4 ms·

Which would be very painful when it heats the skin.

by jeltz 6y ago

nabla9 6y ago
Have you noticed how microwave cooking is not warming up the skin any faster than the insides? The wavelength is selected so that it penetrates and warms water molecules. It would cook your insides according to their water content. Surface of the skin has less water than tissue inside, so you would feel pain beneath the skin.
- ben_w 6y ago
  MythBusters tested that, the outsides (of food) heat up first.
  - kragen 6y ago
    The fact that reverse thermal gradients are often present in microwave thermal processing of materials is quite well known and can be replicated in food in your home microwave, precisely as nabla9 said, regardless of Mythbusters being able to produce a forward thermal gradient. Microwaves attenuate as they penetrate a resistive or high-permittivity substance, because they deposit some of their energy as they pass through. In the limit of infinite depth, no radiation survives and there is no heating. Given a homogenous substance, the heating is always strongest at the surface, decaying exponentially (to a good approximation) as you go deeper. However, heating is not the same thing as temperature. The surface of food in your microwave is exposed to room-temperature air and can therefore cool down by conducting its heat to that air. Food just under the surface can conduct its heat to the surface food, while deeper food cannot. It's easy to set up situations where this results in a reverse thermal gradient penetrating some distance into the food, or even all the way to the center. This is one of the most significant advantages of microwave heating in industrial material processing, because there are many cases where the normal thermal gradient produces cracking and microwave-induced reverse thermal gradients do not. You can set up a forward thermal gradient with microwave heating by some combination of hot air, strong attenuation, short exposure times, shorter wavelengths, and great depth, although unless the air is actually hotter than the highest temperature reached within the food there will always be a reverse thermal gradient present at the surface, since the heat equation always produces a continuous temperature field at t ≠ 0. You may be able to get the reverse thermal gradient to be entirely outside the solid body if you work hard enough, but a much easier way is to only measure the temperature at intervals large enough that the entire reverse thermal gradient is smaller than the first interval. For example, if the thermal maximum is 8 mm under the skin, you could measure the temperature at the surface, 1 cm, 2 cm, 3 cm, 4 cm, and 5 cm. The attenuation is dependent on the attenuating medium and on wavelength. As explained in https://en.wikipedia.org/wiki/Active_Denial_System https://en.wikipedia.org/wiki/Active_Denial_System the ADS uses 95-GHz radio waves with 0.4-mm penetration depth in human flesh, while your microwave oven used 2.4-GHz radio waves with 17-mm penetration depth in human flesh. nabla9 has been downvoted for politely and straightforwardly, if briefly, explaining these perfectly correct and verifiable facts, facts which are central to the discussion. This makes it clear that the voting public at HN has extremely poor judgment and should not be permitted a vote on comments of decent people.
    - JarlUlvi 6y ago
