Hi Group; These wirewound resistors are used in gas filter correlation
(GFC) spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18
ohms cold resistance and are electrified by around 18vdc. They visibly
glow a dull cherry red, and I am aware that they operate by thermionic
emission. The analyzer CO or CO2 absorption wavelengths are somewhere in
the 700nm area. I do not know what is the specific heat or color
temperature of the useful IR emission. My questions to the knowledgeable
group contributors are:

1). Is the wire coated with some variety of phosphors, or perhaps
thoriated?

2). Can anyone recommend a particular type of LED as a possible
replacement?

Thanks all.
J.

"Stackclimber" <stackclimber@cox.net> wrote in message
news:V6Iyj.22728$y05.18137@newsfe22.lga...
Hi Group; These wirewound resistors are used in gas filter correlation
(GFC) spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18
ohms cold resistance and are electrified by around 18vdc. They visibly
glow a dull cherry red, and I am aware that they operate by thermionic
emission. The analyzer CO or CO2 absorption wavelengths are somewhere in
the 700nm area. I do not know what is the specific heat or color
temperature of the useful IR emission. My questions to the knowledgeable
group contributors are:

1). Is the wire coated with some variety of phosphors, or perhaps
thoriated?

2). Can anyone recommend a particular type of LED as a possible
replacement?

Thanks all.
J.


I believe the wavelengths of interest are ~4.25 um for CO2 and ~4.67 for
CO.
These are the fundamental and hence most sensitive absorbance wavelengths.
Any shorter wavelengths will be overtones.

I presume the wirewound thing you describe is a "globar." See wikipedia
reference. The material if SiC which has good IR emissivity, I believe.

Common LEDs don't emit that deeply into the IR. There are some exotic
ones,
that are marketed in the US by BEC. These LEDs are fairly costly relative
to a globar and require relatively fance drive electronics, so one has to
have an application that can absorb this cost.

Sincerely,

John

These wirewound resistors are used in gas filter correlation (GFC)
spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18 ohms
cold resistance and are electrified by around 18vdc.

Stackclimber writes:

These wirewound resistors are used in gas filter correlation (GFC)
spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18 ohms
cold resistance and are electrified by around 18vdc.

If you're needing replacements and interested in rolling your own, the
resistance wire is easily had.

Ah, yes Richard. I have some experience in the calculation and building of
industrial heaters. Plain resistive wire is easy to come by, I agree. But
these resistors are something unique, and from a 30X microscopic inspection
of a new component I can see a crusty coating on the wire that does not seem
to be temperature related exfoliation, but instead seems to be a coating of
some sort. <snip

For the heck of it, while waiting for the correct part to arrive, I
scraped and buffed away the ceramic coating on one side of a regular
20 ohm wirewound power resistor and installed it in the analyzer. I
achieved around 15% of the intensity I needed to run the analyzer.

Ah, yes Richard. I have some experience in the calculation and building of
industrial heaters. Plain resistive wire is easy to come by, I agree. But
these resistors are something unique, and from a 30X microscopic
inspection of a new component I can see a crusty coating on the wire that
does not seem to be temperature related exfoliation, but instead seems to
be a coating of some sort. I wonder if the coating is a phosphor, or maybe
a thorium compound to enhance emissivity.
For the heck of it, while waiting for the correct part to arrive, I
scraped and buffed away the ceramic coating on one side of a regular 20
ohm wirewound power resistor and installed it in the analyzer. I achieved
around 15% of the intensity I needed to run the analyzer.
No, I think John Jarvis is essentially right except that this component
is wirewound, and likely on a silicone carbide bobbin using phosphorized
or thoriated tungsten wire. The classic Globar appears to be not
wirewound, but maybe its most conductive layer is a thick film deposition.
I don't know yet. I'm learning as I go.
The original IR source seems to be made to fail. Its a high-failure item
and very pricey.
Joe

"Richard J Kinch" <kinch@truetex.com> wrote in message
news:Xns9A571A806E35someconundrum@216.196.97.131...
Stackclimber writes:

These wirewound resistors are used in gas filter correlation (GFC)
spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18 ohms
cold resistance and are electrified by around 18vdc.

