Imperfect black body cavity Temperature measurement

[Onno]

I have a question regarding the so called "black body cavity".
I am working with small Molybdenum cups (height 6 mm; d = 4 mm; bottom thickness 2mm; wall thickness 0.2 mm) A set of stainless steel radiation shields (3) is mounted around the cup in its vertical direction. The cup top opening is shielded with 2 thin stainless steel plates with a 1 mm hole in it. The bottom is open and is heated by a 2 mm diameter infrared laser beam to temperatures between 900 and 1300 C. All takes place under ultra high vacuum conditions. We use sapphire windows for access of the laser beam and extraction of the light through the top cover 1 mm holes.
We want to measure the temperature of the cup with a pyrometer using the light emitted through the top holes in the steel cover plates If this system were a perfect black body cavity we would use an emissivity on the pyrometer system equal to eps = 1. However, this cavity is not perfect. As said it is open at the bottom for access of the heating laser beam. It looses heat through the radiation shields around the cups vertical dimension. Furthermore the top hole is 1 mm and therefore is a significant heat loss.
We tried extensively to measure the cups temperature under a laser load by mounting a type C thin wire thermocouple. This did not produce accurate results at all due to what we think is annealing of the thermocouple wire, diffusion of Molybdenum into the wire, and oxidation of the wire after venting to atmosphere.
Are there perhaps any suggestions on how to measure the emissivity and thereby temperature of this "imperfect cavity" using a pyrometer.

Thanks for sharing your ideas and thoughts on this.

 

[Compositepro]

A different type of thermocouple might work, like type N.

For optical type of pyrometer to work you need to use a multiple wavelength principle which calculates temperature from the ratio of energy emitted at differing wavelengths. This eliminates errors due to emissivity and window absorption. It basically measures color temperature. The simplest and original form is the hot wire pyrometer. There are now very expensive electronic versions.

 

[hacksaw]

Have you corrected for the emissivity of the windows?

From you problem description, how is the t/c wire being oxidized if exposed to high temperature in uhv?

Back in the day we used vapor deposited type K's they worked fine, oxidation in uhv was not a problem, but we did have to correct for the intervening emissivities.

How have you calibrated the emissivity factor for your geometry, you realize that there are specialized sources (passive targets) with calibrated emissivities

 

[Onno]

We use type K thermocouples in another set up where a convection oven is used at around 950 C. The type K temperature measurement in that "atmosphere" set up works with great stability. The type K is indeed stable in an oxidizing atmosphere. Howvever, a type K used in vacuum is just not stable. Its characteristics just change and change and change in a "reducing" uhv environment. Type C is mounted in another vacuum oven setup also operating at high temperatures, up to 1800 C. In this set up however the type C is always under vacuum and has a robust diameter of 1 mm. It performs stable. The type C and K we tested in the laseroven were of bare thin wire 0.1 mm and as said degenerate as we speak.

A hot wire pyrometer is not a practical solution since we have to compare the color of the heated object with a standard i.e. the hot wire. This presents a major problem because we are dealing with a heavy duty IR source which is quite harmfull if observed by the naked eye. Any in between filtering will obvious change the color temperature and add additional problems.

The windows we use have a transmission of 99% for visible and IR light so we guess that this window is not too involved in a correct temperature measurement using a pyrometer.

So returning back to the original question: we know we use an imperfect black body cavity, what can we do practically using a simple pyrometer to study the deviation. We had the following idea yesterday. Suppose we mount a small carbon disc at the bottom of the cup. The emissivity of the carbon disc is nearly eps =1. Different measurements using a pyrometer can then bemade on differnt (empty and with carbon disc) cups.

 

[ChipOatley]

This is not my area of expertise, so I should probably shutup, but I'm not that smart.

If ? = ~1 for the carbon disc, then ? = ~1, and the carbon disc will be hotter than the cup material, assuming there is a significant difference.

The closer you can get to a closed cavity, the closer it will act as a black body. Is it possible to close the bottom, leaving only a 2mm dia hole for the laser?

Can you eliminate the top hole and set up the pyrometer to view the cavity off-axis through the opening for the laser beam? An IR filter could be used to view the cavity if the pyrometer works at the unfiltered frequencies.

Finally, is it possible to make the cup a hollow sphere rather than a cylinder? The view factor between the cup and the rest of the world would be lower.

If I'm off base here, perhaps one of the real experts can make this a teachable moment.

 

[hacksaw]

carbon targets work

"Emissivity values in tables have been determined by a pyrometer sighted perpendicular to the target. If the actual sighting angle is more than 30 or 40 degrees from the normal to the target, lab measurement of emissivity may be required.
In addition, if the radiation pyrometer sights through a window, emissivity correction must be provided for energy lost by reflection from the two surfaces of the window, as well as absorption in the window. For example, about 4% of radiation is reflected from glass surfaces in the infrared ranges, so the effective transmittance is 0.92. The loss through other materials can be determined from the index of refraction of the material at the wavelength of measurement."

develop your calibrations over a range of heat loads

 

[Onno]

for. Oatley

The cup is topped with a cover that has a small hole. If I assume a perfect black body cavity, a carbon disc inserted in the cup will be as hot as the cup is. Any deviation from perfect black body behaviour will make the carbon disc less hot that it's cup surroundings I presume. We have thought of closing off the bottom laser entry area as much as possible to reduce radaiation loss. But since we have a multi mode YAG laser that we are not able to manipulate confidently with regard to its beam size we did not put any effort in that yet. For the future if we can measure beam size and shape, maybe. The cup, as said is very small and made of Molybdenum which is rather difficult to machine. So we do not envisage to make a sphere. The sphere also has as disadvantage that mounting radiation shields around the cup will be of sphere shape also. Way too difficult.

for Hacksaw

If one does a pyrometer measurement of an open, heated, flat surface we would indeed rely on emissivity tables. However in our opinion the Molybdenum cup is a cavity and therefore will tend to have an emissivity tending towrd the one one for a black body cavity. The cup will not display an emissivity belonging the bare metal Molybdenum.

It is clear now from several remarks made that we will have to investigate the window properties to get a proper idea of the influence on the transmission of radiation from the cup system through that window.

 

[IRstuff]

I think you are carrying the "perfect" blackbody paradigm too far and, misapplying it in some cases:

"If I assume a perfect black body cavity, a carbon disc inserted in the cup will be as hot as the cup is. Any deviation from perfect black body behaviour will make the carbon disc less hot that it's cup surroundings I presume. "

Even in a less than perfect situation, the carbon disk would be "close enough," which is something that you've yet to elucidate. Just what is your accuracy requirement? 1 degree, 10 degrees? If the latter, you are probably already close enough.

A perfect blackbody cavity is useless for normal usage, since it requires the cavity to be free of openings. All practical blackbody cavities have openings, ala the high-temperature sources, which typically have an opening on the order of 1/2 inch for a cavity that is less than 3 inches in any dimension.

Your openings are already miniscule. Just how much power are you putting into the crucible? A simple area ratio would determine the amount of heat leakage, and the amount of possible heat loss. If it's less than 10%, that's probably close enough, and a pyrometer can simply correct for the difference.

TTFN

FAQ731-376