Infrared temperature sensor for TIG welding

Date 01-18-2008 21:30

Hello vscid, this may be slightly off the question that you are asking, but you should consider this when you are wanting to use an infrared type temperature sensor system. Some of these units can be swayed heavily by the reflective surfaces of the material being tested, some even require the tested surface to be painted with a special coating before taking temperature readings from them. Be sure to check and consider this when you go to select a particular unit. Hope this helps if only a small amount. My $.02. Best regards, aevald

By vscid

Date 01-18-2008 22:25

you are absolutely right. your reply helps me confirm my apprehensions.
the thing is , I cannot use the paint, so is there any other option? other temp. measurement technology?

Rgds

By rlitman (***)

Date 01-18-2008 22:41

Which brings us right back the original question.
Reflectivity is roughly the inverse of emissivity. So, with varying reflectivities, you would need a sensor with variable emissivity (and some way to determine either the reflectivity, or the emissivity) to get accurate readings.

A special coating (or just about any uniform coating or surface treatment), could be used to fix the emissivity to a specific value, making measurement more accurate.
Of course, that wouldn't be conducive to welding over, but it depends on what you're measuring. Is it possible to get temperature readings from the back side?

Keep this in mind though:
For high values of emissivity (which can range from 0 to 1, so .9 would be a high value), changes in emissivity would register as small changes in temperature readings.
Unfortunately, for low values of emissivity (like say .2, which is reasonable for shiny metal), even tiny changes in emissivity can result in vast changes in temperature readings.
So, the shinier your surface, the harder it will be to collect consistently accurate readings.
As an example:
I've got a Raytek mini-temp thermometer (its a small handheld IR thermometer, with a fixed emissivity of .95, but a maximum temperature reading way to low to use for welding).
If I put two pots on the stove, filled with boiling water, the anodized aluminum put would measure around 205F, but the shiny stainless pot might only read as 120F.

If you have a variable emissivity sensor, and some way to calibrate it, it can work well. You just need a good sample to measure off of, that reflects like the weld you will be reading later, but at a known temperature (room temp is ok, but the closer to working temps, the better).
So, if you could determine the emissivity off of a sample welded part taken from an oven, you can get pretty close to the mark, so long as the real part "looks" the same, and with the cleanliness of TIG, that should be a reasonable assumption.

By aevald

Date 01-18-2008 23:18

Hello rlitman, very good information and nicely covered. As I believe you somewhat eluded to, the calibration issue and surface continuity conditions would be the only real trip-ups to implementation of this sort of regimen. I appreciate the explanations and suggestions as I am now armed with a bit more additional information in this area. Best regards, Allan

By Tommyjoking (****)

Date 01-19-2008 07:34

neat topic,

Can I add an aside question (because I am lazy and pressed for time) do lazer temp guns read under the same constraints as infrared guns?

By rlitman (***)

Date 01-20-2008 01:57

Yeah. The laser is just there to aim the lens. Actually, most non cryogenically cooled thermal imagers (aka FLIR), work on the same principle.
There is just a tiny sensor (typically a thermocouple), in an evacuated chamber, with a lens that focuses the IR onto it.
For a laser temp gun, there is only one sensor, but for an imager, there is a grid of sensors called a microbolometer.
With the sensors isolated from each other, and in a vacuum, the only thing affecting their temperature is IR radiation.

All objects with heat emit IR radiation. For a "black body" (the ideal situation of e=1.0) the spectrum emitted is easy to profile.
Objects with lower emissivities emit less, and at lower energy levels. That's why the glass on your thermos bottle is silvered. :)

By Flash Date 01-19-2008 20:35

Good Ol, thermo couples do not suffer from errors associated with shiny surfaces
R
Flash
www.technoweld.com.au

By vscid

Date 01-21-2008 13:47

rlitman,

highly appreciate the painstaking explanation. your example about the 2 pots was simple and very effective.
as regards your question:

"......Of course, that wouldn't be conducive to welding over, but it depends on what you're measuring. Is it possible to get temperature readings from the back side?........"
I am measuring temperature inside a nuclear reactor pipe. No, my welding equipment design does not allow me the measurement of temp. on the back side.

By vscid

Date 01-21-2008 20:07

Digging further into the discussion, I mentioned earlier that I will be experiencing varying emissivity. But what is this varying nature of emissivity realy related to?
Is it more of a function of the alloy chemistry of the filler metal ? (I am using alloy 52M:chemistry: Cr 30, Fe 10, Ni 57, rest other metals: all approximate values). Or is it more of a function of the surface characteristics (whether its a polished or oxidized surface)? The surface of the bead would definitely not be a shiny surface, so should that be assumed as a oxidized surface?
In that case, the margin for error would be reduced, isn't it?

By rlitman (***)

Date 01-21-2008 21:15

Well, this is way outside of my expertise, but let me see how far I can take it comfortably:

Yes, emissivity is partly a function of alloy. A shiny black surface will have a higher emissivity than a shiny white surface. Its sort of related to "color", except that we're talking about color within the far infrared spectrum, and visible color is not necessarily relevant.
I know that a gold mirror has a higher infrared reflectivity than an aluminum one, whereas its the opposite of that in the visible spectrum.
But, I have no clue as to how much slight changes in alloy will effect it (i.e. will those changes have any significance).

On that note, surface oxidation may not be clearly visible (such as the passive layer on top of stainless or aluminum), but I'm not sure how that works with Inconel.
A quick search on "inconel emissivity" on google, shows a number of tables of emissivity, which show approximate values, at different temperatures, and both oxidized and clean.
Some of the charts I saw, seemed to show that inconel's emissivity varied widely depending on temperature, so my advice on calibrating against a room temperature object would be bad.
Anyway, if you have a thermocouple (I'm guessing that's what you're using now?) handy, with a variable emissivity IR sensor, you could calibrate it in the range of your target temperature, and so long as you always aim the lens at similar surfaces, you should be pretty close.
The charts I see, show inconel ranging from around .2 through .4. That's low enough that you will need a sensor which you can set the emissivity (a laser temp gun typically set at .95 will be useless), but not so low that good readings will be difficult (like the .03 of polished aluminum, or the .06 of chrome).

-- Robert