Some questions about thermocouples in a gas forge

Jonas Liebel

Jonas Liebel

Joined
Jan 20, 2018
Messages
34
Hi there folks,

I started using my newly built gas forge in which I installed a thermocouple for heat control.

Screenshot_20180319-164749.png

Now after using the forge a few times, some questions regarding the temperature displayed by the thermocouple arose, mainly:

Is the temperature that is displayed by the termocouple (for example 1472F) the same temperature that the tip of it currently is heated to? You know, if the tip of the termocouple glows orange, is that 1472F the temperature of the orange glowing tip or does it measure the air around it?

Generally I was quite unsure about when my blade had the right temperature, and whether or not it had a higher temperature than displayed by the thermocouple.

Does anyone have some information on that?
It would be of great help!

Thanks guys,
Jonas
 
I can't help but I would like to know exactly what thermocouple and read out you are using.
 
Welcome to Shop Talk.

Answers to your question and some general notes:

The leads on the TC can be shortened as needed. The tip is the only place that reads temperature. Use an 8 or 10 gauge TC.

In a forge it is best to use a ceramic TC sheath. If you don't the TC will have a short life.
https://www.ebay.com/itm/High-Tempe...811983&hash=item1a1d768a00:g:tQkAAOSw9mFWLntX


Use a TC mounting block for best connection.
ttps://www.ebay.com/itm/High-Temperature-Ceramic-K-type-Thermocouple-Terminal-Block/263404024834?hash=item3d541a7402:g:vSkAAOSwzXxaQSF2

Use Type K for forges - TC, block, and wires. They are polarized, so assre proper connections.

The tip is what is being shown on your display. So, if the tip glows what looks orange to you and the display says 1472, then that color is what 1472 steel will look like.
If it is sitting in a flame, it will be showing the flame temp. If it is in a ceramic sheath, it reads the temp of the sheath, which is more like the temp of a blade sitting near the sheath. If your forge is well made and the chamber soaked at heat for 10-15 minutes, the TC and chamber will likely be close to each other.

You can buy the TC, wire harness, connection block, PID, SSR, and heat sink ina package on ebay for about $67. Add a sheath and you have almost everything you need to set up your forge for temperature control. You will need to add a solenoid and some valves to your burner to make it run. There is a lot of info and discussion on this in the Sticky on PID controlled forges.
 
Unfortunately I can't give you a proper description in english since I live in Germany, but the readout says it's "type k"

The thermocouple I use is for measuring the temperature of gases.

Edit:
Thanks Stacy.
I will see if i can get some of these Items in Germany, why will the TC have a short life if i just use a steel pipe?
 
Jonas Liebel said:
Unfortunately I can't give you a proper description in english since I live in Germany, but the readout says it's "type k"

The thermocouple I use is for measuring the temperature of gases.

Edit:
Thanks Stacy.
I will see if i can get some of these Items in Germany, why will the TC have a short life if i just use a steel pipe?
Click to expand...

You mean without a ceramic sheath? Because the probe will oxidize in the forge environment. In my experience, first by giving you unreliable measurements and then by failing completely. I use these in my gas fired forges:

This probe:
https://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=39

This sheath:
http://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=40

The downside is that inside the ceramic sheath, it responds to temperature changes a little slower than an exposed one. That can be accounted for rather easily on a manual set up, I assume the same with a PID.

3fifty7 3fifty7 I use the two things above with this handheld readout in my forge, or with a liquid proof probe for my quench oil and parkerizing tank: https://www.amazon.com/gp/product/B06XC6T8PX/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
 
It will burn up due to hot gas erosion. In an electric Heat Treatment oven, the TC does not need to be in a sheath because there are no combustion gasses moving across it rapidly. In a forge, an unprotected TC may last anywhere from a few weeks to a couple months, depending on your usage. If you are running at welding temps, it won't last more than a few days.

There is another reason that the sheath helps. If you are using a PID to control the burner, the sheath smooths out the flame temp changes which play across an exposed TC tip.
After running for 10-15 minutes, the PID will have "learned" the curve to regulate the chamber temp to as close as +/- 2F.
 
Do you have the TC connected to a PID controller, or are you simply using it to measure temp?

I normally recommend to place the TC underneath the metal in the forge, instead of permanently mounting it. This is the most accurate reading of the metal temp you will get. As Stacy alluded to earlier, you are learning the color/temp relationship. Once you know what color you are looking for, you won't use the TC as often.

