm all night once I started doing that. I imagine it might take longer if there's no snow to set it on.

[huron]

I'm talking fill, deflate, fill, deflate, fill, deflate, give up and put my pack under me.