High temperature measurements using Optical Pyrometer

**BobL** · 2nd May 2013, 09:58 AM

I was going to post this in the smithy forum but thought other metal workers might be interested in see it.

It's what is called a disappearing filament pyrometer and is very useful to measure the temperature of hot metal once it is glowing red hot or hotter.

High temperature measurements using Optical Pyrometer-img_3765-jpg

High temperature measurements using Optical Pyrometer-img_3765-jpg

The pyrohead, meter, camera tripod adapter and a range of focal length lenses all come in this very handy if slightly battered lab padded suitcase.

The only thing missing is the original power supply.
I used the same pyrometer nearly 40 years ago and because it was a bit fiddly to use I became very familiar with using it. I also remember that it uses a variable ~3V DC power supply (PS). It will function on 2AA batteries but it can only be accurately calibrated using a variable PS. I don't have a variable DC that goes down to 3V so I adapted a 5 - 12 V PS and a large variable resistor (Grey box on the left below in picture below) to suit.

Here's how it all looks when it's setup
High temperature measurements using Optical Pyrometer-img_3764-jpg

High temperature measurements using Optical Pyrometer-img_3764-jpg

The way it is used is as follows.
The pyro head (black unit with the eyepiece and lens) is a low power telescope that produces a magnified (~x10) image of the target object, and also contains a thin incandescent filament that overlays the image.
Looking through the pyrohead eyepiece the target is brought into focus using the silver knob.
Then the head is moved using the small X-Y adjusters so that the filament is directly over the top of the target.
The black knob on the side of the head is turned which alters the brightness of the thin filament. When the filament colour matches that of the image the filament seems to disappear - this is the point where the temperature can be read from the scale.

Here is what it looks like through the lens.
The image on the left has the the filament slightly brighter than the object so the meter will be reading too high a temperature - adjust the filament current down.
The image on the right has the the filament dimmer than the object so the meter will be reading too low a temperature - adjust the filament current up.
In the middle image the filament brightness and colour matches that of the object (ie looks like it disappears) so the meter will be reading correctly.
High temperature measurements using Optical Pyrometer-pyrofilament-jpg

High temperature measurements using Optical Pyrometer-pyrofilament-jpg

High temperature measurements using Optical Pyrometer-img_3767-jpg

As you can see there plenty of range and sensitivity but I doubt I will be needing any more than the first range (650 to 1400ºC). My experience is that the object needed to be at ~750ºC before it could be measured reliably but that is still a useful starting temp. The unit has a couple of calibration points on the scale whereby it has to be drawing so many mA at specific points in the scale. These points can be checked with a mA meter and the PS V tickled up and down til it shows the right current.

My forge is awaiting a gas line to my shed so I can't play with it yet.

One advantage of a pyrometer like this is it can sit well back from a furnace or forge and still accurate measure the surface temperature of areas of specific objects or parts of objects as small as 1mm^2 to +/- 5ºC so it can be used to check the uniformity of heating of objects like blades. In contrast IR optical scope type thermometers, or thermocouples located on the walls or floor of a furnace, can usually only measure bulk temperatures
Of course it does require having the door of a furnace open so these devices are often used in conjunction with sensors like thermocouples.
I will report back when I make a few measurements

**Kraehe** · 2nd May 2013, 12:06 PM

Do you need perfect colour vision to use one of these?

K.

**BobL** · 2nd May 2013, 12:27 PM

Originally Posted by Kraehe

Do you need perfect colour vision to use one of these?
K.

The readings are related more to intensity than to colour. For ranges two and three a red colour filter is dialled in over the viewfinder and it operates entirely in the red colour range. if you have red/green colour blindness it won't matter as you are not discriminating between red and green but between shades or intensities of red.

**RayG** · 2nd May 2013, 01:27 PM

Hi Bob,

How do you allow for emissivity variations with an optical pyrometer?

I've got a IR thermometer that's good to 1650 C, but you need to set the correct emissivity, which makes it useless for things like aluminium.

Regards
Ray

**.RC.** · 2nd May 2013, 01:48 PM

So does the meter measure resistance of the filament or voltage going to the filament?

**Stustoys** · 2nd May 2013, 02:13 PM

Oh great...... something else I dont have.

