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[Amps] Centurion cooling improvement ideas for AM service

To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] Centurion cooling improvement ideas for AM service
From: Will Matney <craxd1@ezwv.com>
Date: Thu, 26 Aug 2004 10:32:40 -0400
List-post: <mailto:amps@contesting.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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