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PV=nRT tricking me?

I have a physical situation that so far I'm unable to figure out a reason for; I'm assuming there's a simple answer that I haven't thought of yet.

We have a kid's inflatable pool this summer- holds about 200-250 gallons, and measures about 1m x 3m x .8 m. Due to raccoon problems, we've found we need to empty it and bring it inside at night(we're on the third one of the summer before I was willing to do that; now I waste water on their pool and let the lawn die, except around the pool of course. Kids are more important than lawns, right?)

Anyway, for the past few nights I've been observing this. I empty the pool just after dark and bring it inside. Our temperatures over the past week have been 90ish days, 60ish nights, low humidity. I've been bringing the pool in around 10 at night, and out around 10 am. Outside T is probably 65 when I bring it in, and upper 70's, maybe 80 when I bring it out. (All T's in Fahrenheit)

When I bring it in, the pool feels limp, low on air. When I bring it out again in the am, it feels taut. There are no leaks, and no one has filled it.

The pool is ordinary plastic, made of three stacked inflatable 'rings', roughly circular, maybe a 20 cm. dia, approx. ( I haven't measured that). If the particular plastic is important, I still have the box, I could find it.

Now, I thought that the opposite should happen. The pool sits in the sun all day, the air in the channels heats up. The outside air cools off; I have been assuming that the air in the channels cools slower than the air outside of it, as it's trapped inside and at least somewhat insulated. Not much, but some. So- if air inside is warmer than air outside, why is pressure inside not greater than pressure outside?

And, why is it 'tightening up' overnight? The air in the house probably stays around 70, 75 at night- we keep screens open to cool off at night. Inside, though, it does sit in an area that gets morning sun, beginning

5:30, 6-ish if I open the blinds(and morning heat even if I don't open the blinds). And from 4 pm or so on, it sits in shade outside. Is that important?

So- I'm missing something. Like I said in the beginning, I'm sure it's something that will be a "DOH" moment when I hear or read it- but I'm not seeing it, right now.

TIA, k wallace

Reply to
k wallace
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The temperature in the house at night is warmer than that of the water during the day. As long as there's water in the pool, the air in the pool's chambers will be closer to the water temp than the air temp.

Ned Simmons

Reply to
Ned Simmons

You are forgetting that the air in the pool is sitting ontop of the ground, which would act like a heat sink. In addition to that, you have heat stored in the water in contact with ground. Since you have all of that contact with the ground, your pool cools down quickly. (Just for fun, empty the water out a several hours before it gets dark and see what happens.)

This may have to do with the ground cooling the air to below 65. The temperature inside of the tubes may be warmer than the ambient air if the walls are colored. Think of your car in the summer with it's windows rolled up.

Reply to
YouGoFirst

I would think that captured air in a 90 deg. ambient coupled with the heating of the plastic from the sun and the greenhouse effect on the inside air, would probably be as hot or hotter than the surrounding air (think of the inside of your car in the summer). The water in such a small pool would eventually heat up too.

I was going to guess that the plastic is expanding from the heat of the direct sunlight (it probably gets hot to the touch--over 110 F I'll bet). When it is out of the sun, it cools to room temperature and shrinks. The problem with this theory is that the plastic would have to expand a *lot* to keep up with the equivalent air exapansion which, as a gas, would be much more...unless there is a big difference between the plastic temperature and the air temperature, and it's unlikely that it would be that great.

I guess if the garage actually stays hotter at night than the tube air greenhouses up in the day, or your kids are playing in cold water right out of the tap...?

Don Kansas City

Reply to
eromlignod

Unless I'm missing something, the answer is right there. If you deflate it at

65F, the air will have expanded a bit when it is warmed up to 75-80F.

-Paul

Reply to
Paul Skoczylas

65F, the air will have expanded a bit when it is

I don't deflate it/reinflate it daily. Just fill and empty with water. The water is tap-cold (not sure of the temp of that, but cold to touch) and gradually warms up during the day, but is consistently cooler than the air. I think that's what I wasn't considering- the heat sink of the water. Maybe. I'll ponder that some more. k wallace

Reply to
k wallace

yes- but it doesn't heat up to the temp. of the surrounding air. Even in the evening when I empty it, the water is cool compared to the ambient air. It's cold when filled- not sure what tap temp. is, but pretty cold. I think I was not considering the heat sink of the cool water in the pool, perhaps.

I don't know which is going to be dominant- the convective and radiative heating of the air greenhousing in the tubes (which are med. blue in color, so don't reflect all the sunlight) or the conduction from the water, which is a cool mass.

The garage isn't that hot at night, as the door is open in the evening until I close up the doors around midnight. I don't think it's a plastic expansion issue; I'm going with the water idea- that the water is a large, cool heat sink, which, while it heats up in direct sunlight, also cools back down in the shade/at dusk until I empty the pool. Being that it's in direct contact with the tubes of air, it cools them off, and cools off the air in the tubes, which 'condenses' those gases. Once the pool is emptied, the air tubes are surrounded on all sides by ambient air at between 60-70 for the whole time it's empty, which re-heats the air and expands it.

that's what I'm going with, anyway.

thanks- k wallace

Reply to
k wallace

Why is it firmer in the morning? Because it's hotter!

Doesn't it get colder than ambient each night when it gets wet and the water evaporates into a clear night sky?

Brian Whatcott

Reply to
Brian Whatcott

Doesn't what get colder than ambient? The air in the pool? I'm not sure exactly what you are saying, but from reading of your posts here I'm sure you are saying something that makes sense. Excuse me if I'm appearing dense; I'm just not sure what you are referring to. regards, kw

Reply to
k wallace

I would guess that V (Volume) is not constant. The rubber pool is hotter in the evening, when you take it in, and due to thermal expansion of the rubber the air volume is increased. Even though the air temperature is higher in the evening, the volume must be even higher, so that T/V is less than 1, and the pressure is lower.

Just a guess. Now it's time to hook up a pressure gage and some thermocouples to find the real answer.

Dave

Reply to
dgp

I was supposing the trapped air in the inflatable is cooler than ambient at night, then warms in the morning.

Brian Whatcott

Reply to
Brian Whatcott

Yeah, I agree with that theory. Chances are, the water you fill your pool up with has been sitting underground in a pipe for the last several hours, and is probably between 50F and 65F. Additionally, even though it's a small pool, it takes ALOT of energy to heat up water even a little. Per weight or per volume, water's got alot more thermal capacity than steel. So by the time you throw out the water, it's probably only heated up a few degrees. Evaporation during the day also helps keep the water cool. So when you take in the pool, the air inside is still probably cooler than your garage at night.

Dave

Reply to
dave.harper

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