I awoke in the night the other month with my mat feeling distinctly deflated. Having spent more time than I'd rather stomping on it in the bath and looking for bubbles, I have singularly failed to find a puncture.

From a bit of internet research, I understand that mats can go soft due to air cooling inside them (which would be fair enough, as it was pretty close to freezing when I was out). That said, I've inflated it at home, kept it in a warm room, and it's still lost pressure and gone floppy over a few days.

Anyone noticed how long it takes the average mat to lose pressure?
(Exped Downmat HL if that makes any difference.)

Sorry not to answer the Q, Bear, because I don't usually have, er, problems going soft, so I'm told anyway, but just gerrit posted off to them and they'll find the leak and do a nice fix FOC....

My physics isn't up to calculating the exact pressure loss per volume per degC.

"the air in the mattress tends to condense".

"Thermarest article says it's because the warm air from your breath cools and thus decreases in pressure" (eh?).

"Regardless, something to consider is that all inflatable air mattresses stretch. So for like the first 10 times you use it, the material is still stretching out. The air isn't actually leaking, the volume is just getting bigger" (eh?)

"The ideal gas law says that PV = nRT, so as pressure, amount of gas (n) and the ideal gas constant remain constant, if temperature goes down, so will volume. Be careful though, the temps for this equation are in Kelvin. If you work it out, if you start at 20 deg C (293 K) with 1 mol of gas the gas will have 24.04 L volume. Dropping the temp to 0 deg C (273 K), it gives a gas with 22.40 L volume, a loss of over a liter and a half or about 7%. Any more loss than this percent wise and you've almost certainly got a leak". A 20C decrease only causes 7% loss - very unlikely a 20deg temperature drop after inflation?

I've just read a number of different explanations on tinternet so I'm remaining to be convinced about the exact physics. And if the air temperature inside the mat has lowered, then the external temperature must be the same or even lower so there's something not quite adding up somewhere.

Wonder how big an effect external barometric pressure has?

And lung air will be warmer than pump air but to what effect? "exhaled breath temperature (EBT) for normal participants is 34.45 °C on average". You breathing or pumping, Chris? But surely breathed air cools rapidly in the mat so topping up after a short while removes this issue?

All very interesting . Nah, you've sprung a leak .

I've found that Exped mats can leak very slightly through the valve. I recall Hux pointed it out to me and suggested smearing the valve plug with spit before inserting it (stop it at the back) and hey presto, it seems to work.

Might try that. Over 8 hours the other night the mat needed reinflating 3 times. Patched a small hole before but either it didn't work or there's another. Grr.

I fancy injecting a good old-fashioned BBB digression in here. I reckon the "lower temperature causes mat to deflate" story is similar to the "hot drink warms you up for bed" story. I'm not convinced by either.

"Adding a hot liquid to your body does increase your body temperature, but the warming effect on your body is minimal. Humans are 70 percent water. A 170-pound person contains about 15 gallons of water. Imagine adding eight ounces of warm water to 15 gallons of warm water—the temperature won't really change that much".

Reckon psychological effects have a lot to do with all this stuff.

I had this issue Bearlegged. Thermarest Neoair roughly 2016 model. A few tiny punctures here n there which had been sealed with some puncture repair patches.

Last year overnight with pressure on it, felt as though it was losing pressure. Like throughout the night about 10% or less would come out. Spoke to TR who said to send it back but I never did. Kept sleeping on it and suddently it's stopped doing that. Stays inflated now.

Maybe it was the cooler air or something around the patches that needed adhering. I'm as baffle-d as you. But haven't sent it back and the initial response which looked positive means I'll be buying another Neoair when a deal comes calling.

I've found that Exped mats can leak very slightly through the valve. I recall Hux pointed it out to me and suggested smearing the valve plug with spit before inserting it (stop it at the back) and hey presto, it seems to work.

There's your answer right there Bear... Check the shop to see if Stu has some spit sachets remaining

redefined_cycles wrote: ↑Tue Mar 12, 2024 10:52 pm I had this issue Bearlegged. Thermarest Neoair roughly 2016 model. A few tiny punctures here n there which had been sealed with some puncture repair patches.

Last year overnight with pressure on it, felt as though it was losing pressure. Like throughout the night about 10% or less would come out. Spoke to TR who said to send it back but I never did. Kept sleeping on it and suddently it's stopped doing that. Stays inflated now.

Maybe it was the cooler air or something around the patches that needed adhering. I'm as baffle-d as you. But haven't sent it back and the initial response which looked positive means I'll be buying another Neoair when a deal comes calling.

Oh... to add to that, it's using the new Flextail pump and not breath-air that this newfound phenomena has been reached. May my breath wasn't dense enough

redefined_cycles wrote: ↑Tue Mar 12, 2024 10:54 pm There's your answer right there Bear... Check the shop to see if Stu has some spit sachets remaining

Looks to be all out of regular spit, but I'm sure he could knock up a pack of lightweight special?

Yep smear a bit of spit, shove it in and twist it a bit. no more leaks!