Monitoring Heat in a Propane Forge

Before I bought my electric oven, I used my forge to heat treat. One thing I did try was making a round pipe muffle with 270 degrees coverage of insulwool (coated in satanite), soft fire brick caps on both ends, and a long thermocouple probe that reached to where the blade was. The blade sat on a soft fire brick surface inside the muffle. Using this setup, I was able to maintain a 10-20 degree temperature range during heat treat which was good enough for what I was doing (O1 and 5160).
 
I'm not sure what your forge is like but the heat of a propane forge normally isn't even enough to use a temp probe.
just my 2 cents

any experienced makers agree? If I am wrong I may get some for my forge. :p
 
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The Omega probe in the post #1 link is similar to the sort of thing I use, though I actually go longer and thicker.

In that range, I'd go for a TJ36CAXL14G24,

For preference though, I use a KHXL14GRSC24.

The Mineral-Insulated construction in conjunction with the proprietary Omega XL sheath material gives very good resistance to "drift". Type K thermocouples are particularly prone to drift at temperatures above about 1000 degC (1832 degF): a progressive loss of accuracy with time and temperature.

Having a 24" handheld probe allows you to profile the temperature distribution in the forge. The 1/4" diameter means that it is rigid enough to do this, even when horizontal and hot. The Grounded junction is relatively slow to respond compared to an Exposed or Grounded junction, but is much more tolerant of rapid temperature changes: You may need to wait 20-30 seconds at each point you measure for the reading to stabilize.

For general-purpose use, I like the Type K with the XL sheath because the Type K tables go up to 2500 degF, 1372 degC. Since around 1300 degC (2372 degF) seems to be about right for a welding forge, the type K with XL sheath (rated to 1335 degC, 2440 degF) makes a pretty good general-purpose probe for knifemaking and bladesmithing.

For slightly lower temperatures, my preferred choice is a type N thermocouple, again MI and with either a widely-available Nicrobell or Omega-proprietary XL sheath. Type N was developed to be a base-metal thermocouple (i.e. relatively cheap. Types B, S and R are used to around 1760 degC, 3200 degF, but use Platinum-based alloys and cost accordingly), without the high-temperature-drift issues of type K. The downside is that it is "only" good to 1300 degC, 2372 degF. I always use type N thermocouples in HT ovens for their stability.

The high-temperature type K probes with sectional ceramic sheaths, like the Auber link from post #2, are good if you want a permanently-installed thermocouple. For use in reducing forge atmospheres, they need a ceramic outer sheath as well. To give useful information, they need to be placed somewhere that is not in the way of the workpiece, but that accurately represents the temperature at the workpiece. Most of the forges that I have seen have shown quite a lot of temperature variation throughout the chamber, so I'd probably want to do a scan with a movable probe first to establish a suitable location.

For a temperature readout, the post #1- linked Auber controller will work fine in conjunction with a type K thermocouple, though it does not support a type N input.

Use of panel-mount controllers for temperature indication ties you to a power supply: not much of an issue if you are running a blown burner and need to power the blower anyway. It's more of a hindrance if you are running a fairly mobile Naturally Aspirated forge.

I usually use a cheap handheld pyrometer unit from ebay. This needs the miniature plug which comes on the end of the curly cable on the handheld probe. If using one with the Transition Junction probe, you'd need to fit, or have fitted, a suitable plug.

I used to use TM902C handheld readouts at under 5 bucks delivered. These only read in degC but are effectively foolproof: they are either on or off. I have access to a calibrator and always check accuracy of any instrument I use. The first 20 or so TM902Cs that passed through my hands were as accurate as big-name units costing more than ten times as much, all the way from -50 degC to 1365 degC, even though most showed a maximum range of either 800 degC or 1300 degC on the front. In 2016 I bought 10 of them and they looked slightly different to the previous ones when they arrived. The new ones were fine to 800 degC, but had increasing errors above this temperature, showing 1370 degC at an input value of 1290 degC. It's not possible to tell the difference between the good and the bad ones on an ebay description, so I no longer buy TM902Cs.