      When the US military sets up an AN/TPQ-36 Firefinder radar, or similar variants, it is typical for these to have taped off (like police tape) sections where the energy is dangerous to humans. These devices, although mounted on trailers typically, are fairly old, and not designed for a point location, but rather distance. I was told that being overly close to these devices in operation will lead to brain injuries
    - iudqnolq 6y ago
      I'm not sure why my sibling is downvoted. See for example page E-17 of this manual > When the radar transceiver is energized, it poses a microwave radiation hazard to personnel. The hazard distance for the Q-47 is much greater than the distance for the Q-37. The hazard distance for troops extends in front of the radar for 217 meters over the radars full 1600-mil area of scan. The radar also poses a hazard for electrically detonated explosives. Figure E-14 depicts the radar’s radiation hazard area. https://web.archive.org/web/20141226051243/http://armypubs.army.mil/doctrine/DR_pubs/dr_a/pdf/fm3_09x12.pdf https://web.archive.org/web/20141226051243/http://armypubs.a...
      - kragen 6y ago
        I think your sibling is [dead]ed (not downvoted) because it is posted by a shadowbanned account (see their comment history of posting a lot of content-free flamebait: https://news.ycombinator.com/threads?id=JarlUlvi&next=16619847 https://news.ycombinator.com/threads?id=JarlUlvi&next=166198...) and not enough people have vouched for the comment. IIRC workers in front of radar antennas noticed chocolate bars melting in their pockets during WWII, and that observation inspired the development of microwave ovens. Not sure how long it took for people to realize that their internal organs were doing something similar to the chocolate bars...
  - jacquesm 6y ago
    MythBusters notwithstanding I have an RF burn on one of my fingers that was barely visible at the surface when it happened but which totally cooked the tissue up to about 6 millimeters inwards. It's a pretty weird spot, 2.5 mm across and even 35 years later it hasn't healed, the zone is simply dead to the touch even though the flesh has recovered. You can also still see exactly the shape of the original burn. Note to self: pay attention when trimming HF transmitters about where you keep stray digits and where the locations of the tops of trimmers are relative to those digits.
  - binbag 6y ago
    Oh well it must be the case then? Microwaves resonate with water molecules. If the surface is moist it will heat first. If not... Well it won't!
    - ben_w 6y ago
      They’re not resonating with water: https://physics.stackexchange.com/a/71868 https://physics.stackexchange.com/a/71868 If they were tuned to water molecules, they’d be great at defrosting, and unable to do this: https://youtu.be/xwEQZw3KPWg https://youtu.be/xwEQZw3KPWg
  - stjohnswarts 6y ago
    Yeah this is why I usually get better results a 70% or so setting rather than 100% as it gives the heat a chance to spread a bit better than nuking at full power.
- PhasmaFelis 6y ago
  > Have you noticed how microwave cooking is not warming up the skin any faster than the insides? No. Microwave hot pockets are famous for coming out boiling on the outside, cold on the inside (if you don't let it sit for a couple minutes to equalize).
  - staticassertion 6y ago
    Basing an argument on hot pockets feels weak.
    - PhasmaFelis 6y ago
      It's an example, not an argument. An argument isn't really needed; the way microwaves heat things is well-documented. For example: https://culinarylore.com/food-science:do-microwaves-cook-from-the-inside-out/ https://culinarylore.com/food-science:do-microwaves-cook-fro...

