Forge and Pyrometer

[Jedediah]

So I'm wrapping up my little forge build and a friend had a type k pyrometer with a ceramic sheath he gave me. What I'm wondering is to use one of these things where is the best place to put it and mount it and how much needs to be exposed. I was thinking about putting a second hole some where on the shell and then welding a tube to work as a holder. hopefully these pics will show up and this will all make sense. So any tricks, tips or hints would be much appreciated. 

[Latticino]

I'm planning on doing this with my updated gas forge as well.  A couple of suggestions from my experience with using these thermocouples and covers in glass furnaces:

1. Thermal expansion can be a problem.  The metal will expand and shrink at a different rate than the ceramic sheath.  Suggest you wrap the exterior of the sheath with refractory blanket and leave a reasonable gap between it and the metal tube holder.
2. I'm not sure how well the sheath will respond to rapid thermal cycling.  My glass furnace ran continuously and used to warm and cool over a period of days.  Keep an eye on it and look out for cracks.
3. Type K thermocouples aren't really rated for the high temperatures you will see in a forge used for forge welding (type R or S are better, but quite expensive).  I'm not sure if mine degraded quickly due to the temperatures, but they certainly don't last long term.  The thicker the thermocouple wire of the thermocouple itself the better.  The tips start to look glassy when they are failing.
4. You probably know this already, but you can't just wire it to the pyrometer with regular copper wire.  It needs to be wired up with special thermocouple wire, as used for type K units.
5. An inch or two inside the inner wall should work fine.  You won't get a highly accurate reading, but you should get relative data that will be helpful (i.e. when the thermocouple reads 1500 deg. F the knife blank has just hit the transition zone, the blank might actually be at 1450, but it is all relative anyway)

Good luck, post photos when you finish.

[Alan Evans]

Following advice from IaninSA on here, I found a CEM IR temperature gun which goes from -50˚C up to 1600˚ C (and will also read in ˚F, -58 to 2912) for around £130.00 on Amazon uk. It seems to be very sensitive at high and low temperatures and is much more convenient and versatile than a built in thermocouple...so far I have used it to check my furnace temperatures 1350˚C and a chiller cabinet I was setting up for deer at 4˚ C .  Another useful feature is, it has a very small focus point so you can take a reading off various places along your workpiece if you need to.

Thoroughly recommend one even if you do have a freebie thermocouple to play with.

Alan

[SmoothBore]

Concur.

The portable Infra Red Gun is a far better system.

[John in Oly, WA]

How accurate are those Infrared guns?

[Alan Evans]

Short answer is no idea. Need another one to compare it with!

It seems to give reasonable results...ambient temperature of objects in the room  etc.

My one has a trimming system to cope with the emissivity of different materials, I presume the more expensive ones would also have a similar way to calibrate them.

According to the book of words, less than+/- 3.8% at the top of the range +/- 5˚C between 1300 and 1600˚C

And +/- 1.5%  or +/- 2˚C between -20 to 200˚C

Alan

[Sanderson Iron]

Why do you need a pyrometer?  Are you heat treating with this furnace and need to maintain a constant temperature over a long time? The best pyrometer for hot metal is an eyeball, used centuries before numbers were applied to heat.  It helps for forging aluminum, but the forge will be hotter than the metal, so having a pyrometer in the fire won't help that much.  I've used a pyrometer that's held against the metal, and that reads the actual metal's temperature rather than the fire's.  It helps educate the eye, but the eye is still the result.  

I'm not meaning to down play a fun toy.  Since you've got it, it'll help you in the long run.  Kinda like weighing your iron before and after forging to figure your scale loss--it's an educational thing, and the tool is eventually abandoned for experience.  

That sure wasn't very helpful.  How 'bout put it in the fire occasionally, rather than leave it there to die;  that'd help you learn and save the tool.  Kinda like a rangefinder that way.  

joel