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My question is: which thermocouple type should I get and which sheath material should I choose? Does anyone have any specific recommendations? The use case is accurate temps during forging and initial heat treats. I tend to over complicate and/or over build. I have a 2 burner Mighty Forge. The blacksmith shop I purchased it from said they have measured it at our elevation at 10psi at 2600° - So, it gets way hotter if needed. I want to be able to get it this hot and maybe hotter without worrying about burning up my probe. 




I have been poking around for a total of about 8 hours on thermocouples for forges and reading a ton. From what I can understand there are many types of thermocouples that could work in a forge, depending on the planned use and operating temperature. The most common high temperature thermocouples are (K, N), (R, S, B), and (C, D, G).


If anyone has an conflicting information on the atmosphere types these devices may operate within, please let me know. I actually found many places where information conflicted. For example, some places state that a type C can exist within reducing atmospheres, where others only mention inert or vacuum as acceptable. 


Nickel Alloy Probes:


Type K (Type N is similar to K but not as performant, so we will skip its details)

  • Nickel-Chromium 10% / Nickel-(Aluminum and Silicon 5%)
  • -454 to 2300°F
  • Max continuous temp: 2000°F
  • Inexpensive, reliable, accurate, wide range and very common
  • Not supposed to be used in reducing atmosphere
  • Cannot be used in a vacuum
  • Green-Rot phenomenon may occur due to low oxygen level for the thermocouples which are used between 815°C to 1040°C (1500°F to1900°F).



Platinum Rhodium / Platinum Probes:


Type R

  • Platinum Rhodium 13% / Platinum
  • -50°f to 2700°F
  • High accuracy and stability
  • Slightly better than type S in terms of accuracy and stability
  • Can be used in inert and oxidizing atmospheres 


Type S

  • Platinum Rhodium 10% / Platinum
  • Very High Temp, short term up to 3092°F 
  • Continuous 2912°F
  • Thermocouple grade wire -58 to 2700°F
  • High accuracy and stability
  • Can be used in inert and oxidizing atmospheres 


Type B

  • Platinum Rhodium 30% / Platinum Rhodium 6%
  • 32 to 3100°F
  • Highest accuracy and stability at high temps
  • Best to use at 1100°F and above
  • Cannot be used sub-zero, very low response time under 500°F
  • Inert and Oxidizing Applications


Tungsten-Rhenium 5% / Tungsten-Rhenium 26% Probes:


Type C (Type D is so very similar we won't discuss it separately Type D is 3%/25%, Type G is Pure Tungsten / 26% Rhenium) 

  • -32 to 4208°F 
  • Can only be used in a reducing or inert atmosphere
  • Do not use in an oxidizing atmosphere, they rapidly oxidize
  • Not for use below 750°F 


Now, all the above temp limits and atmosphere requirements are for bare wire. If you want more technical information, this is a good read. However, none of these would work correctly in home forge conditions without a sheath as I understand the findings. So, now we have complicate matters greatly and figure out what type of sheath we need combined with the probe. This is where it gets confusing. I found a great solution for $2,400, that is tad bit over my $200 budget. 


Insulation & Sheaths:


I got tired of writing everything out in this post, so I lazily just copied and pasted from a reliable source:











Crazy extra things:






So, after looking at this, hafnium oxide insulation with a molybdenum junction and sheath 




So, unless I am just way to extra, as my kids would say, I would want a type C probe with hafnium oxide insulation, a molybdenum junction and sheath. I have RFQ's out to two companies and Omega has a version for $329. Expensive for sure. I would welcome any thoughts. The one I settled on that would be long enough to enter the forge and have enough clearance to not burn up the leads was XMO-W5R26-U-125-30-H-RP-6, you have to read the data sheet to put that together. This would allow me to operate the forge as hot as it could go and have room to spare. I am going to continue searching but I am hoping someone just tells me I am crazy and to order some $50 probe and be done. 


Side note: If I were picking a probe for a HT oven, I would NOT choose this probe type. That would be different. I would end up with a Type R of some sort. 





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I used a type K for my forge. It was an OmegaCladXL from Omega. Resistance to scale was excellent.


I now use a kiln and put a type S in. It's impervious to oxidation, but being made out of ceramic, it could break if not careful.


Edit corrected a typo

Edited by Joël Mercier
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Yeah, you're WAY overthinking things. :)  This gets asked every so often, and the main thing to remember is you're not going to be leaving the probe in the forge at way over welding heats for long periods.  If you plan to do that, you need a thermowell and things get nuts, expense-wise.  A handheld Omega type K with OmegacladXL for the probe, choose your length based on comfort and HT forge size.  Mine is 24" long to cover a 48" furnace.  I wouldn't recommend going over 2200F very often with it, but unless you're doing some sort of lab calibration stuff or writing a scientific paper about cast steel, once you're over 2000 F you can eyeball it close enough.  You won't be leaving stuff in the forge for long at those temps, unless you're HT-ing some stainlesses.  


For continuous monitoring of heat treat temperatures, a type K Omegaclad will last forever in the forge. The readout part is where it gets expensive here.  A lot of guys use a cheap one from Auberins, I have a nice two-channel handheld from Omega that cost around $120. The probe was $65, so the whole package was sub-$200 and it does exactly what I need it to do.  


Here's a LOT of info:




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So, yes, I was envisioning something that would stay inside the forge at all times, like drilled through and permanently mounted. I did read some of those posts before posting. No one seems to be talking about the environments the probes are designed to tolerate much. This is a very important aspect of selection. Type K probes are not supposed to be used in reducing atmospheres. I was planning on using an Arduino, which I already have many of, and an Adafruit MAX31856 ($20) to read the temp and then output on an LCD screen or segmented display, which I also have. I would have to do some modifications to the library to read Type-C probes with the Adafruit board, but I think I could get it to work. I am under the assumption that I can put the probe I found directly into the forge with no temp well and be just fine. 


I guess I also need affirmation of my assumption that the forge will be operating in a reducing atmosphere 99% of the time.

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