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Alan Longmire

thermocouple recommendations?

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I recently scored a sword HT rig from fellow forumite Dennis McAdams, based on the highly successful one built by Jesus Hernandez.  

While I know I can use decalescence, and I'm pretty good at that, the limitations of my shop mean I'll be doing this outdoors in daylight.  This makes me think a decent thermocouple probe and thermometer is in order.  I have a crappy HF unit that I don't trust, so I'm thinking of upgrading to an Omega unit.  They have a couple in the sub-$100 range that look good.  I would like to have a 24" probe type K thermocouple to go with it, though, and the neat ones they sell are 18" max.  

So:  Those of you who use a portable probe system, what are your thoughts, please?  I'd rather not drill a bunch of holes down the length of the furnaces.

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Thanks!  That is a nice unit at a great price, but the furnace is 48" long.  I need to know the temperatures from center to end, and a 12" probe will not do that unless I drill holes in the shell.  I may end up doing that since the price is right, but for this and other applications I'd prefer a longer probe.  I may actually make one, as I have six feet of heavy wall 1/4" stainless tube.  I'd just prefer something readymade so I don't accidentally screw it up before I start.  Bare thermocouples are cheap, but plug-and-play appeals to me on electrical stuff.  I will happily make my own steel from rocks, but wrangling electrons makes me uneasy...:lol:

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I've had pretty good results with a few of the cheap Chinese pyrometers. I have access to a thermocouple calibrator and always check the accuracy of the pyrometers across the intended working range before using them in anger.

Until a couple of years ago, I used to recommend the TM902C, available for around 5 bucks delivered. I'd had maybe 20, ordered in ones and twos over maybe 4 or 5 years. They were boringly accurate from 0 degC to 1370 degC (32 degF to 2500 degF) and came with a glassfiber-insulated bead probe that was good to 400 degC (750 degF) and was flexible enough to be closed in an domestic oven door to check tempering temperatures. 

A couple of years ago, I ordered ten of them. When they arrived, I checked them on the calibrator and found they were accurate up to 800 degC but then became progressively inaccurate. I can't remember whether they showed 1370 degC at an input of 1290 degC, or showed 1290 degC at an input of 1370 degC: either way I was not happy with them. They were very consistent and all read the same on any given input signal.

Side-by-side, there were external differences between the good ones and the bad ones, but these would not be obvious on an ebay listing so I stopped using or recommending them.

I have since used DM6801 digital thermometers and found them boringly accurate. 10-20 bucks delivered from China. The supplied bead-type probe is PVC insulated and only good to a little over the boiling point of water: not much use to most of us.

Whatever pyrometer you get, the supplied thermocouple will not be any use in a forge and you'll need to go to a thermocouple specialist.

The "best" probe I have used for "our" purposes is an Omega KHXL-14U-RSC-24 which is a handled type K Mineral Insulated probe 1/4" diameter and 24" long below the handle. The sheath material is Omegas proprietary "Super Omegaclad XL", which is not really necessary for Heat-Treating, but seems to be the best I've found for surviving being used to occasionally check forge temperatures without going to painfully expensive Type S, Platinum-based, thermocouples. The assembly includes a curly cable and miniature plug.

https://www.omega.com/pptst/KHXL_NHXL.html

Using the part number builder on the page does not allow anything longer than 18", but putting the desired part number into the search box, top-right, will get you a price. For sword-length stuff, a 36", 48" or even 60" might even be worthwhile.

I am cheap and tend to use similar thermocouples from the company we use at work. However, these "only" have a type 310 Stainless steel sheath, which is rated to 1100 deg (2012 degF) and will last pretty much indefinitely for HT, but doesn't last many cycles to 1300 degC-plus when used for checking welding temperatures are being reached.

