Gear Review
Therm-a-Rest NeoAir Sleeping Pad
Science Lesson
By Kretzky
Ranked #11 - Lightweight Sleep Pads
September 30, 2009
4 Stars cause it costs more than spending a night in a 4 star hotel.
With that said, I love this thing. Definitely the most comfortable I have been sleeping outside. I didn't slide around on it at all, in fact it made rolling around in my sleeping bag a little more challenging. I had no issues with being cold down to 35 deg. I haven't gone colder than that with it yet, not sure I'd' want to go too much cooler without adding something else underneath.
Blowing it up isn't too bad. There is one thing to be aware of though, some people have complained that this thing leaks, or deflates on its own. This is wrong, to debunk this - little science lesson coming up... PV=nRT (That's pressure x Volume = moles x constant R x Temperature)
Don't worry about n & R they're constants. So Basically Pressure x Volume = Temperature. What that also means is that PV/T = PV/T in the same closed container at two different temperatures, obviously. So basically, what I'm getting at is the air you're putting into this thing is 98 degrees. Let it sit out at camp for a while before you go to bed and it's going to cool off. Pressure inside isn't going to change, but you better believe the filled volume is. Based on the geometry of this thing, it's about 3000 cu in. So if you fill it up with 98 degree air, and then let it cool to say, 60 degrees, it's only going to be about 1800 cu inches - yeah, it'll look like it lost 40% of its air!
Some of you may have followed that, some people’s eyes may have glossed over reading it, but if it made sense to 1 person I'm happy. Hope it helps.All in all this thing is awesome and I recommend it if you can afford to spend all that money on it. You just might have to top it off with a little more air before you go to bed.
**Since there's been a few comments and my memory of the ideal gas law was a bit rusty...
Temperature must be expressed in Kelvin, so 98.6 deg Fahrenheit = 310K and 60 deg Fahrenheit = 288K.
What this means is that the bag will look like it lost 7% of it's air, not 40% (that did seem like an awful lot). Obviously the colder it gets, the more air it will seem to have lost. At 0 deg Fahrenheit it will have "lost" 18% of it's air.**
View Details: Therm-a-Rest NeoAir Sleeping Pad
Helpful Votes: 50 Yes
Change me.
20 Comments Last Reply: September 3, 2011 By: a.w.c100148142
ok everybody.... we all think we're all smart. now lets move on to just reviews
Helpful Votes: 0 Yes
To sle4131021's comment "In a closed system, where air molecules cannot enter or leave, as the temperature increases and decreases, the pressure will increase and decrease respectively. You will not lose any air unless there is a leak. As it gets colder the air simply contracts, reducing the pressure inside the mattress." The pressure will be constant while you're lying on the mattress as long as you're not resting directly on the ground. If the area of your body doesn't change and your weight doesn't change then as long as the mattress keeps you off the ground it will be exerting the same psi (weight/area) on you which corresponds to psi of internal pressure (I should probably be using Pascals). Since pressure is held constant, volume varies with temperature.
Helpful Votes: 1 Yes
Someone suggested only a place like the Mojave would have temp swings of more than 15-30%. I live in KS where it is very typical to have highs in the upper 90z, even 100 then drop to the 60z at night. So such temperature changes occur in more places than you would imagine.
Helpful Votes: 1 Yes
awesome
Helpful Votes: 0 Yes
Brad - you asked where I camp that it starts at 98 degrees and drops to 60 degrees overnight. No where. But I use my lungs to blow this thing up, and the air that comes out of me is 98 degrees. So it's always going to start at 98 degrees and drop to whatever the ambient air temperature (or temperature in the tent) happens to be. Hope that makes a little more sense.
Helpful Votes: 1 Yes
Kretzky: where do you camp that it's 98F when you set up camp, and then it drops to 60 overnight?! Must be the Mojave Desert. Few places would be that extreme. A 15-30 deg. swing would be more typical. I recently used the NeoAir with a 20-deg swing of outdoor ambient, but I doubt it was that great a range inside (warmer overall), and especially with my bag and body atop the pad as the outside temp. cooled to the upper 30s. Seemed like the pad lost a greater % of its air than your equation would have predicted. I'm not convinced that it doesn't leak a little.
Helpful Votes: 0 Yes
hahah this is great
Helpful Votes: 0 Yes
You say the pressure inside the mattress isn't going to change as the temperature cools off. But it will decrease. RickP, In a closed system, where air molecules cannot enter or leave, as the temperature increases and decreases, the pressure will increase and decrease respectively. You will not lose any air unless there is a leak. As it gets colder the air simply contracts, reducing the pressure inside the mattress.
Helpful Votes: 3 Yes
I may be wrong since it has been a few years since I took college chemistry, but shouldn't the equation use kelvin temps instead of fahrenheit? Just doing the math real quick in my head makes me think that, by using degrees F, the ratios will be skewed enough to give you a much more pronounced pressure/volume loss.
Helpful Votes: 0 Yes
You guys commenting that the thing might implode had better hope it does not do that too violently. Enough of an implosion might cause the formation of a black hole, which could suck up the earth, and then you would not have the opportunity to review other gear, unless you figured out a way to get the reviews back out past the event horizon! So don't let any extremely heavy people sleep on it. Marmot from Thailand
Helpful Votes: 0 Yes
Physics is a lot like Chinese food, awful filling but doesn’t stay with you too long.
Helpful Votes: 0 Yes
Great application of thermodynamics to explain a common misconception. The ideal gas law can explain slight volume changes in normal sleeping pads as well if the pad is allowed to fill and closed during the day. The night temperatures will cause the volume of the air to decrease.
Helpful Votes: 1 Yes
Hah! Haven't check this in a little while. Nice Work guys, it's been a few years since college - well done. Thumbs up all around.
Helpful Votes: 0 Yes
BTW- no dis intended....just having fun with the math. You get what you pay for, and the NeoAir is no exception.
Helpful Votes: 4 Yes
Sorry! You had to get me started. Below 0 and it becomes a vacuum. Far enough below 0 and I guess it implodes, then you still freeze your ass off, because you end up sleeping on the ground. Now I'm done.
Helpful Votes: 4 Yes
That was just too hard to resist, but...you can't divide 0 into 98 parts and it would be flat. or: 255/309=18% pressure loss when expressed in Kelvins
Helpful Votes: 5 Yes
According to the updated Kretzky Law of further analysis, you would freeze your ass off for $149.95
Helpful Votes: 5 Yes
Almost, but not quite... For your calculations, the temperature should be on an absolute scale, e.g. Kelvin. 98 Fahrenheit and 60 Fahrenheit are 309 and 288 respectively on the Kelvin scale. So, the volume is reduced to 288/309 = 93% of the original volume when you lower the temperature from 90 to 68. I.e, you lose 7% of the air. (If you really could use Fahrenheit temperatures, what would happen to the calculation when the temp dropped below 0?)
Helpful Votes: 9 Yes
A little update to this review. My neoair had it's first sub-freezing experience this weekend. Temps dropped into the low twenties overnight. When I got up in the morning I could definitely feel the coolness from the ground working up through the pad. I'd say anything below 30 deg F - add a Z-lite pad underneath this to keep yourself insulated from the ground.
Helpful Votes: 2 Yes
A little physics goes a long way.
Helpful Votes: 38 Yes