Oven temp verification.

J

Jcdelamater

Joined
Oct 5, 2022
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40
So I got a new thermocouple for my HT oven (K-type unsheathed). I know my prior (sheathed) TC read lower than actual oven temp. Does anyone have suggestions on how to determine if the new TC is accurate? Or if not, how to apply some correction factor?

Also any recommendations on what height in the oven the probe should be at?
 
If you don't have a high end multimeter an ice slurry should read 0c and boiling water although less accurate should read at least 100c while boiling.

The probe being close to the blades without getting in the way would be the most ideal location.

Testing on my air fryer the back right corner is the hotest with the left front corner the coolest, I haven't tested my fan forced tempering oven yet as I need to wire the PID in to bypass the termostat and timer.

There may be a setting in your PID to calibrate your probe, I don't think mine does so I would need to add or subtract to the desired tempreture to get it dialed in.
 
That's a good way to check more normal temperatures, but I'm looking for a way to double check temperatures between 1400-2000F. I don't totally trust that just because it is accurate at 0C and 100C that it will be accurate at 1000C.
 
I think tempreture sensative croyons are the go for you then, they are hard to find above 600F but they will be out there.
 
Take alare stainless steel spoon (tablespoon or serving spoon) and put a teaspoon of rock salt in it.
Preheat the oven to 1450, put in the salt. Soak for 30 minutes. Check the salt.
Raise 10° and soak 30 minutes again.
Keep raising and checking until the salt melts.
The salt melted somewhere between the last two settings, so considefr it halfway between them. If the oven was set at 1480° when the salt melted, it is probably dead on. 1475°=1475°.
If the salt melted at 149°, the oven reads about 10° too high. 1485° = 1475°
If it melted at 1470°, it is reading about 10° too low. 1465° = 1475°
 
Stacy E. Apelt - Bladesmith said:
Take alare stainless steel spoon (tablespoon or serving spoon) and put a teaspoon of rock salt in it.
Preheat the oven to 1450, put in the salt. Soak for 30 minutes. Check the salt.
Raise 10° and soak 30 minutes again.
Keep raising and checking until the salt melts.
The salt melted somewhere between the last two settings, so considefr it halfway between them. If the oven was set at 1480° when the salt melted, it is probably dead on. 1475°=1475°.
If the salt melted at 149°, the oven reads about 10° too high. 1485° = 1475°
If it melted at 1470°, it is reading about 10° too low. 1465° = 1475°
Click to expand...
This sounds pretty good, especially the slow ramping, with a couple caveats. In chemistry labs we often test the melting point of a synthesis product. If you slowly ramp the temperature and it melts abruptly at the compound's known mp, then the abrupt transition points to high purity, while the mp corroborates the identity. If is slightly impure, then the melting point will be a bit lower and less abrupt. Rock salt is typically only 95-99 percent pure, so it might start sagging and melting a few degrees (say 5-10), below its nominal melting point.
 
Is there much of a difference between table salt and rock salt? I'll have to give this a go when I get back after Christmas. I'll let you all know how it goes.
 
For our purposes either table salt or rock salt is sufficient. Kosher salt is probably the purest.
 
Run coupons and find your peak AQ hardness, does not matter what the oven temp is telling you it is how the steel is responding. Once you find your peak hardness you will know where to set your oven temperature.
 
Seedy Lot hives basic good advice but it requires some well controlled parameters.
The peak hardness depends on more than just the temperature of the oven. It also is controlled by the structure before HT and the quenchant and quench method. It also requires a calibrated and accurate hardness tester to do the comparison tests.

What many makers do is run a test HT on a batch of seven identical stock reduction blades ground from the same batch of steel. 4" is enough blade length and a 4" tang gives enough grip.
Quench in the same quenchant at the same temperature. Start 15° below your target and raise the oven temperature in 5° steps up to 15° above it (allow the oven a 10-minute stabilization at each new temperature).
They don't need to be fully functional or "knife-shaped" knives - just grind a bevel and put an edge on a bar of steel. Temper all in one batch together at 400°F.
Put duct tape on the tangs for a handle. Now you can finish the bevels as closely identical as possible (120 grit is fine), sharpen them the same, and test in many ways to see how the edges hold up.
Do cutting tests on manila rope, cardboard, wood, and do an edge flex test (brass rod test). If you have access to a hardness tester, grind the tangs flat and parallel and have them Rockwell tested.

TIP -
When doing multiple test blades, use stamps to stamp each tang on both sides with a letter or number. Stamp deep so it will be there after clean-up. Make a log with the numbers, HT parameters, etc. for each blade.
This is also a good idea on anyone doing multiple blade batches or using several types of steel. Sharpies and paint pens will rub or grind off too easily ... if you even remember to mark the blades. Stamping stays on the tang pretty much forever and can be quickly re-stamped if needed after heavy reduction.