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1 hour ago, Alan Longmire said:

Thanks!  That is a nice unit at a great price, but the furnace is 48" long.  I need to know the temperatures from center to end, and a 12" probe will not do that unless I drill holes in the shell.  I may end up doing that since the price is right, but for this and other applications I'd prefer a longer probe.  I may actually make one, as I have six feet of heavy wall 1/4" stainless tube.  I'd just prefer something readymade so I don't accidentally screw it up before I start.  Bare thermocouples are cheap, but plug-and-play appeals to me on electrical stuff.  I will happily make my own steel from rocks, but wrangling electrons makes me uneasy...:lol:

Isn't the thermocouple a long piece of stainless steel with heat probes attached to it from my understanding?  Wouldn't you be able to add something to one to sort of extend it?  Not sure if that would work or even be useful any.  Just a thought.

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Alan, McMaster Carr has a thermocouple with a sectioned ceramic shield that I think you can make any length you need.  With these if the end burns out you just cut back to clean metal, twist the ends back together and TIG it.  The reading actually comes from the very tip. I believe that the metal shields are Inconel not just stainless steel.  

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Thanks Tim, I was hoping you'd respond given that you're the resident guru of process controls for hot stuff.  I will check that out.  I like the idea of cheap and "boringly accurate" for the pyrometer.  I agree the Omegaclad XL is overkill for HT, but I can see myself doing something stupid like sticking it down a smelter (which really would need a type N).

Wayne, I've used those ceramic ones and find them too fragile.  Plus I don't have a TIG machine to connect the wires...

Andy, there's thermocouples and then there's thermocouples.  Lots of variety and materials used for everything from food processing to pottery kilns to checking the pouring temperature of molten platinum, not to mention the one in your oven.  Just gotta match the probe to the application, which is where I was stuck.  

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2 hours ago, Alan Longmire said:

Thanks Tim, I was hoping you'd respond given that you're the resident guru of process controls for hot stuff.  I will check that out.  I like the idea of cheap and "boringly accurate" for the pyrometer.  I agree the Omegaclad XL is overkill for HT, but I can see myself doing something stupid like sticking it down a smelter (which really would need a type N).

Wayne, I've used those ceramic ones and find them too fragile.  Plus I don't have a TIG machine to connect the wires...

Andy, there's thermocouples and then there's thermocouples.  Lots of variety and materials used for everything from food processing to pottery kilns to checking the pouring temperature of molten platinum, not to mention the one in your oven.  Just gotta match the probe to the application, which is where I was stuck.  

Ah makes sense, I guess you could say I'm more accustomed to the food industry thermocouples.  Just stick one in a 400 degree oven and let it sit there.  I don't even think I could put one on the break drum lol.

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On 11/4/2018 at 4:23 AM, Alan Longmire said:

Wayne, I've used those ceramic ones and find them too fragile.  Plus I don't have a TIG machine to connect the wires...

I too find them to be be too fragile.  And oxy acetylene works well, too!  For short use (i.e. single heat) you can even get by with just twisting the wires together.  

When we do surveys of our HT ovens at work, we use the wires with the ceramic weave shielding.  If I were in your situation, that is what I would get and just leave the ss tube you have in the HT chamber and slide the wire up and down in that tube to get temperatures within the chamber.  

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Thanks, Jerrod!  I may use that as a backup plan, since the thermocouples themselves are not that expensive, it's the probe housing.  My SS tubing is 1/8" ID x 1/4" OD, and I think it's 316L.  Might be 304.  As long as the bead is small enough to fit that would work fine.  So much research to do...

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On 11/5/2018 at 1:25 PM, Alan Longmire said:

Thanks, Jerrod!  I may use that as a backup plan, since the thermocouples themselves are not that expensive, it's the probe housing.  My SS tubing is 1/8" ID x 1/4" OD, and I think it's 316L.  Might be 304.  As long as the bead is small enough to fit that would work fine.  So much research to do...

Did you ever find what you desired?

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Yesterday, in fact.  I went with Tim's suggestion.  I may yet use the stainless tube as a thermowell in the tempering oven, we shall see.

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