On Thu, 26 Aug 2004 08:18:43 +0100 "Ian White, G3SEK"
<G3SEK@ifwtech.co.uk> writes:
Hi Will & Ian,
> Will Matney wrote:
>
> >> The Fluke Model 61 is $109. It shines a laser beam where it is
> >>measuring the temp.
<snip>
> However, the three types of measurement that we're most interested in
> happen to be the three very worst cases for any kind of IR thermometer
-
> even the most expensive.
>
> 1. Air temperature: you can't measure it with an IR thermometer -
you'll
> always see the temperature of the surface behind the air current.
> Alternatively, you have to put some small object into the air current
so
> it takes up the same temperature as the air, and then measure that
> object.
That's a great idea. A small black painted target would be best.
>
> 2. Shiny objects: anything that looks like a mirror has a very poor
> emissivity. It also *is* a mirror, so the IR energy may be coming
> from something else, reflected by the object you're trying to measure.
(I
> guess a laser pointer might be a good warning that this may be
> happening.)
One can paint a small black spot on the target. I did a bit of work
with a company that developed an Optical IR unit about 20 years ago;
They sold cans of standard emissivity black to mediate this source of
error.
> 3. Glass - even worse than metal!
>
> All these weaknesses come together if you're trying to measure the
anode
> temperature of a 4-Something or a 3-500Z. *You* can see the anode
> glowing behind the glass, but what the IR thermometer sees will be
> anybody's guess.
IR doesn't radiate "through" standard glass. It heats the inside and
then travels by conduction to the outside. So, you can not measure the
temperature of the anode structure even though you can see it.
Again, painting a black target will facilitate measurement of the
outside temperature of the glass. Of course, that black spot will
radiate better than the rest of the glass envelope and therefore be a bit
cooler. I leave it to the thermodynamics department to calculate the
correction factor. :-)
73 & Good morning,
Marv WC6W
*
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