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Science Forum Index » Optics Forum » temperature of filament
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| Allan Adler |
 Posted: Mon Jan 07, 2008 2:32 pm |
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What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
For example, can one use a digital camera and analyze the pixels in
a digital photo to accomplish this?
--
Ignorantly,
Allan Adler <ara@zurich.csail.mit.edu>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston. |
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| Don Klipstein |
| Posted: Mon Jan 07, 2008 2:32 pm |
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In article <y934pdpjxk8.fsf@nestle.csail.mit.edu>, Allan Adler wrote:
Quote: What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
For example, can one use a digital camera and analyze the pixels in
a digital photo to accomplish this?
Measure the voltage and current to determine the resistance when it is
hot. Measure the resistance at a known low temperature (without heating
up the filament). Find the ratio. Get a "properties of tungsten" table,
which includes resistivity as a function of temperature. Hope that your
filament temperature is uniform.
- Don Klipstein (don@misty.com) |
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| Helmut Wabnig |
| Posted: Mon Jan 07, 2008 7:02 pm |
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On 07 Jan 2008 13:32:55 -0500, Allan Adler <ara@nestle.csail.mit.edu>
wrote:
Quote: What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
For example, can one use a digital camera and analyze the pixels in
a digital photo to accomplish this?
You need a pyrometer.
w. |
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| urs bögli |
| Posted: Tue Jan 08, 2008 4:06 am |
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Allan Adler schrieb:
Quote: What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
For example, can one use a digital camera and analyze the pixels in
a digital photo to accomplish this?
This depends very much on the accuracy you're looking for.
The resistance=f(temperature) method id very cheap and fine.
There exists also color charts to guess the temperature (orange ~1000C,
red~800°C)
You could use a spectrometer to find the maxima of the emission. Then
you can calculate the temperature.
Good Luck
Urs |
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| Don Klipstein |
| Posted: Wed Feb 13, 2008 7:49 pm |
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In article <y93k5l8yvrl.fsf@nestle.csail.mit.edu>, Allan Adler wrote:
Quote: Allan Adler <ara@nestle.csail.mit.edu> writes:
What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
I seem to think that usually the "best bet" is determining resistance at
room temperature and resistance at operating temperature. Find the ratio
of these, then find a "properties of tungsten" chart.
This will oversimplify things by assuming the filament temperature is
uniform from one connection point to the other, but my experience is that
this does better than anything else.
The main shortfall here is that not quite all of the filament will be at
"full operating temperature". I seem to think that actual "operating
region" of the filament will have temperature only a few to a few 10's
of K hotter than indicated by resistance ratio between hot and cold when
filament lenghth is nice and long so that no more than a small percentage
is greatly cooler than "operating temperature". The biggest error I found
here so far is contact resistance to lamps whose cold resistance is being
determined.
- Don Klipstein (don@misty.com) |
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| Allan Adler |
| Posted: Thu Feb 14, 2008 1:03 am |
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Allan Adler <ara@nestle.csail.mit.edu> writes:
Quote: What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
How cheaply can I get a pyrometer that is adequate for this purpose?
The FP-400 tube had a pure tungsten filament, while a random tube might
not. Also, the FP-400 apparently had a hole in the anode so that one could
use a pyrometer to measure the temperature of the cathode. I doubt that a
random tube would have that property. So, I'd have to assume I was viewing
the filament from various oblique angles. I don't know how that would
affect the measurements. I didn't think up this technical detail myself;
the book of Melissinos specifically draws attention to this feature of the
FP-400, which he assumes one is using in the experiment. I don't know whether
the hole is mentioned as a convenience or as a necessity; I'd believe the
latter, but it is better to ask to be sure.
--
Ignorantly,
Allan Adler <ara@zurich.csail.mit.edu>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston. |
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| Phil Hobbs |
| Posted: Thu Feb 14, 2008 5:25 pm |
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Don Klipstein wrote:
Quote: In article <y93k5l8yvrl.fsf@nestle.csail.mit.edu>, Allan Adler wrote:
Allan Adler <ara@nestle.csail.mit.edu> writes:
What is the easiest and cheapest method to determine the temperature of
a filiment in a light bulb or electron tube (vacuum or gas filled)?
I seem to think that usually the "best bet" is determining resistance at
room temperature and resistance at operating temperature. Find the ratio
of these, then find a "properties of tungsten" chart.