Originally Posted by RayG

How do you allow for emissivity variations with an optical pyrometer?

Does emissivity come into it once you start talking about temps high enough for visible light? You're measuring the color of the surface not what is radiating from it(??). I'd always thought that a piece of red hot copper was at the same temperature as a piece of red hot steel? am I wrong again?

Stuart

**BobL** · 2nd May 2013, 02:16 PM

Originally Posted by .RC.

So does the meter measure resistance of the filament or voltage going to the filament?

It measures current.

Here is the basic circuit from the PYRO website. Optical Pyrometer temperature sensor for non contact high temperature measurement
High temperature measurements using Optical Pyrometer-pyro-jpg

High temperature measurements using Optical Pyrometer-pyro-jpg

**BobL** · 2nd May 2013, 02:49 PM

Originally Posted by Stustoys

Oh great...... something else I dont have.

Does emissivity come into it once you start talking about temps high enough for visible light? You're measuring the color of the surface not what is radiating from it(??). I'd always thought that a piece of red hot copper was at the same temperature as a piece of red hot steel? am I wrong again?

Stuart

Yep emissivity is important especially because it varies with temperature and wavelength being measured . My understanding is that around the 1000ºC mark the intensity of the red wavelengths can be used because the emissivity of most metals at these temps in the visible red areas of the spectrum are similar. This is why a incandescent metal filament can be used as a comparator. Above 1400ºC a red filter is used to knock out the other visible wavelengths to prevent them washing out the red wavelengths I have cross checked this pyrometer against a TC and they agreed within 20ºC up to 1500ºC and I did not feel the need to go any further.

**RayG** · 2nd May 2013, 03:15 PM

Originally Posted by BobL

Yep emissivity is important especially because it varies with temperature and wavelength being measured . My understanding is that around the 1000ºC mark the intensity of the red wavelengths can be used because the emissivity of most metals at these temps in the visible red areas of the spectrum are similar. This is why a incandescent metal filament can be used as a comparator. Above 1400ºC a red filter is used to knock out the other visible wavelengths to prevent them washing out the red wavelengths I have cross checked this pyrometer against a TC and they agreed within 20ºC up to 1500ºC and I did not feel the need to go any further.

That's a nice instrument to have around,

I can see that being very handy especially for temperatures above the K type TC 1300C limit. I got the high temp IR with adjustable emissivity, thinking it would work for Aluminium, but no such luck, for Al, I think K type is the best bet. The emissivity problem is with things like Aluminium, which don't generally get up to 1000 C.

I remember a plant tour of Alcoa year ago, the rule was "don't touch anything" Aluminium at 300 C looks just exactly the same as 20 C

Regards
Ray

PS What TC type did you use at 1500C ?

**PDW** · 2nd May 2013, 03:53 PM

Originally Posted by RayG

I remember a plant tour of Alcoa year ago, the rule was "don't touch anything" Aluminium at 300 C looks just exactly the same as 20 C

Yeah, it's a lot of fun oxy-acetylene welding it. First it's not hot enough then there's a hole since it's 'hot short'. We had to be able to repeatably & reliably butt weld 1.2mm aluminium together, why I do not know as nobody in their right mind did it even 30 years ago. All part of the welding ticket at the time.

PDW

**BobL** · 2nd May 2013, 05:46 PM

Originally Posted by RayG

That's a nice instrument to have around,

I can see that being very handy especially for temperatures above the K type TC 1300C limit. I got the high temp IR with adjustable emissivity, thinking it would work for Aluminium, but no such luck, for Al, I think K type is the best bet. The emissivity problem is with things like Aluminium, which don't generally get up to 1000 C.

I remember a plant tour of Alcoa year ago, the rule was "don't touch anything" Aluminium at 300 C looks just exactly the same as 20 C

Yep - working around Al welding BIL he said the first rule in his workshop is to assume ALL AL IS HOT but I still managed to scald myself - fortunately not badly, but enough for me to remember next time.

PS What TC type did you use at 1500C ?

It was a Type B (Pt/Rh) - not very sensitive (~10 µV/°C) so a reasonably good quality meter is required. As you can imagine in a University Physics Lab we had some very fancy meters that went down to sub pico volts. Several that went to 10^-14 V were run in a Peltier cooled stable temp chamber.

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