Judging by your forge picture, you will have considerable difference in temp between the left/right side, front/back of the forge. You should try to adjust the angle of the burner to swirl the flame more, this will give you more even temps in the chamber.
 
kuraki said:
You mean without a ceramic sheath? Because the probe will oxidize in the forge environment. In my experience, first by giving you unreliable measurements and then by failing completely. I use these in my gas fired forges:

This sheath:
http://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=40

The downside is that inside the ceramic sheath, it responds to temperature changes a little slower than an exposed one. That can be accounted for rather easily on a manual set up, I assume the same with a PID.

3fifty7 3fifty7 I use the two things above with this handheld readout in my forge, or with a liquid proof probe for my quench oil and parkerizing tank: https://www.amazon.com/gp/product/B06XC6T8PX/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Click to expand...

hmm I never thought of the ceramic sheath...
mine is an Omega with an inconel sheath rated for 2100F -

from the Omega specs
"...while Inconel is rated to 1150°C (2100°F). Both materials are rated “Very Good” in oxidizing, vacuum and inert atmospheres, and rated “Good” in Hydrogen atmospheres. "
 
That may be ok? Outside of a welding forge. I doubt mine is Inconel. Another nice thing about the ceramic sheath is I have one cemented in the welding forge, another in the heat treat drum forge, and I can just slip the probe out of one and into the other, without removing the sheath. Eventually when I do some PID controls I'll want them to have their own but for now doing it manually that's convenient.
 
Inconel or not, a TC rated at 2100F means that it can survive that temperature before some sort of melt failure occurs. In a gas forge the problem isn't the temperature, it is the hot gases. They are extremely corrosive. Just like a cellphone that says it is waterproof and good up to 120F,... stick it in 100F sulphuric acid and it will be gone soon.I have tested some very robust metal clad TCs and they still burn up is a pretty short time. I have found nothing that will protect better than a ceramic sheath.

There are several sizes of sheaths. For a 1/4" metal clad TC, a small sheath with a 3/8" ID will work. The standard 3/4" OD sheaths are more robust and cheaper, though.

I suggest an 8 gauge type K TC with 3" to 4" of ceramic spacers on the leads. The standard sheath holds that nicely. If you get a long sheath, it can be cut down by carefully filing around it with a tri-square file or use a diamond cut of disc on a Dremel. Plug the sheath behind the ceramic spacers with small wads of kao-wool., keeping the leads from touching. Mount a TC block right against the end of the sheath, and cut off the TC leads about 1/2" past the block. Hook up type K TC wires to the block and plug them into the readout or PID control.

Placement of the tip of te TC can vary, but the places to avoid are hot or cold spots. As Charles said, somewhere close to, or under, the blade is a good location. Having a burner and forge chamber designed to get a good swirll and even heating is important. Letting the forge chamber full soak ( absorb heat) is also important.

While discussing the corrosive gases in a forge, a forge muffle will only last so long before it gets eaten up. Don't make it too permanent. I did that mistake and built a stainless muffle into the ends of a forge. It looked good, but eventually I had to tear out the ends and replace it. It did HT wonderfully, though. It was placed down the center of the forge with the flame swirling around it on all sides. The TC was placed in the muffle under the blade. I have a bigger version of this forge put away in parts right now and will be bringing it out to finish up for the new shop, .... hopefully this coming winter.
 
Last edited:
Stacy E. Apelt - Bladesmith said:
While discussing the corrosive gases in a forge, a forge muffle will only last so long before it gets eaten up. Don't make it too permanent. I did that mistake and built a stainless muffle into the ends of a forge. It looked good, but eventually I had to tear out the ends and replace it. It did HT wonderfully, though. It was placed down the center of the forge with the flame swirling around it on all sides. The TC was placed in the muffle under the blade. I have a bigger version of this forge put away in parts right now and will be bringing it out to finish up for the new shop, .... hopefully this winter.
Click to expand...

Better work fast, Stacy, winter is over in about 6 hours. . .

I know it's way off topic, but the other day I saw an all stainless forge on eBay. It had a stainless chamber. I remember wondering how long that would last.
 
Well, I may be able to answer that in a year, too. The HT forge I have set up to build will have two pieces of stainless pipe. One is 7" ID and the other is 11" ID. I plan on pouring Cast-o-lite 30 between them to make a refractory sleeve with a stainless inner chamber wall. I'll wrap that in refractory wool and put on a sheet metal shell. I figure that if the inner wall deteriorates, I will just peel it out when it gets pretty badly eaten away. The idea was to get a very turbulence free flame swirl around the heavy wall 3" ID muffle. We'll see how it works.
 
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