On Aug 26, 2004, at 12:18 AM, Ian White, G3SEK wrote:
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.
Ian -- Agreed, however, the solution is: do not have the dull-gray
anode stem chrome-plated on the 3-500Z.
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.
The Fluke 61 has a laser beam pointer.
end
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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