It is always a good idea to stamp the tangs on your blades. Harbor Freight has a letter/number set for $15. Stamping the tang on every blade and keeping a logbook can make identification easy during construction or later on when doing a repair. Consider it the Westen equivalent of a Japanese mei. A good system is to use a letter for the year and numbers for the blade number that year. If you started in 2010 with A, a blade marked G-31 was the thirty-first bade make in 2016. Look it up in you book and you know the day you made it, steel type, HT info, handle material, and maybe the original purchaser or other notes. While making a spread sheet is the modern way, a physical logbook is more impressive and less likely to be lost because of a computer crash. The more info you put in your logbook the more you become consistent in knifemaking.
 
Stacy E. Apelt - Bladesmith said:
Seedy Lot hives basic good advice but it requires some well controlled parameters.
The peak hardness depends on more than just the temperature of the oven. It also is controlled by the structure before HT and the quenchant and quench method. It also requires a calibrated and accurate hardness tester to do the comparison tests.

What many makers do is run a test HT on a batch of seven identical stock reduction blades ground from the same batch of steel. 4" is enough blade length and a 4" tang gives enough grip.
Quench in the same quenchant at the same temperature. Start 15° below your target and raise the oven temperature in 5° steps up to 15° above it (allow the oven a 10-minute stabilization at each new temperature).
They don't need to be fully functional or "knife-shaped" knives - just grind a bevel and put an edge on a bar of steel. Temper all in one batch together at 400°F.
Put duct tape on the tangs for a handle. Now you can finish the bevels as closely identical as possible (120 grit is fine), sharpen them the same, and test in many ways to see how the edges hold up.
Do cutting tests on manila rope, cardboard, wood, and do an edge flex test (brass rod test). If you have access to a hardness tester, grind the tangs flat and parallel and have them Rockwell tested.

TIP -
When doing multiple test blades, use stamps to stamp each tang on both sides with a letter or number. Stamp deep so it will be there after clean-up. Make a log with the numbers, HT parameters, etc. for each blade.
This is also a good idea on anyone doing multiple blade batches or using several types of steel. Sharpies and paint pens will rub or grind off too easily ... if you even remember to mark the blades. Stamping stays on the tang pretty much forever and can be quickly re-stamped if needed after heavy reduction.

It is always a good idea to stamp the tangs on your blades. Harbor Freight has a letter/number set for $15. Stamping the tang on every blade and keeping a logbook can make identification easy during construction or later on when doing a repair. Consider it the Westen equivalent of a Japanese mei. A good system is to use a letter for the year and numbers for the blade number that year. If you started in 2010 with A, a blade marked G-31 was the thirty-first bade make in 2016. Look it up in you book and you know the day you made it, steel type, HT info, handle material, and maybe the original purchaser or other notes. While making a spread sheet is the modern way, a physical logbook is more impressive and less likely to be lost because of a computer crash. The more info you put in your logbook the more you become consistent in knifemaking.
Click to expand...
I like this, been thinking on the general idea for a little while. And actually, 2023 is just around the corner so that's when it makes sense to officially get a log/portfolio started👍 It's also the end of my first, (technically) year of knife making. So glad I'm doing it, no looking back! Buuuut...if I HAVE to look back, a log would be handy!
 
Stacy E. Apelt - Bladesmith said:
Take alare stainless steel spoon (tablespoon or serving spoon) and put a teaspoon of rock salt in it.
Preheat the oven to 1450, put in the salt. Soak for 30 minutes. Check the salt.
Raise 10° and soak 30 minutes again.
Keep raising and checking until the salt melts.
The salt melted somewhere between the last two settings, so considefr it halfway between them. If the oven was set at 1480° when the salt melted, it is probably dead on. 1475°=1475°.
If the salt melted at 149°, the oven reads about 10° too high. 1485° = 1475°
If it melted at 1470°, it is reading about 10° too low. 1465° = 1475°
Click to expand...
Ive been wanting to find a similar test, but for higher temps. Closer to 2000. Haven't had much luck.
 
Yeah, I don't have access to a hardness tester. That makes sense, run a DOE varying temps and keep some notes on the results. I'll have to do salt and copper. I actually have a bunch of copper pipes I'm considering melting down and using somehow.

Thanks for all the advice, I think this well help improve my HT process.
 
Aluminum melts at 1221 F and Borax at 1369 F. This is a bit lower than usual temps, but more data points is always helpful.
 
If using metals to test temperatures, they have to be 100% pure. Most metals you get have some alloying. Also, metals oxidize at high temps and go through a slump phase while melting. Crystaline materials that don't form oxides, like salt and borax, melt more accurately.

I didn't go into it, but there are kiln cones called pyrometric cones for almost every temperature, and other temperature testing crayons and cones that are used in the metal industries. These are relatively cheap and easy to use. Tempil is who I get my testing crayons from.
 
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