Thread: Vacuum Insulation

Hello,

Question:

1) Took a Stainless Vacuum Thermas and poured boiling hot water into it, 30 seconds later the outside is cold to the touch.

2) Drill a hole and let all the vacuum out.

3) Pour boiling water in, guess what happens in 30 seconds?

4) You are right, the outside is hot

5) Place a pump on the hole and pump out the air to 28HGVAC

6) Pour boiling water in again, guess what happens in 30 seconds?

7) Wrong, the outside is hot!

What do you think the problem is?

Maybe the flimsy inner flask deformed and now touches the outer part.

If there's insulation (foam?) that could have blocked your pump.

My response would be that the 28" Hg isn't enough of a vacuum to act as an insulation. There are still a fair number of molecules of air inside at that point so it does not act as a vacuum insulation, but rather as a low air pressure insulation. I wouldn't know what the pressure needs to be in order to for the thermos to act as a vacuum insulator, but I would guess you need to get into the single digit torr ranges or lower. Search google for thermal conductivity of air vs. density to get a better feel...

Solveer, your anser sounds correct. All the guages that are avaiable only go to 30hg. I read somewhere that 30hg is the lowest that you can go. How much vaccum could they put into a 6 dollar thermos.

Originally Posted by johnadaniels

Hello,

Question:

1) Took a Stainless Vacuum Thermas and poured boiling hot water into it, 30 seconds later the outside is cold to the touch.

2) Drill a hole and let all the vacuum out.

3) Pour boiling water in, guess what happens in 30 seconds?

4) You are right, the outside is hot

5) Place a pump on the hole and pump out the air to 28HGVAC

6) Pour boiling water in again, guess what happens in 30 seconds?

7) Wrong, the outside is hot!

What do you think the problem is?

Because you didn't wait until the outside of the Thermos cooled down...duh!

Vacuum is a perceived effect based on atmosphere pressure. The higher the ambient pressure outside of the vacuum vessle, the higher the vacuum. It's inversely proportional to ambient pressure. 0 psi-ambient = 29.9 in Hg. Atmospheric pressure is 14.7 psi. So if you have zero psia in the Thermos and 14.7 psia in the atmosphere, you have a full 29.9 of vacuum. But it's outside pressure that's trying to push in, not the vacuum pulling anything in.

Regardless, I doubt any Thermos at 28 in Hg would be hot after only 30 seconds. There's just not enough air molecules to transmit the energy to the outside Thermos wall. I doubt it would be hot after 30 seconds even if it was filled with air. It's 2 minutes until 5PM, so I'm not going to do the calculations using the gas laws to figure out actual conductivity. LOL

Originally Posted by johnadaniels

Solveer, your anser sounds correct. All the guages that are avaiable only go to 30hg. I read somewhere that 30hg is the lowest that you can go. How much vaccum could they put into a 6 dollar thermos.

Light bulbs are typically evacuated to 10^-7 bars. and the small ones are only $0.25. The thermoses are also evacuated using a diffusion pump to about the same. Your 28 mmHg (1.4*10^-2) had a ways to go. According to a paper I read, convective heat transfer becomes a factor at 2*10^-6 bars and higher. Below that number, most of the heat transfer is radiative from wall to wall.

Cypress and Gen1GT thanks for adding your replies, you sound very smart. You saying that 28HG evacuated is not enough. What HG do you think would be enough? 30HG 32HG 40HG?

Do you know what the lowest HG that you can evacute is?

Using HG because that is what shows on a vacuum gauge from auto parts store to measure the vacuum from the engine. The gauge reads from 0 (no vacuum) to 30HG. That was the reason I was asking is if 30HG the lowest vacuum you can achieve.

If 30hg is the lowest then do you think that the 2hg difference could be the reason that the insulation is not working. Kind of like boiling water gets to a certain point and then BAM water starts to boil.

