tube shields

[Kenneth G. Gordon]

On 13 Nov 2007 at 16:04, Patrick wrote:

> For what is worth...I have measured the temperature of tubes with
> black tube shields (Military) and shiny ones and the difference is
> minimal, between 1 to 3%, depending on tube type.

Where did you measure the temperature? Outside, on the shield, 
or inside, against the glass of the tube?

> On the other hand,
> painting the shiny tubes with ultra flat black paint obtained the same
> results. The measurement was done using one of these new laser
> temperature gadgets and the paint used was Americas Finest by
> Rust-Oleum HD2876 ultra flat black..

If you used one of those laser gadgets, then I cannot see how you 
could measure anything but the surface temperature of the outside 
of the tube shield. If that is what you did, then I agree that there 
would be a minimal difference.

I will have to dig out the military report, but I believe they used 
some sort of thermocouple device as is often used in metallurgy, 
resting it, or clamping it against the glass of the tube.

I might add that they also measured the temperature of the tubes 
with no shield.

The problem with the shiny shields was that they reflected heat 
back into the tube. Either SHIELD would get to about the same 
surface temperature after cooking long enough.

I repeat, according to the report, the black ones radiate it away, 
while the shiny ones reflect a significant amount back into the tube.

I might add that the shiny shields also include a significant 
insulating air-gap between the metal and the surface of the tube in 
many cases, while the IERC shields closely hug the glass.

As far as painting the OUTSIDE of the shield is concerned, I would 
agree that leaving it shiny would be best so that more heat was not 
absorbed from outside sources. At least that seems logical.

However, if there were no outside sources, as I remember it, there 
was a very slight increase in the amount of heat radiated away for 
a single tube in a completely open environment.

I am wondering if perhaps the explanation might be something on 
the order of the shield would act more like the theoretical "black-
body".

I'm just guessing here, since I do not have the report in front of me 
at the moment, it has been a long time since I last read it, and I 
have periodic bouts of CRS disease.

Ken Gordon W7EKB

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