Reasons why a thermocouple are indispensable

leifjl

leifjl

Joined
Nov 27, 2011
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A couple of things I've discovered since purchasing a hand held thermocouple.

1. My evenheat kiln runs 30-50 degrees hotter than what the readout is displaying! I thought it might be off a little, maybe 10-20 degrees. I've altered my H.T. schedules accordingly.

2. Surprisingly the oven I use for tempering (A precision Quincy 1000 degree) Was never off by more than 3 degrees. I thought, if anything, the evenheat would be the more accurate of the 2.

3. There can be significant differences in a kilns temperature just based on where you're placing your knife. Knowing where these hot and cold spots are is critical.

4. And just for curiosities sake I took a reading of my dry ice and alcohol mix. Ranged from -97 to -99 over a period of about 20 minutes.

I know a lot of this has been covered before, but thought it was interesting and important enough to share.

By far, one of the best purchases I've made in a while.
 
Yes very important tool for me too . I was having a little trouble with a bar of steel I bought recently. It allowed me to rule out a few things just by having one. Must have in my opinion.
 
For what they cost, they should be the first purchase for anyone doing their own heat treating. I was able to test even temperatures in a 10 brick forge, and fine tune burner angles to even out heat. Removes a lot of guessing.
 
Thermocouples are not the most accurate measuring element. Each one can be 1 - 2% different in reading. Be aware of that if you ever have to change it.

Craig
 
As Craig says, there are tolerances on thermocouples, even when new, and the readings obtained can vary by several degrees. A large part of this is down to small differences in the alloy composition of the two wires that make up the thermocouple.

In industry, type K thermocouples, such as those used in most HT ovens, are known to be prone to "drift" when used for long periods at temperatures above about 1000 degC (1832 degF).

A different type of base-metal thermocouple, type N, was developed to overcome this. I use type N in my homebuilt HT ovens and the industrial ramp/soak controllers that I use can accept them. However, most of the mainstream oven controllers seem to accept only type K inputs.

It is probably a good idea to change the type K thermocouples fairly regularly; perhaps annually for a heavy user working with stainless, though a hobby maker working with Carbon steels will probably never need to change one. Buying several spare thermocouples at the same time, to ensure the wires are from the same melts, can reduce the potential variation when a thermocouple is changed, by eliminating any changes in alloy composition.
 
Are y'all using an IR one? Which model?
 
leifjl, how old is the kiln you are speaking of and how would you use a hand held thermocouple
in a kiln?
Eddie
 
gredpe3 said:
leifjl, how old is the kiln you are speaking of and how would you use a hand held thermocouple
in a kiln?
Eddie
Click to expand...

I don't remember the manufacturer off hand but it's a type k thermocouple with a range of something like -200 TO 1300 degrees celcius
My kiln is about a year and a half old.
It's got dual probes with independent read outs, so I tested different areas in the kiln by inserting them through the exhaust vent. First probe 1, then probe 2 in the same area, just to check them against each other.
 
Fiddleback said:
Are y'all using an IR one? Which model?
Click to expand...

IR has some quite severe limitations for "our" purposes. The main one is that the emissivity of the surface to be measured is usually unknown and is critical to accuracy. The emissivity can also change quite dramatically during the sort of processing "we" tend to undertake.

For a better explanation of emissivity than I can provide:

http://www.reliableplant.com/Read/1...-difference-between-apparent,-actual-ir-temps

I had a fairly expensive 0-1600 degC (0-2912 degF) IR thermometer

http://www.digital-meters.com/tempe...red-thermometer-50-to-1600-c-cem-dt-8859-p250

I found it to be of little use due to the emissivity issue. To get a useful reading, I needed to measure the actual temperature of a workpiece by some other method, measure it with the IR thermometer, adjust the emissivity setting on the IR thermometer until it agreed with the known temperature. All of the faffing about with the IR unit was just timewasting and didn't give me anything particularly useful.

A friend who makes knives (rather nice ones) finds it much more useful. From what I understand though, he uses it more as a consistency/QC check than as a means of accurately determining temperature; for example, he has found that when the billet is at the right temperature to take to the power hammer, the IR thermometer gives a reading between X and Y.
 
Can you put an additional item with an emissivity of close to 1 in the furnace with the knives and read off of that?

Which/what do you use. I knew about the emissivity issue, and had never bought a thermocouple because of that. You're just sticking a k-probe into the furnace?
 
I think we are using the wrong terms here.
A thermocouple is a device with two dissimilar wires fused at a junction to create an electric current when heated. The voltage is read by some sort of meter and that reading is translated into an approximate temperature reading. The quality of the thermocouple and the reading device will determine the accuracy of the reading.
A 1/8" lead TC with a ceramic sheath in a forge, or bare in an electric oven, can read the chamber temperature quite accurately. If the TC is properly wired to the reading unit ( often a PID) with TC wires ( there is a different type of wire for connection these devices), these are very good for doing HT, and are the industry standard. A good setup can be had for around $100.

One issue in taking a reading in an oven or forge is what you are reading. If there is a big flame in the gas forge, you may be reading the flame, not the blade. If there are hot electric coils in an oven, the IR emitted by the coils may be affecting the accuracy. The best readings are taken near or in contact with the blade. Also, allow the oven/forge to soak long enough for all the refractory to come up to full temp, so you are reading the true temp of the whole chamber. If the blade sits in a 1500F chamber for five/ten minutes, chances are good that it will also be 1500F. The metallurgical lab guys who want to know exactly what the blade temperature is tie a good grade TC to the blade with binding wire to read the blade's temp. For really serious tests, they embed the TC in the blade.
To keep the flame reading from skewing my HT, I turn off the flame temporarily when taking a temperature reading in the gas forge.

A pyrometer is a device to read temperature. Some have a thermocouple to read the temperature, and others are the non-contact type that uses a reading of the IR and other light being emitted by the object. These can be as cheap as $15 bucks for ones that can do fairly well for checking the oil in a quench tank or turkey fryer, to lab units with emissivity adjustments that cost well over $1000. The lower cost IR/laser guns from HF and such are fine for things below 900F, but as soon as any item starts to glow, the readings get harder to be accurate. This is the range we work in for HT, and such pyrometers, even the lab units, are of little use.
 
This post is troubling for me,I recently purchased a Evenheat Oven KF series.The reason was to get more accurate and even heat for the blades I heattreat.Now I guess I'll be getting on the ball and get that Rockwell Tester.It's the only way to be sure.
Eddie
 
Thanks Stacey. Can someone recommend a unit? It seems like a good tool to have around.
 
Now we're getting somewhere! thanks Guys! This thread has been an education.
 
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