On 4 Dec 2006 06:07:44 -0800, "Denny" wrote:

....
So I purchased an Infrared measurement gun...

fascinating little instrument - did you know that on a clear day with
the air temperature +12.1 F and zero wind, that a 5 foot diameter
black rubber tractor tire facing the clear sky to the North can have a
surface temp of -2.6 to -3.0 F? I didn't but that is what I found...
OK, I digress again

It is not such a digression.

You should do make measurements of the temperature of different
materials that you know are at the same temperature, and see what
results you get from your non-contact thermometer.

The emissivity of the surface is an important factor that limits the
absolute accuracy of these things.

It is an interesting experiment that you have described, though I am
not sure that surface temperature (if it is accurate) alone is a good
indicator of the power flow to the air. For example, would you expect
that the temperatures of natural coloured and black aluminium heatsink
to be the same if dissipating the same (non zero) power in the same
environment? Extending that to your experiment, is dark enamelled
copper wire (as may be used in a coil) a better black body radiator
than bright aluminium (as may be used in a capacitor).

Owen
--

Owen Duffy wrote:
On 4 Dec 2006 06:07:44 -0800, "Denny" wrote:

...
So I purchased an Infrared measurement gun...

fascinating little instrument - did you know that on a clear day with
the air temperature +12.1 F and zero wind, that a 5 foot diameter
black rubber tractor tire facing the clear sky to the North can have a
surface temp of -2.6 to -3.0 F? I didn't but that is what I found...
OK, I digress again

It is not such a digression.

You should do make measurements of the temperature of different
materials that you know are at the same temperature, and see what
results you get from your non-contact thermometer.

The emissivity of the surface is an important factor that limits the
absolute accuracy of these things.

It is an interesting experiment that you have described, though I am
not sure that surface temperature (if it is accurate) alone is a good
indicator of the power flow to the air. For example, would you expect
that the temperatures of natural coloured and black aluminium heatsink
to be the same if dissipating the same (non zero) power in the same
environment? Extending that to your experiment, is dark enamelled
copper wire (as may be used in a coil) a better black body radiator
than bright aluminium (as may be used in a capacitor).

Owen
--

And of course, the magnitude of the surface area over which the power is
being dissipated will also influence the temperature rise of the small
area being measured by the instrument.

Measured temperature differences of the same material and at the same
distance (under different experimental conditions) are likely to be more
accurate than absolute measurements with unknown emissivity.

Chuck

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Owen Duffy wrote:
On 4 Dec 2006 06:07:44 -0800, "Denny" wrote:

...
So I purchased an Infrared measurement gun...

fascinating little instrument - did you know that on a clear day with
the air temperature +12.1 F and zero wind, that a 5 foot diameter
black rubber tractor tire facing the clear sky to the North can have a
surface temp of -2.6 to -3.0 F? I didn't but that is what I found...
OK, I digress again

It is not such a digression.

You should do make measurements of the temperature of different
materials that you know are at the same temperature, and see what
results you get from your non-contact thermometer.

The emissivity of the surface is an important factor that limits the
absolute accuracy of these things.

It is an interesting experiment that you have described, though I am
not sure that surface temperature (if it is accurate) alone is a good
indicator of the power flow to the air. For example, would you expect
that the temperatures of natural coloured and black aluminium heatsink
to be the same if dissipating the same (non zero) power in the same
environment? Extending that to your experiment, is dark enamelled
copper wire (as may be used in a coil) a better black body radiator
than bright aluminium (as may be used in a capacitor).

Owen
--

Actually, unless the heat sink gets pretty hot relative to its
surroundings, a large percentage of the heat loss is convective, not
radiative, assuming we're operating in air at normal atmospheric
pressures. It all gets much more interesting in a good vacuum.

If I got the numbers right, for example, blackbody radiation at 280K is
about 35 milliwatts/cm^2, and at 300K it's about 46 milliwatts/cm^2.
So 20C above roughly room temperature ambient gets you a whopping net
11 milliwatts/cm^2 to radiation. (Fins facing each other don't help
radiation, but do help convection.)

Cheers,
Tom

Owen Duffy wrote:
On 4 Dec 2006 06:07:44 -0800, "Denny" wrote:

...
So I purchased an Infrared measurement gun...

fascinating little instrument - did you know that on a clear day with
the air temperature +12.1 F and zero wind, that a 5 foot diameter
black rubber tractor tire facing the clear sky to the North can have a
surface temp of -2.6 to -3.0 F? I didn't but that is what I found...
OK, I digress again

It is not such a digression.

You should do make measurements of the temperature of different
materials that you know are at the same temperature, and see what
results you get from your non-contact thermometer.

The emissivity of the surface is an important factor that limits the
absolute accuracy of these things.

It is an interesting experiment that you have described, though I am
not sure that surface temperature (if it is accurate) alone is a good
indicator of the power flow to the air. For example, would you expect
that the temperatures of natural coloured and black aluminium heatsink
to be the same if dissipating the same (non zero) power in the same
environment? Extending that to your experiment, is dark enamelled
copper wire (as may be used in a coil) a better black body radiator
than bright aluminium (as may be used in a capacitor).

Owen
--
(I apologize for not thinking of this in time to include it in my
previous posting...)

Remember, too, that what looks "black" to us in the visible spectrum
may be quite different over the spectrum including most of the
radiation, especially when the object is only about 250K to 350K. At
300K, the peak of black body radiation occurs at about 10 microns, less
than 1/10000th the frequency of visible light. I suppose heat sinks
are dyed black more because we think they should look black than
because it does any good thermally. And avoid buying heat sinks that
are PAINTED black; the paint just adds to the thermal resistance.

Cheers,
Tom

K7ITM wrote:
Owen Duffy wrote:
On 4 Dec 2006 06:07:44 -0800, "Denny" wrote:

...
So I purchased an Infrared measurement gun...

fascinating little instrument - did you know that on a clear day with
the air temperature +12.1 F and zero wind, that a 5 foot diameter
black rubber tractor tire facing the clear sky to the North can have a
surface temp of -2.6 to -3.0 F? I didn't but that is what I found...
OK, I digress again

It is not such a digression.

You should do make measurements of the temperature of different
materials that you know are at the same temperature, and see what
results you get from your non-contact thermometer.

The emissivity of the surface is an important factor that limits the
absolute accuracy of these things.

It is an interesting experiment that you have described, though I am
not sure that surface temperature (if it is accurate) alone is a good
indicator of the power flow to the air. For example, would you expect
that the temperatures of natural coloured and black aluminium heatsink
to be the same if dissipating the same (non zero) power in the same
environment? Extending that to your experiment, is dark enamelled
copper wire (as may be used in a coil) a better black body radiator
than bright aluminium (as may be used in a capacitor).

Owen
--
(I apologize for not thinking of this in time to include it in my
previous posting...)

Remember, too, that what looks "black" to us in the visible spectrum
may be quite different over the spectrum including most of the
radiation, especially when the object is only about 250K to 350K. At
300K, the peak of black body radiation occurs at about 10 microns, less
than 1/10000th the frequency of visible light. I suppose heat sinks
are dyed black more because we think they should look black than
because it does any good thermally. And avoid buying heat sinks that
are PAINTED black; the paint just adds to the thermal resistance.

Cheers,
Tom

Shoot, make that "less than 1/10th the frequency of visible light," but
still darned little radiated in the visible region.

There's cool 'speriment using a spherical flask of visibly opaque
liquid to focus IR radiation to a point where you can feel it very
easily. What you see in the visible spectrum isn't what IR "sees."

Cheers,
Tom