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Sleeping Gear The Sleeping Gear forum is for the discussion of sleeping gear (bags, mats, quilts...).


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  #1  
Old 09-30-2006, 10:56 AM
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badfishgood badfishgood is offline
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Join Date: Aug 2006
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Air mattresses and changes in temperature...

Is it possible that an extreme change in temperature could affect the "air volume" of an inflatable mattress pad?

There reason I ask...

Last week I was on a trip in the high Sierra (11k+) and experienced some extreme temperature changes. One night it was in the 30s and the next night is was below 10 degrees. I was camped in the same spot and didn't think to "top-off" my pad the second night. Upon waking up the second morning I found my pad to be significantly less inflated than the previous morning. A function of not topping it off? Or of a change in air temperature?

Cheers.
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  #2  
Old 09-30-2006, 11:17 AM
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hoosierdaddy hoosierdaddy is offline
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Join Date: Mar 2006
Location: Western Washington
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Probably just the fabric stretching somewhat after inflation or perhaps pinhole leaks. All three of my different brand inflatable pads are not as inflated the next morning as they were when I went to sleep no matter what the temperature is.
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  #3  
Old 09-30-2006, 01:50 PM
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WildlifeNate WildlifeNate is offline
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Backpack: Osprey Atmos 50
Sleeping Gear: DIY down quilt
Shelter: ENO Doublenest Hammock, WB Bugnet, GG Tarp
 
Join Date: Mar 2006
Location: Nacogdoches, TX
Posts: 1,618
(Pressure) x (Volume) = (number of molecules of gas) x (ideal gas constant) x (Temperature)

So, the pressure a given amount of gas exerts in a container most certainly does vary according to temperature (all other things being equal). It doesn't even have to be an extreme change in temperature. Any change in temperature (of the air inside the pad) will result in a change in the pressure. You're not really talking about the volume here, since the volume is a constant (the interior of your inflatable pad).

There is the possibility that the fabric might change, but if anything, it would contract, making the volume slightly smaller. The only time it would expand would be if it got warmer, which in this case, it did not.
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Old 09-30-2006, 03:52 PM
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Lonewolf22 Lonewolf22 is offline
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Wouldn't the air mattress be somewhat warm from your body heat during the night? If so then the mattress wouldnt get "smaller" correct?
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  #5  
Old 09-30-2006, 07:13 PM
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Dusty Boots Dusty Boots is offline
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Backpack: Depends on trip conditions. I have 4 to choose from. 2 Keltys, 1 Gregory, 1 MEC pack
Sleeping Gear: POE Max Thermo pad/MEC -3 Merlin
Shelter: TarpTent Double Rainbow
 
Join Date: Apr 2006
Location: Ontario, Canada
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It happens quite often with certain Brands

I know a few hikers that have had their BA mats go flat, or had a significant loss of air in cooler/cold temps and it happened with the replacement mats as well so they took them back for a refund and bought something else.
It seems to happen more often with air mats, then the self inflators.
Air looses it's 'volume' when chilled and air mats contain more air(by volume) than the self inflators, so I guess that is why they seem to be affected more by cooler temps.

One possible solution is to use a fairly thick CC pad underneath your pad, to help insulate it from the cold ground.

FWIW, .... I have spent quite a few nights out in the -35*C range and never noticed much change in my ThermaRest pad(S) that I have used at different times. (a 1st generation LE and one of the old orange ones) I haven't tried out my expedition model in cold temps yet, as the lowest I've had it out in is about -16*C


Dusty
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  #6  
Old 09-30-2006, 10:07 PM
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WildlifeNate WildlifeNate is offline
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Backpack: Osprey Atmos 50
Sleeping Gear: DIY down quilt
Shelter: ENO Doublenest Hammock, WB Bugnet, GG Tarp
 
Join Date: Mar 2006
Location: Nacogdoches, TX
Posts: 1,618
Your body would warm the pad to some degree, but it's not going to be by a lot. That heat will be lost pretty quickly to the ground unless you're using a closed cell pad beneath the air mat. That's why the BA mats are temp rated to about 15 deg F.

This isn't just bull I'm spouting...it's the ideal gas law.

In this case, the significant temp change without putting more air into the pad to account for it is why the pad went 'flat' overnight. I've had the very same thing happen to bicycle tires. Inflated them when the bike was warm in the garage. Drive half an hour to an hour in the cold to where I'm going to ride...the bike has cooled, and the tires pressure is now insufficient to ride.

Thermarests are a bit different in their construction. They use a MUCH smaller volume of air and have a better insulator (open-celled foam) to retain heat better than the inflatables like the BA and Insul-mats. Still, even with a TR, I have noticed a slight overnight loss of pressure when the morning temp is significantly colder than the temp when I inflated the pad.
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  #7  
Old 09-30-2006, 10:27 PM
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mn_backpacker mn_backpacker is offline
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You can observe this exact same temperature effect here in MN in the early part of winter. People's car tires get slightly flatter when it gets cold here. You can even feel it when you drive. In the frigid north, you have to air up a little when winter hits if you wish to maintain the same pressure. Usually for every 10 degrees of temperature drop, your tires will loose about 1 psi.
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  #8  
Old 10-01-2006, 10:02 AM
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badfishgood badfishgood is offline
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Join Date: Aug 2006
Location: Bay Area
Posts: 103
Quote:
Originally Posted by WildlifeNate
(Pressure) x (Volume) = (number of molecules of gas) x (ideal gas constant) x (Temperature)

Thanks Nate.

The pad in question is the BA Insulated Air Core. I love how comfortable it is and I love how it packs, so I'm going to stick with it for a while.

I was concerned at the time that I might have acquired a pin hole leak. Now that I'm back home and the pad has maintained a relatively constant pressure over a few days, I'm ruling out a leak.

Next time I will make sure to reinflate the pad before a second night of straight use, if the problem still presents itself after that, I'll have to consider the CC pad idea (which I already have for use in snow).

With that... I'm going to take a hike.
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  #9  
Old 10-02-2006, 12:20 PM
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chicote chicote is offline
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Join Date: Jun 2006
Posts: 66
Air Volume

I went out for a 1 night trip this weekend. Immediately when I arrived I went to set up camp. I after I ate dinner and was hanging out with the other campers I decided to go grab my Insul-mat Max Thermo to use as a seat down by the fire. From the use of my own hot air to inflate and the 15 degree drop in temperature I noticed the pad wasn't fully inflated. I opened up the valved and put a couple more puffs in and was set. The temp dropped some more over the night but I don't think it had much of an effect on the pad because of the body heat. If left alone I bet it would have deflated a little bit more.

Last edited by chicote : 10-02-2006 at 12:22 PM.
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  #10  
Old 10-11-2006, 05:05 AM
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jasonklass jasonklass is offline
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Join Date: Mar 2006
Location: Denver, Colorado
Posts: 485
This happens to me all the time; especially when winter camping. I usually over-inflate my pad a little right before I go to sleep because I know the air will condense when the temp. drops overnight. I don't think body heat really mitigates this.
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