I then switched to TM6801A units, again from ebay. These have been accurate and, to date, reliable over the eight units I have had. They are switchable between degC and degF readings, so are pretty close to foolproof (it's usually fairly obvious if you have the wrong units selected). Those that I have tried also read the full type k range (to 2500 degF), despite only saying 1300 degC (2372 degF) on the front. The 6802 unit in Scott's Amazon link seems to be a dual-channel version of the single-channel 6801.

I also use GM1312 readouts. These will read several different thermocouple types (I frequently use types K, N and S) to high accuracy, but this introduces the possibility of selecting the wrong input type and getting erroneous readings.
 
For a propane forge you *NEED* the ceramic sheath thermocouples if you want them to last more than a few sessions, especially at the upper limit of the probe temp range.


The Auberin's high temp K type thermocouples have a little bit of extra-leeway outside of the spec, and I've regularly had some of mine up at 2450+ for extended periods of time. It's not optimal, as mentioned, there are other types that are superior for temps above 2375ish.


To be clear, you not only want the K type thermocouples from Auberins that have sectional ceramic links around the wires themselves, but the test-tube shaped "sheath" they sell, that the whole probe slides down into.


You can permanently mount (mortar) the ceramic sheath into the forge, and simply slide the thermocouple itself in and out as needed. Although typically, it's best not to monkey with them. The ceramic sheath itself isn't super fragile, but it's not super durable either. I've broken a bunch of them smacking them with billets, etc.


Also, their "Universal Thermometer" works great for this application, to read the temp. You can set an alarm band, i.e. start beeping once you hit a certain temp, and stop once you go below a certain temp, or the opposite, depending on your needs. I wouldn't bother with the PID, unless you're trying to setup some automated temperature controlled forge, which IMO, is very complicated, and never very elegant, with propane forges, which one of the major advantages is typically, atmospheric control, but is difficult to maintain when you've got constantly fluctuating air/fuel ratios when using solenoids to regulate temp. Unless you're building a setup like this, the PID serves you no purpose, and is more complicated to use, and needs to be housed preferably in a project box.


A good needle valve for your fuel, and large sliding gate valve for a HIGH pressure blower (the only easily available ones on the market for us are the "Forge Blowers" that Kayne & Son's (Blacksmith's Depot) sells, and are an excellent value, and the only thing I'd recommend for a blown burner forge of any type), will let you nail just about any temp and hold it, once you understand how to tweak it, and your forge equalizes, that your burner design will allow.


If you follow this link, you can get the pyrometer, thermocouple, and sheath, all in the same page for $90, make sure to check the box for the sheath below the listing: https://www.auberinc.com/index.php?main_page=product_info&cPath=17&products_id=282


Yes there are plenty of other options to geek out on, given time, and a thorough understanding of your needs and logistical caveats, but you can feel secure just buying the above for now, and know you didn't waste your money.. Can you find potentially the same/similar components, for cheaper? Yes, but Auberin's stands by their stuff, tons of us in the community pay a few extra bucks for the piece of mind that they're not selling junk, and will fix it if there's a problem.
 
I have simple, cheap units available on my web store for a fraction of the cost of the Auberins units. I put these together because most makers don't need the accuracy of the more expensive units and this units reads in °F up to 1999°. For that matter, most makers rarely use them when heat treating once they've learned the color/temp correlation. Once you learn what color is 1475°F, that's all you need. When that time comes, you still have a decent multimeter.

http://www.atlasknife.com/product/100mm-thermocouple-set/
To use the thermocouple, I recommend placing the probe under the metal
as it sits in the forge. This will give the most accurate reading.
DT33c_thermo_700px-600x600.jpg
 
I use these on my forge for heat treating. Worth every cent IMO.

PID: https://www.auberinc.com/index.php?main_page=product_info&cPath=1&products_id=3
Thermacouple: https://www.auberinc.com/index.php?main_page=product_info&cPath=20_3&products_id=39
Ceramic sheath: https://www.auberinc.com/index.php?main_page=product_info&cPath=20_3&products_id=40

I let my forge soak at temp for about 45 mins to an hour before placing the blade in a stainless steel muffle. My forge will maintain a temp of +-5* just setting it manually. I have plans on upgrading it with an SSR and solenoid to control it, I just haven't yet.
 
