Since the assembly must heat up after a while, it seems like a larger set would have higher temps.... sorry if this sounds crazy, I'm just curious.

I've never felt a shadow mask get hot. It's only ~1 inch or so from the CRT face, and CRT faces don't even get warm, far as I know.

Let's say the CRT operates at 25KV and you're running it pretty bright at 1ma emission. That's 25 watts of disappation. Some of it in light, the rest in heat. That's not much power over such a large area. But would warm the mask to deform it. They tension it and use low CTE materials to minimize the effect to keep the registration.

I'd guess that size doesn't matter since you probably have the same power per unit area.

I'll try to look up the mask temperature when I get to work.

John

O.K. This is from Meas. Sci. Technol. 8 (1997) 1328–1332. The temperature is in degrees C. It show's the temperature distribution. The peak temperature seems to be about 70 degrees C.

John

They get hot if the whole TV gets hot... Especially tube sets in metal cabinets. The shadow mask is very thin, and is convex, so I doubt it would change shape with heat; at least not enough to throw off the purity. After all, they cook the tubes for a day at 500 degrees or some hot temp, and they don't get malformed in that process.

Charles

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Something tell me that this would be a good thread. So far you guys have made me realize the more obvious answer; that ambient heating from the rest of the set heats the thing too....didn't even consider that. Also, it is neat to hear that the supports can actually compensate for the heating/expansion of the mask.

Something tell me that this would be a good thread. So far you guys have made me realize the more obvious answer; that ambient heating from the rest of the set heats the thing too....didn't even consider that. Also, it is neat to hear that the supports can actually compensate for the heating/expansion of the mask.

The heating is from the electron beam. The mask sits in vacuum and being thin doesn't conduct heat very well along the surface. The only way it cools itself, except for the edge, is by blackbody radiation. 70 deg C is pretty warm, it's about 158 deg F.

The Stefan Boltzmann law is of the form P=k(T1**4-T2**4). In this case T1 is the temperature of the mask and T2 is the ambient temperature of the glass around the mask. P is the power radiated. It has to come into equilibrium with the power being deposited by the electron beam. So you could solve it for the temperature of the mask. The temperature would go as the fourth root of the power so it wouldn't change a lot with a change in beam current. If you doubled the current the temperature would increase by a factor of 1.19

The increase in temperature of the CRT bulb from the set heating it contributes, but isn't a major factor in the temperature of the mask since the faceplate is usually near room temperature.

John

True, I have never felt a crt faceplate that I can describe as 'abnormally warm to the touch' although after hours of operation some do get slightly warm.

158F sure is hot. Can you explain blackbody radiation as it pertains to heat dissipation in a vacuum? Kind of lost me there.... If there are no air molecules in there, where does the heat go?

True, I have never felt a crt faceplate that I can describe as 'abnormally warm to the touch' although after hours of operation some do get slightly warm.

158F sure is hot. Can you explain blackbody radiation as it pertains to heat dissipation in a vacuum? Kind of lost me there.... If there are no air molecules in there, where does the heat go?

It radiates the heat away. Like a light bulb except it's in the far infrared. It's what you feel when you stand in front of a fireplace. As the temperature of an object increases the wavelength of the radiation gets shorter until it gets into the visible as in a light bulb.

Because there are no molecules to transport the energy is the reason it gets so hot. It can only dissipate the energy by radiation.

John

True, I have never felt a crt faceplate that I can describe as 'abnormally warm to the touch' although after hours of operation some do get slightly warm.

158F sure is hot. Can you explain blackbody radiation as it pertains to heat dissipation in a vacuum? Kind of lost me there.... If there are no air molecules in there, where does the heat go?

The sun is in a vacuum, yet we do get a lot of radiation from it.
If you get the concept of photon, you will be on the way to an
understanding of radiation in a vacuum. Sorry I'm in a hurry,
as someone said this is an interesting thread, see you later...

The idea that the sun's radiation heats this planet opened my eyes to the whole thing. I thought something else was going on in the vacuum but I was in error.

Even your body, at its temperature of about 98 deg. F (37 deg. C) is emitting
some black body radiation right now, and since its temperature is much lower
than that of the sun (or of a running shadow mask!), the peak of this radiation
is at a microwave frequency of a few GHz. Medical diagnostic
techniques have been experimented based on this phenomenon:

http://www3.interscience.wiley.com/journal/114083730/abstract?CRETRY=1&SRETRY=0