Temperature finding question

[ahidley]

OK. I want to heat treat something. The way I did it in the past was to harden it to max then temper it via a chart I have. The chart has colors, i.e. straw, light brown, brown, light blue, blue, etc, and it has temps 450, 500, 550, 600, etc... I know a specific color equals a specific temp.
If I were to get a laser thermometer that has a Temperature Range : -32C to +550C (-25F to 1022F);Accuracy : +/-1.5% and take a reading when the flame from the torch is on it would it read the flame temp or the metal temp? I understand it needs to soak through for a specific time per thickness, but thats not the question.
The thermometer used is Laser Temperature Gun Non-contact Infrared IR Thermometer

[lakeside53]

It will read the flame... take the flame away from the work for a second.

[Willy]

I'm not sure how well the IR temp guns respond to picking up the heat off of a flame as I have not tried it myself. It would be a simple task to find out. It would also seem simple to just briefly move the flame away from the area that needs to be monitored for temperature.

Also bare in mind that an IR temp gun functions by picking up the infrared energy radiated by the material under scrutiny and that different materials vary greatly in their ability to radiate this energy. Just as important is the finish of the material being measured. Huge difference in emissivity between a rough dark piece of steel and one that is polished or in the "white". Not compensating for this will lead to disappointment as the readings will not be anywhere close.

Lot's of folks are not aware of this and blindly accept the gun's readings or think the gun has a fault because they know the reading must be wrong.

Have a look at pages 8&9 in this short pdf file to get an idea of the great differences in emissivity values of steel with differing finishes.

http://www-eng.lbl.gov/~dw/projects/...missivity2.pdf

[boslab]

As said, I'd get a digital multimeter with a type k socket, or a dedicated temp meter along with a metal sheathed K probe, the metal clad are a bit slower but ok, wire wrap the thermocouple to the bit of metal,
I've even used a digital panel meter jury rigged before now
Mark

[Arcane]

Or you could use temperature indicator sticks.

[lakeside53]

I have sticks, thermocouple and "laser" ir... I reach for IR 'cos it's so easy (and dirt cheap). But +10 on surface emissivity issues... I ran into that last week heating up a cylinder block. Big difference between machined and "cast black" surfaces.

[MichaelP]

Most of the IR thermometers have an ability to adjust readings for different materials. On top of it, you can make further corrections by measuring the known ambient temperature of the same part you're going to heat treat. This will not guarantee the perfect results anyway, but should be more than sufficient for your needs.

Keep it in perspective. People manage to heat treat successfully based on the color chart. A decently set IR gun will be much more precise than that.

[MattiJ]

Keep it in perspective. People manage to heat treat successfully based on the color chart. A decently set IR gun will be much more precise than that.

I'm not so sure of that. Emissivity values for steel varies wildly depending on surface condition.

Two of most inaccurate and mis-used temperature measurements in industry are surface thermometers and IR thermometers!

[J Tiers]

I'm not so sure of that. Emissivity values for steel varies wildly depending on surface condition.

Two of most inaccurate and mis-used temperature measurements in industry are surface thermometers and IR thermometers!

Yeah. Seen any red hot aluminum recently?

[wombat2go]

Induction heat treat machines typically use the optical ratio ( 2 color) pyrometers for critical automotive jobs.
They are more accurate than the brightness types, but cost about 3* more.
Steel in air below 600 C ( 1000F) is difficult to measure accurately with a brightness optical pyrometer.
I would suggest the Tempilaq paints and or crayons for home shop/jobbing shop use.

[MattiJ]

Induction heat treat machines typically use the optical ratio ( 2 color) pyrometers for critical automotive jobs.
They are more accurate than the brightness types, but cost about 3* more.
Steel in air below 600 C ( 1000F) is difficult to measure accurately with a brightness optical pyrometer.
I would suggest the Tempilaq paints and or crayons for home shop/jobbing shop use.

2-color pyrometers also often make errors that are magnitude worse than single-wavelenght pyrometer.
2-color pyrometer works nicely if the emissivity value is stable accross wavelenghts (what they call grey-body). Pretty often you have to fiddle the non-greyness/slope adjustment to get any reasonable result because the emissivity is not same for both wavelenghts.
And all hell breaks loose when the relative emissivities change during process, ie bright stainless steel is starting take tempering colors.

Multi-wavelenght...yuick. don't get me started
(been calibrating, repairing and commissioning pretty much every brand and model of pyrometers for last 10 years)