[Amps] Centurion cooling improvement ideas for AM service

[Will Matney <craxd1@ezwv.com>]

I thought about this when I was looking at the instrument but figure 
Fluke would have taken this into account. Measuring air flow 
temperature, I use a small pocket held stem thermometer that us used in 
the HVAC industry. They were designed for this and can be mounted over 
the tube or chimney if it's extended. It also comes in handy for 
measuring the core temp on transformers too. Of course if one was doing 
a study using the digital version, the items could be painted with flat 
black engine paint. That should give less reflection on any surface. It 
would work on glass too if you didn't care to put a big spot on a tube. 
Of course it could be removed later.

Will Matney


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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