thermocouple placement in a forge and lenght

[Vulcanite]

Bout to order some a PID, thermocouple and ceramic protector for it for my ceramic brick heat treating forge so I can accurately monitor temperatures for hardeining etc.

I think Auber has the probes in length of either 6 or 12". The rectangle shaped forge will have interior dimensions of ca 4" wide x7" tall x 19" deep.

Is a 12" probe too long? I mean, does the whole probe have to be in the forge or just the tip? I dont' know if I would put the probe in lengthwise or through a hole i the side or what??

At first, I will just leave a small opening in the back end and blow a torch into the mouth. Later it will feature PID controlled venturi burners, but for now I will be controlling it all manually.

Just want to make sure i order the correct thing.

Thx

[Fred.Rowe]

Place the K thermocouple as much to the center of the forge body as possible. Only the very tip is needed for accurate readings. The less probe you have exposed to the forge heat the better; less to burn up.
I run mine through the side with only the last 1/2 inch exposed. I insulate around the hole where the probe comes through the wall.
Keep the probe tip out of the direct flame from the burners.

[Vulcanite]

THx. I guess the 6" probe will probably be easier to use.


I guess I'll run it through side wall near the top.

I'll lay the blade across a couple little 1 or 2 inch tall pieces of ceramic brick and I'll blow the flame in the forge mouth, above the blade and toward the little gap at the other end. There is space in the top of the forge, above the forge mouth for the probe to be in.

Is it bad to blow the flame right onto the blade? Propane...

[timgunn1962]

The thermocouple is made of two different metal wires, welded together at the tip. A (very small) voltage is generated at the welded junction, which varies with temperature. It's this voltage that is measured by the PID controller and displayed as temperature. The important thing is that the junction is where the temperature sensing takes place, so the overriding requirement is that it be somewhere in the forge that is at the same temperature as the workpiece.

In a gas forge, you'll need the ceramic protection tube between the junction and the forge atmosphere. This has the effects of slowing the thermocouple response and averaging the reading over maybe 2 or 3 sheath diameters, so you need to ensure the last 3 sheath diameters are at the workpiece temperature. I'd aim for about 5-6 sheath diameters of insertion depth if no other constraints apply.

Apart from that, it's really just a case of what is most practical for your application. Sometimes side-entry is best. Often top-entry is best, especially when using sectional insulators on exposed-junction thermocouples in electric furnaces, as they don't offer much support for side-entry.

[Bigblue17]

This has the effects of slowing the thermocouple response and averaging the reading over maybe 2 or 3 sheath diameters, so you need to ensure the last 3 sheath diameters are at the workpiece temperature.

Really!?!?

Thank you!

-Peter

[twalsh341]

I'm hoping to forge and HT 1095 and W series correctly, I'm familiar enough with PID and thermocouple, but can anyone explain a little more about the "sheath diameters" how to measure etc?

[SteelSlaver]

Here is a link to Auberins thermocouple sheath http://www.auberins.com/index.php?ma...products_id=40

It has a ID of 1/2" and OD of 3/4". It will protect your thermocouple in a forge atmosphere. At least 3" or better 4" of it inside the forge will work. It will slow the response time of the setup a little, but once up to temp should keep you within a few degrees. I run one in my HT oven and it has 2" of fire brick and a inch of Kaowool so I run a 12" with 4" of sheath inside the oven 3" through the insulation and a bunch sticking out the top. I used to run a bare thermocouple and changed for 2 reasons. My thermocouple should last longer and inside the sheath it is not as effected by the radiated heat from the coils. The coils in an oven must be hotter than the air in order to raise the heat. While this may effect the response time some it hold fine when at temp with a good PID. When I went to the sheath I got an over run of about 10f while I only had 3 or 4 bare. I compensated for this by using a programable PID and make a set point about 20f lower than my target and then it ramps up the last 20 slowly.

[Fred.Rowe]

Omega makes thermocouples to your specified length.

[Willie71]

Thank you so much everyone. I was going to ask this question today, as I got the last parts for my PID forge in the mail yesterday.