Originally Posted by johnadaniels

Cypress and Gen1GT thanks for adding your replies, you sound very smart. You saying that 28HG evacuated is not enough. What HG do you think would be enough? 30HG 32HG 40HG?

Do you know what the lowest HG that you can evacute is?

Using HG because that is what shows on a vacuum gauge from auto parts store to measure the vacuum from the engine. The gauge reads from 0 (no vacuum) to 30HG. That was the reason I was asking is if 30HG the lowest vacuum you can achieve.

If 30hg is the lowest then do you think that the 2hg difference could be the reason that the insulation is not working. Kind of like boiling water gets to a certain point and then BAM water starts to boil.

Oh I provided that answer 1.6*10^-6 bar is enough... thats 4.7*10^-5 inHG absolute pressure. If atmospheric pressure is 30.000000 inHG on the day you do this, then it would be 29.999953 inHG vacuum. 28 inHG is not nearly enough, there is still way too many molecules in the void space. An oil diffusion pump can get down to 10^-10 bar. Other diffusion pumps can get down to 10^-14 bar which is about the limit.

Thanks for the reply again cypress.

Could you please translate the following:

What are the following in HG:

10^-10 bar = ??HG

10^-14 bar = ??HG

Originally Posted by johnadaniels

Thanks for the reply again cypress.

Could you please translate the following:

What are the following in HG:

10^-10 bar = ??HG

10^-14 bar = ??HG

It's a negative exponent. Just add that many decimal points to the original number.

Originally Posted by johnadaniels

Solveer, your anser sounds correct. All the guages that are avaiable only go to 30hg. I read somewhere that 30hg is the lowest that you can go. How much vaccum could they put into a 6 dollar thermos.

they measure that in Torr, typically fractions of it. A simple vacuum gauge would read 0 PSIA, -14.7 PSIG, 29.92 in. Hg, etc. for a perfect vacuum. When you see a Torr measurement, it may be something like 1.3 x 10E-3. For a near perfect vacuum, it would be more like number x 10E-12

1 Torr = 1/760 of an atmosphere.

There seems to be some confusion as to the units used to measure vacuum (at least by some). A vacuum measures, just like a pressure.
Consider where the units come from: inches of mercury, milimeters of mercury, etc. If you have a U-shaped tube and partially fill it with mercury (Chemical Element: Hg) the level of mercury on either side of the tube will be the same height if both ends are open. Now close one end and start applying a vacuum to it. The height difference (measured in mm or in) will be 'vacuum'. Why mercury? Simple: Its very dense and has a low vapor pressure (so that it can be used for quite low pressure readings). For more sensitive readings (at higher pressures) you would use water since it is less dense (so the difference becomes more pronounced).
The standard atmosphere is defined as having a normal pressure equivalent to 760 mm Hg (29.92126 in. Hg) so if you think about it, the lowest vacuum possible (absolutely zero atmosphere) is equivalent to 29.92126 inHg. Why does your scale have 30"Hg? Simply because that is close enough. Once you get past about 28" your accuracy is not there anyway, so forget using a piece of equipment like that. One extra piece to mention is that the usual (automotive) vacuum meter will be what's called a bourdon type meter (see wikipedia for details). If you use a meter like that, the underlying assumption is that the vacuum you are measuring is relative to 1 standard atmosphere. If the outside air pressure is different from that (1013.25 mBar) you will have to add/subtract it to get what your meter is really reading.

Hope this helps explain where the units come from (incase there was some confusion).

I've been doing research on vacuum insulated panels (VIPs) and found this thread.

Would encapsulating standard rigid insulation in a moderate level of vacuum (maybe around 150 mBars?) make any significant difference in it's insulating properties?

Current VIPs are prohibitively expensive because they use exotic core materials like aerogel and vacuum pressures around 50 mTorr, but they achieve R-50/inch insulation. I'm wondering if there's an inexpensive middle ground?

Steve

Interesting