Calling all fuel canister engineers!

[Gary Dunckel]

“…so IR will be reflected back up to the bottom of the canister where the liquid fuel is”.

I’m not quite sure what you mean, Jerry. The aluminum floor will do that without also including the ti foil disk. My thinking is that the ti disk will retain a small amount of the heat that it absorbs, which might then be slowly conducted to the base of the canister that sits on it. But maybe it doesn’t do that, in which case the ti disk is not necessary at all.

[Bob Moulder]

Re Ti disk…

It depends what it is sitting on — snow/ice would be a heat sink and would likely have a negative effect. Metal/glass table (flat)… somewhat negative; cast-iron pattern… much less because of far fewer contact points.

Best would probably be Ti foil atop CCF. For snow I use only 3/8″ CCF.

On any hard surface, however, the contact area of the canister rim is so small that the overall effect is very minuscule. Obviously, snow is much worse because it conforms to the concave base. Sitting on a rock there is very little heat loss because the contact points are very few.

[David Thomas]

Ti is a poor conductor. Thin Ti even more so.

Does a mirror reflect your image less well when it’s cold?  Nope. Shiny, bare metal reflects the IR that lands on it. The reflected IR is not a function of the metal’s temperature.

This is why an IR thermometer pointed at a hot (or cold) bare metal pipe only shows the temperature of the surrounding objects.

[Gary Dunckel]

I agree with you, Bob, about the CCF. I also use a 6″ x 6″ piece of 3/8″ foam, which is the base that everything sits on. For my patio tests I don’t use one, but rather just set things on my aluminum table (talk about a heat sink at 0* F!). I figure that if things work there they will certainly work better if things are sitting atop the piece of CCF.

One thing I am curious about–isn’t snow/ice a great insulator, and a few millimeters below the surface it should be about 30-32* F, right? So how much of a heat sink is it, really, when ambient is below 0* F?

You are right about the minimal contact between the small titanium disk and the canister–just the outer rim of the base of the canister. But I still think that there’s a possibility that the ti disk would be warmed a bit from all those IR photons blasting it. And being such a poor conductor it should somewhat retain a bit of that heat, and maybe pass a little of it to the canister. That metal certainly wouldn’t cool as fast as the aluminum reflector would when the stove is shut off. Maybe a full titanium disk that fills up the entire inside of the aluminum reflector would be most efficient? Then there is the question of whether the titanium disk should be placed on top of the reflector’s aluminum floor, or under it, between the floor and the CCF.

So many questions, and so few remaining frigid winter days to do the tests and get the answers.

[Roger Caffin]

This is why an IR thermometer pointed at a hot (or cold) bare metal pipe only shows the temperature of the surrounding objects.
ONLY if the reflectivity is 1.00 (or 100%), but nothing is that good. Even a silvered telescope reflector will be a few percent below perfect.  Ordinary steel has a reflectivity of only 10% to 20% and will register quite well on an IR thermometer once you have calibrated it for the steel emissivity. Glowing red steel is measured very well. (We normally use ’emissivity’, not reflectivity, but no matter.)

–isn’t snow/ice a great insulator, and a few millimeters below the surface it should be about 30-32* F, right?
No, wrong. Ice is not a good insulator, although the AIR between snowflakes might be – until the snow packs down and gets rid of the air.
And no, the temperature a few mm below the surface can be very cold if the ambient is cold. There is nothing to stop it getting to -20 C if that is where the ambient is. Nothing magic here.

Cheers

[Gary Dunckel]

What you wrote about snow being a poor insulator is interesting, Roger. When we winter camp, it often gets down to +10* F (-12* C). Before we slip into our sleeping bags we fill our 1 liter Nalgenes with water and shove them upside down into a snow bank. We pile snow on top and place sticks to mark where the bottles are so we can find them. In the morning they are never frozen, except for maybe a small bit of ice on top. It seems like snow insulates pretty well to me. Ice…likely not.

[Roger Caffin]

Hi Gary

Yes, that works because you are dealing with fresh snow with a lot of air in it. The air is doing the insulating, not the ice. It would be a different matter if you stuck them down a bore-hole in ice.
Mind you, I just stick my water bottles near the end of my quilt, on the foam mat down there (inside my double-skin tent). That works fine too.

Cheers

[David Thomas]

When I IR measure copper pipe with 120-130F water inside it, in a 72-73F room, it reads as 77F.

It’s like, “What color do you see in a mirror?”  The colors in the room.

Actually, it’s exactly like that because IR thermometers simply measure the “color” (frequency) of the IR photons.

[Roger Caffin]

Hi David

Well, I don’t know what sort of IR thermometer you have, so that makes it a bit difficult.

However, to the best of my knowledge, there are no IR thermometers which measure the frequency of the IR photons. For a start, the radiation from the pipe will follows Plank’s law, and it will have a quite broad distribution of frequencies. There is no single frequency to measure.

Most IR thermometers measure a ratio of the energies at two different narrow frequencies bands, and then they convert that ratio to an equivalent black body temperature (using Planks law). However, since the radiation they are measuring is not coming from a ‘black body’, they have to correct this result using the emissivity of the surface. If you vary this emissivity factor, the displayed temperature will change. It sounds to me as though your IR thermometer has the wrong emissivity for a copper pipe, and it needs to be calibrated for copper.

Cheers

[Gary Dunckel]

Roger, how do you explain that the interior of a snow cave/quinzee/igloo seems to remain at about freezing temperature, even though ambient can get well below that? There must be some sort of insulating quality of the white stuff, even though the snow walls and “roof” are packed and pretty much devoid of air. Sure, body heat warms up the inside, and perhaps a bit of candle warmth, but any heat seems to remain inside. It seems to me that something is insulating something here.

[James Marco]

The thermal resistance of ice is around 2.5 depending on density or somewhat better at removing heat from you than Granite at ~2.0.
The thermal resistance of snow is around 0.5 depending on temperature/density. This is roughly the same as blown in fiberglass insulation.

Plus, the internal area of an iglo/snow cave is small. It will easily get you to around 0C with a couple bodies and a candle while using a small vent.

[Roger Caffin]

I think James got most of it.
What warms up in an igloo is the air. It has very low thermal mass, so that is easy. If the air is largely trapped, which is what everyone tries for, then you are in business for comfort. Of course, the air will hit the wall and cool, while the wall warms up. As soon as the snow reaches 0 C it turns to water and that either dribbles down the wall into the floor or it gets sucked back into the snow. Either way, the inside SURFACE of the wall sits at 0 C fairly easily when the igloo is occupied.
As for the snow blocks used to make an igloo – there will be a lot of air in them still. Try picking up a 1 foot cube of snow and comparing the weight with a 1 foot cube of water or ice. Big difference! And if there was no air in the snow block, it would be solid ice, and you would sure know it!

Cheers

[Author]

Posts