This will oversimplify things by assuming the filament temperature is
uniform from one connection point to the other, but my experience is that
this does better than anything else.
The main shortfall here is that not quite all of the filament will be at
"full operating temperature". I seem to think that actual "operating
region" of the filament will have temperature only a few to a few 10's
of K hotter than indicated by resistance ratio between hot and cold when
filament lenghth is nice and long so that no more than a small percentage
is greatly cooler than "operating temperature". The biggest error I found
here so far is contact resistance to lamps whose cold resistance is being
determined.
- Don Klipstein (don@misty.com)
Pyrometers are best for things like furnaces, where the radiation is
coming from a wide solid angle and is reasonably uniform with respect to
position. Unless you're putting the pyrometer inside the light bulb,
you can't get those conditions in this measurement.
A colour temperature meter is one approach (subtract about 200 C from
the colour temperature to get the filament temperature--hot tungsten is
more emissive at short wavelengths).
The suggestion to use the published properties of tungsten is not a bad
one. Why do you want to measure this? How accurate a measurement do
you need? Do you care about temperature variations along the filament?
Cheers,
Phil Hobbs |
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| Allan Adler |
| Posted: Fri Feb 15, 2008 11:22 am |
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Phil Hobbs <pcdhSpamMeSenseless@pergamos.net> writes:
Quote: Pyrometers are best for things like furnaces, where the radiation is
coming from a wide solid angle and is reasonably uniform with respect to
position. Unless you're putting the pyrometer inside the light bulb,
you can't get those conditions in this measurement.
Melissinos specifically calls for a pyrometer. But the tube he wants one
to use (FP-400) is designed so that one has a clear view of the center of
the filament.
Quote: A colour temperature meter is one approach (subtract about 200 C from
the colour temperature to get the filament temperature--hot tungsten is
more emissive at short wavelengths).
Thanks for the suggestion. How cheaply could I get one and from whom?
Quote: The suggestion to use the published properties of tungsten is not a bad
one. Why do you want to measure this? How accurate a measurement do
you need? Do you care about temperature variations along the filament?
Using the resistivity of tungsten to measure temperature is the other method
that Melissinos recommends. However, if you don't have a FP-400 tube (and
I don't) then you don't necessarily have a pure tungsten filament, so the
calibration curves for tungsten don't necessarily help.
The book of Melissinos (Experiments in Modern Physics) has an experiment
involving thermionic emission in a vacuum tube. One measures current against
voltage and temperature and fits the data against various general formulas
to determine some constants. So, the accuracy has to be good enough to
facilitate this. Melissinos mentions that temperature varies along the
filament and can be 1/3 as much at the ends. He says one has to compensate
for this but so far I haven't seen how one does that; but I'm still reading
the experiment.
I'm doing this on my own, with essentially no equipment at the moment,
and very little to spend on it. I don't intend to purchase anything until
I have thought through how I will do everything and what it will cost.
I have the RCA tube manual and a tube vendor's catalogue, but neither has
information about the composition of the filaments. I also have various books
on circuits involving tubes. So, I expect that I will be able to at least
build some simple circuits (e.g. a simple radio receiver) using tubes. The
next step will be trying to do the experiment on thermionic emission from
Melissinos.
If I get reasonably good at that, it would make sense to go more deeply
into it. For example, maybe, by focusing three digital cameras on the
filament from different angles, I can get a 3-dimensional representation
of the space curve that is the filament. If the digital cameras can also
be used to determine the temperature at each point, then the space curve
can be mapped with the temperature values at each point. That could make
it possible to do very detailed studies of what happens along the filament
in the tube. But at the moment, I still have to learn how to get a tube
to work, once I buy a tube. The tubes I buy to build a radio probably won't
be ideal for the thermionic emission experiment but will give me practice
with tubes and, if I happen to have devices for measuring temperature, voltage
and current, I can also get practice using them on these tubes. After that,
I will be a little better informed and can return to the details of planning
to do the experiment in Melissinos. So, I'd like to try to get inexpensive
ways of measuring temperature even though I'm not ready to do the thermionic
emission experiment yet; it's part of preparing myself to do the experiment.
--
Ignorantly,
Allan Adler <ara@zurich.csail.mit.edu>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston. |
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