If you're needing replacements and interested in rolling your own, the
resistance wire is easily had.

Years ago I discovered (by accident!) how to put this coating on. Hang
the resistance wire
vertically then just ramp up the power supply till the wire glows
white hot for a few seconds. When
it cools down again you will find a grey oxide coating - this enhances
the IR emissivity. Even better,
however, it enables you to close-wind the heater without any ceramic
spacers to prevent adjacent
turns short-circuiting. Magic!

Brian

----------------
I remember reading some stuff about IR sources in a several years old
Oriel catalogue. This is the best I can find now. They seem more concerned
here with avoiding departure from true blackbody radiation. The whole page
might interest you but particularly the last paragraph headed Emissivity.
http://www.newport.com/store/genproduct.aspx?id=376889&lang=1033&section=Detail

This Product Description of Quart Tungsten Halogen sources mentions doped
tungsten filaments.
http://www.newport.com/store/genproduct.aspx?id=378263&lang=1033&section=Summary

The Tech Note near the end of this page mentions modified emissivity of
ceramic elements to suppress out of band interference and then links back
to the products described in my first link above:
http://www.newport.com/store/genproduct.aspx?id=381843&lang=1033&section=Summary

Various sources here. "high emissivity in the Infrared spectral region"
mentioned.
http://www.hawkeyetechnologies.com/irproducts.htm

Here modification of emissivity by control of surface texture feature size
relative to emission wavelength is described under "The Technology in the
Infrared Filaments":
http://www.metax.co.uk/infrared_source.htm

Overall it seems to me a wirewound infrared source mainly needs a finish
that's good and 'black' in the spectral region of interest, although
suppression of emission in other regions may be arranged to reduce
interference. I can't find any other references to doping tungsten for
infrared emissivity purposes.

Scrim
----------------


"Stackclimber" <stackclimber@cox.net> wrote in message
news:_iJzj.12819$QC.8792@newsfe20.lga...
Ah, yes Richard. I have some experience in the calculation and building
of industrial heaters. Plain resistive wire is easy to come by, I agree.
But these resistors are something unique, and from a 30X microscopic
inspection of a new component I can see a crusty coating on the wire that
does not seem to be temperature related exfoliation, but instead seems to
be a coating of some sort. I wonder if the coating is a phosphor, or
maybe a thorium compound to enhance emissivity.
For the heck of it, while waiting for the correct part to arrive, I
scraped and buffed away the ceramic coating on one side of a regular 20
ohm wirewound power resistor and installed it in the analyzer. I achieved
around 15% of the intensity I needed to run the analyzer.
No, I think John Jarvis is essentially right except that this component
is wirewound, and likely on a silicone carbide bobbin using phosphorized
or thoriated tungsten wire. The classic Globar appears to be not
wirewound, but maybe its most conductive layer is a thick film
deposition. I don't know yet. I'm learning as I go.
The original IR source seems to be made to fail. Its a high-failure
item and very pricey.
Joe

"Richard J Kinch" <kinch@truetex.com> wrote in message
news:Xns9A571A806E35someconundrum@216.196.97.131...
Stackclimber writes:

These wirewound resistors are used in gas filter correlation (GFC)
spectroscopy (CO2 and CO gas analyzers). They have maybe 16 to 18 ohms
cold resistance and are electrified by around 18vdc.

If you're needing replacements and interested in rolling your own, the
resistance wire is easily had.

Brian schrieb:
Years ago I discovered (by accident!) how to put this coating on. Hang
the resistance wire
vertically then just ramp up the power supply till the wire glows
white hot for a few seconds. When
it cools down again you will find a grey oxide coating - this enhances
the IR emissivity. Even better,
however, it enables you to close-wind the heater without any ceramic
spacers to prevent adjacent
turns short-circuiting. Magic!

Brian

I think you have just oxidized the filament. This improves the emissivity
in the IR a lot and is also a protection against further changes
(=stabilization)

Have a look also to this product:
http://www.leister.com/axetris/sensors/irsource/index.html

Urs