Sciencemadness Discussion Board

Why is there no electronic optical pyrometer ?

metalresearcher - 3-1-2021 at 03:05

An optical pyrometer is a device with an incandescent filament observed by the user against the background incandescense color. The voltage can be regulated until the filament becomes invisible and then it has the correct temperature.

Such a device can also be used through a window of a transparent medium, like glass or quartz, e.g. a furnace peephole.

But the very convenient infrared pyrometers don't work that way, they measure the temperature of the glass in between. That is why I cannot measure temperature through the peephole of my Kanthal furnace or the window of my wood stove.

Why is there no device with a small camera in it which senses the color of the target object and compares it to the known RGB values of color temperatures ? Obviously, when the color deviates too much (e.g. green or magenta) it cannot measure.
Just a color comparison, so when such a hypothetical device is pointed at a tomato (where it is not meant to be used for) would show about 700 C as a tomato color is very close to the color temperature of about 700 C, despite being not the actual temperature.

unionised - 3-1-2021 at 03:27

I think you have answered your own question.
" a hypothetical device is pointed at a tomato ...would show about 700 C a... despite being not the actual temperature.".

I suspect that, until you get things very hot, the RGB values would be all R and not much G or B which would make it difficult.


You could, of course, build an optical pyrometer.
If you know the current and voltage of the heated filament then you can calculate the resistance.
If you compare that to the resistance when it's cold, you can find the temperature.

Twospoons - 3-1-2021 at 13:34

If you make the window of your furnace peephole from silicon then your optical pyrometer should work just fine.

If you wanted to work off color then the best bet would be a spectrometer, with some software to find the best matching blackbody radiation profile.

[Edited on 3-1-2021 by Twospoons]

densest - 3-1-2021 at 22:08

Inexpensive optical detectors are not evenly sensitive over their designated colors. Black body emission is mostly IR and red below (IIRC) 800C or so.

There are optical pyrometers which compare the intensity of visible red against near IR or deep red vs. lighter red. They're not inexpensive.
One system I've seen uses a single detector viewing through an opaque wheel with multiple windows in the wheel containing different filters.
The filters would have to be carefully matched against the expected temperature range.
Someone could probably build something like that for you.
Calibration is left as an exercise for the user.

macckone - 3-1-2021 at 23:43

Try searching non-contact infrared thermometer.
There are inexpensive versions.
Less than US$20 for temperatures between room temp and 150C.
Above that the price goes up.
The get one that goes up to 2000C cost about US$70.

metalresearcher - 4-1-2021 at 01:41

Quote: Originally posted by macckone  
Try searching non-contact infrared thermometer.
There are inexpensive versions.
Less than US$20 for temperatures between room temp and 150C.
Above that the price goes up.
The get one that goes up to 2000C cost about US$70.


Well, this is exactly what I have and works flawlessly until 1500 C and is OK for most cases, *as long as* I don't measure through glass.
And using a similar device for measuring through glass, that is what I am looking for.

Twospoons: Probably you mean silica (quartz) as silicon is opaque.
[s]But where can I obtain small (i.e. 30x30mm is OK) quartz windows ?[/s]
EDIT: Found oon ebay.

https://www.ebay.com/sch/i.html?_dlg=1&_jgr=1&LH_Pre...

But do these sheets really transfer IR so the pyrometer reading is not affected, unlike ordinary glass ?



[Edited on 2021-1-4 by metalresearcher]

Metacelsus - 4-1-2021 at 03:51

Quote: Originally posted by metalresearcher  

Twospoons: Probably you mean silica (quartz) as silicon is opaque.


Actually silicon is transparent in the IR, so it could work.

unionised - 4-1-2021 at 05:30

Given that silica absorbs quite a lot of IR and silicon is transparent to a lot of IR, I suspect that twospoons meant what he said.

Fairly obviously, a silicon window won't transmit IR that would be detected by a silicon photodiode type detector but it should be OK at longer wavelengths.
You can buy silicon windows- for example
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=39...

but a thermocouple thermometer might be cheaper, at least for most of the range. .

macckone - 4-1-2021 at 09:33

A B-type thermocouple will withstand temperatures that will melt fused silica.

Fused silica has a higher melting point than silicon.

In very high temperature ranges a calibrated optical photometer is used for one or two wavelengths.
These are very specialized and they are usually for a single process, like melting iron or bronze.
Different materials give off different spectra in the higher ranges due to quantum effects.
Infrared can be problematic as well but it is less so.

There are number of companies that sell these but they are not general purpose.
They basically say 'process has reached required temperature', not what the temperature is.
They are also not cheap.
Somone was working on an app for android that would measure visible light and determine temperture but I don't think the project got off the ground.

Twospoons - 4-1-2021 at 12:21

I absolutely meant silicon, not silica. Theres a reason silicon is used as a optical material for CO2 lasers - it becomes transparent for wavelengths >1.1um. ZnSe is also used, and has a slightly higher melting point.
The pyrometers have sensors meant to pick up the longer IR wavelengths >5um.

I agree with the comment about using a thermocouple - its much simpler and more reliable. Type B goes to 1700C, R to 1480C, S to 1600C. All based on alloys of platinum and Rhodium, and therefore somewhat pricey.

[Edited on 4-1-2021 by Twospoons]

metalresearcher - 4-1-2021 at 13:16

Quote: Originally posted by Twospoons  
I absolutely meant silicon, not silica. Theres a reason silicon is used as a optical material for CO2 lasers - it becomes transparent for wavelengths >1.1um. ZnSe is also used, and has a slightly higher melting point.
The pyrometers have sensors meant to pick up the longer IR wavelengths >5um.

I agree with the comment about using a thermocouple - its much simpler and more reliable. Type B goes to 1700C, R to 1480C, S to 1600C. All based on alloys of platinum and Rhodium, and therefore somewhat pricey.


Well, I have temperature measurement, I have a few type S thermocouples with alumina sheathings and work excellently in my propane furnace for melting cast iron up till 1550 C and also in my Kanthal furnace up till 1200 C. I prefer S rather than K as the latter can only withstand > 1000 C for short periods.
And I use the infrared pyrometer when I have direct access (i.e. without a window) to the target.

I just quickly tested a silicon wafer (which I have) and pointed the pyrometer first directly to a hotplate which showed 300 C and through the wafer to the same hotplate and it was 180 C, so this worked partially.

But I think I should give up the idea of getting an infrared window for the pyrometer. The high temperature measuring devices I have are already great.

Twospoons - 4-1-2021 at 19:09

Out of curiosity, was it a doped wafer you tried? Doping might degrade transmission in the mid IR.
If you're not sure, test for electrical conductivity - pure silicon is a poor conductor.

[Edited on 5-1-2021 by Twospoons]