Re: [Amps] Centurion cooling improvement ideas for AM service

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Will Matney wrote:

> > The Fluke Model 61 is $109.  It shines a laser beam where it is 
> > measuring the temp.
> > http://www.newark.com/product-details/text/CD121/49571.html
> Will miracles never cease! I never thought I would see the day a Fluke 
> instrument would be that cheap. I figured that being a Fluke would have 
> been $350 to $450 dollars. Thanks for the info on this Rich, I may just 
> get one of these myself! The model 65 is only $289 and has memory with 
> temp conversion. The 61 is really all ones needs without getting 
> extras. That would come in handy around the house checking for heat or 
> cooling leaks around windows and doors. I would like this for not only 
> checking tube temperatures but the temps on transformer coils and cores.

Sorry, but miracles are still in short supply. IR thermometers have 
several weaknesses, and all the cheaper models (including the Fluke 561) 
have an additional major problem: they assume a fixed value for the 
thermal emissivity of the surface they're looking at. If that fixed 
value is incorrect, the computed temperature reading will be incorrect 
too.

The Fluke 61 assumes 0.95, which is good for many kinds of dull 
surfaces, but the emissivity of bright metal surfaces can be 
significantly lower than that, so the meter will read low too.

You can of course calibrate the meter by pre-heating a similar surface 
to a temperature that you're also measuring some other way, but on a 
fixed-emissivity instrument the temperature error will not be constant - 
the error itself will vary with temperature. With a more expensive 
variable-emissivity instrument, you still have to do the same 
calibration, but there is an emissivity setting that you can adjust to 
make the meter show the true temperature; it will then read true across 
the whole range.

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.

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.)

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.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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