You need a ceramic sheath to use a probe in a forge. A bare probe can burn up in a week, especially at forging temps.
 
scott.livesey said:
enjoy. you could have found the same thing for half the price but oh well.
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Yeah same here. You seeing a pyrometer, a ceramic insulated and sheathed k-type thermocouple on McMaster for half the price? Cause I'm not seeing anything similar, and specifically, that's what he needs. Not an a bare probe or inconel sheathed one with a multi-meter. I'm not even seeing a single pyrometer or multimeter that'll serve as one at that link for less than $100.


You can find the same components I linked a bit cheaper on ebay, but you don't get the support that auberin's offers. Is it really worth taking a chance to save $20 for some random ebay seller?
 
You link still doesn't work, because of the way McMaster's page is, but I found the thermocouple. Still need a ceramic sheath ($10-20 depending on size and quality), if you've been using one with a bare tip for 2 years in a propane forge I can guarantee you that it's not remotely accurate anymore.

Personally you couldn't pay me to use a battery powered pyrometer/display either. It's why I don't recommend multi-meters for this purpose, most of them are battery power only and have auto-shutoffs, and/or shit battery life, most of them also don't have constantly illuminated displays, many of the cheap ones aren't illuminated at all. The above probe in the kit is also much thicker guage, which while it raises response time, it's much more durable.

Just the way I work, but I need a constantly powered, constantly illuminated display. The one I linked above is bulletproof, dead simple, always on, always bright enough to read even when the forge is brighter than the sun at 2300+ degrees, and offers alarms, which are very useful if you learn how to use them.


So yeah, as I mentioned above, yeah, you can get something cheaper, but you're making sacrifices. Bear in mind that you're also gonna eat probably $10-20 in shipping from McMaster, which btw, never tells you how much the shipping is upfront, and while they're not exorbitant, they don't eat much of the shipping costs. Then he's gotta get connectors from ebay. Price on the display is fine, but is the shipping free if he doesn't have prime?


Regardless, he made his choice, and I think it was a smart one. A turn-key, guaranteed solution, ordered from one location, of known quality, and a stellar reputation in our community. It's not like we're talking about a $5,000 piece of equipment. You guys can nickle and dime yourself to death if you want, but peace of mind, let alone time, would be more valuable to me in this case.
 
javand said:
You link still doesn't work, because of the way McMaster's page is, but I found the thermocouple. Still need a ceramic sheath ($10-20 depending on size and quality), if you've been using one with a bare tip for 2 years in a propane forge I can guarantee you that it's not remotely accurate anymore.

Personally you couldn't pay me to use a battery powered pyrometer/display either. It's why I don't recommend multi-meters for this purpose, most of them are battery power only and have auto-shutoffs, and/or shit battery life, most of them also don't have constantly illuminated displays, many of the cheap ones aren't illuminated at all. The above probe in the kit is also much thicker guage, which while it raises response time, it's much more durable.

Just the way I work, but I need a constantly powered, constantly illuminated display. The one I linked above is bulletproof, dead simple, always on, always bright enough to read even when the forge is brighter than the sun at 2300+ degrees, and offers alarms, which are very useful if you learn how to use them.


So yeah, as I mentioned above, yeah, you can get something cheaper, but you're making sacrifices. Bear in mind that you're also gonna eat probably $10-20 in shipping from McMaster, which btw, never tells you how much the shipping is upfront, and while they're not exorbitant, they don't eat much of the shipping costs. Then he's gotta get connectors from ebay. Price on the display is fine, but is the shipping free if he doesn't have prime?


Regardless, he made his choice, and I think it was a smart one. A turn-key, guaranteed solution, ordered from one location, of known quality, and a stellar reputation in our community. It's not like we're talking about a $5,000 piece of equipment. You guys can nickle and dime yourself to death if you want, but peace of mind, let alone time, would be more valuable to me in this case.
Click to expand...

just what i used to do for a living. saved company over $50k first year just by shopping different sources for materials. been dealing with McMaster for over 30 years, I like the way that your order is packaged and waiting for the truck in less than an hour. unit referenced above will work too, I would rather spend the $$ on steel
 
scott.livesey said:
just what i used to do for a living. saved company over $50k first year just by shopping different sources for materials. been dealing with McMaster for over 30 years, I like the way that your order is packaged and waiting for the truck in less than an hour. unit referenced above will work too, I would rather spend the $$ on steel
Click to expand...

Fair enough. ;)
 
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