Which would be very painful when it heats the skin.

by jeltz 6y ago

Which would be very painful when it heats the skin.

Have you noticed how microwave cooking is not warming up the skin any faster than the insides? The wavelength is selected so that it penetrates and warms water molecules. It would cook your insides according to their water content. Surface of the skin has less water than tissue inside, so you would feel pain beneath the skin.

ben_w 6y ago
MythBusters tested that, the outsides (of food) heat up first.
- kragen 6y ago
  The fact that reverse thermal gradients are often present in microwave thermal processing of materials is quite well known and can be replicated in food in your home microwave, precisely as nabla9 said, regardless of Mythbusters being able to produce a forward thermal gradient. Microwaves attenuate as they penetrate a resistive or high-permittivity substance, because they deposit some of their energy as they pass through. In the limit of infinite depth, no radiation survives and there is no heating. Given a homogenous substance, the heating is always strongest at the surface, decaying exponentially (to a good approximation) as you go deeper. However, heating is not the same thing as temperature. The surface of food in your microwave is exposed to room-temperature air and can therefore cool down by conducting its heat to that air. Food just under the surface can conduct its heat to the surface food, while deeper food cannot. It's easy to set up situations where this results in a reverse thermal gradient penetrating some distance into the food, or even all the way to the center. This is one of the most significant advantages of microwave heating in industrial material processing, because there are many cases where the normal thermal gradient produces cracking and microwave-induced reverse thermal gradients do not. You can set up a forward thermal gradient with microwave heating by some combination of hot air, strong attenuation, short exposure times, shorter wavelengths, and great depth, although unless the air is actually hotter than the highest temperature reached within the food there will always be a reverse thermal gradient present at the surface, since the heat equation always produces a continuous temperature field at t ≠ 0. You may be able to get the reverse thermal gradient to be entirely outside the solid body if you work hard enough, but a much easier way is to only measure the temperature at intervals large enough that the entire reverse thermal gradient is smaller than the first interval. For example, if the thermal maximum is 8 mm under the skin, you could measure the temperature at the surface, 1 cm, 2 cm, 3 cm, 4 cm, and 5 cm. The attenuation is dependent on the attenuating medium and on wavelength. As explained in https://en.wikipedia.org/wiki/Active_Denial_System https://en.wikipedia.org/wiki/Active_Denial_System the ADS uses 95-GHz radio waves with 0.4-mm penetration depth in human flesh, while your microwave oven used 2.4-GHz radio waves with 17-mm penetration depth in human flesh. nabla9 has been downvoted for politely and straightforwardly, if briefly, explaining these perfectly correct and verifiable facts, facts which are central to the discussion. This makes it clear that the voting public at HN has extremely poor judgment and should not be permitted a vote on comments of decent people.
  - JarlUlvi 6y ago
    When the US military sets up an AN/TPQ-36 Firefinder radar, or similar variants, it is typical for these to have taped off (like police tape) sections where the energy is dangerous to humans. These devices, although mounted on trailers typically, are fairly old, and not designed for a point location, but rather distance. I was told that being overly close to these devices in operation will lead to brain injuries
  - iudqnolq 6y ago
    I'm not sure why my sibling is downvoted. See for example page E-17 of this manual > When the radar transceiver is energized, it poses a microwave radiation hazard to personnel. The hazard distance for the Q-47 is much greater than the distance for the Q-37. The hazard distance for troops extends in front of the radar for 217 meters over the radars full 1600-mil area of scan. The radar also poses a hazard for electrically detonated explosives. Figure E-14 depicts the radar’s radiation hazard area. https://web.archive.org/web/20141226051243/http://armypubs.army.mil/doctrine/DR_pubs/dr_a/pdf/fm3_09x12.pdf https://web.archive.org/web/20141226051243/http://armypubs.a...
    - kragen 6y ago
      I think your sibling is [dead]ed (not downvoted) because it is posted by a shadowbanned account (see their comment history of posting a lot of content-free flamebait: https://news.ycombinator.com/threads?id=JarlUlvi&next=16619847 https://news.ycombinator.com/threads?id=JarlUlvi&next=166198...) and not enough people have vouched for the comment. IIRC workers in front of radar antennas noticed chocolate bars melting in their pockets during WWII, and that observation inspired the development of microwave ovens. Not sure how long it took for people to realize that their internal organs were doing something similar to the chocolate bars...
- jacquesm 6y ago
  MythBusters notwithstanding I have an RF burn on one of my fingers that was barely visible at the surface when it happened but which totally cooked the tissue up to about 6 millimeters inwards. It's a pretty weird spot, 2.5 mm across and even 35 years later it hasn't healed, the zone is simply dead to the touch even though the flesh has recovered. You can also still see exactly the shape of the original burn. Note to self: pay attention when trimming HF transmitters about where you keep stray digits and where the locations of the tops of trimmers are relative to those digits.
- binbag 6y ago
  Oh well it must be the case then? Microwaves resonate with water molecules. If the surface is moist it will heat first. If not... Well it won't!
  - ben_w 6y ago
    They’re not resonating with water: https://physics.stackexchange.com/a/71868 https://physics.stackexchange.com/a/71868 If they were tuned to water molecules, they’d be great at defrosting, and unable to do this: https://youtu.be/xwEQZw3KPWg https://youtu.be/xwEQZw3KPWg
- stjohnswarts 6y ago
  Yeah this is why I usually get better results a 70% or so setting rather than 100% as it gives the heat a chance to spread a bit better than nuking at full power.
PhasmaFelis 6y ago
> Have you noticed how microwave cooking is not warming up the skin any faster than the insides? No. Microwave hot pockets are famous for coming out boiling on the outside, cold on the inside (if you don't let it sit for a couple minutes to equalize).
- staticassertion 6y ago
  Basing an argument on hot pockets feels weak.
  - PhasmaFelis 6y ago
    It's an example, not an argument. An argument isn't really needed; the way microwaves heat things is well-documented. For example: https://culinarylore.com/food-science:do-microwaves-cook-from-the-inside-out/ https://culinarylore.com/food-science:do-microwaves-cook-fro...