Thermocouple position in oven

[Natlek]

I was searching some knife forums on East for some clever ideas and i come to one thread in one Czech forum ....... I can not find picture /have no idea in which folder i save it/ but when i find it I'll set it up here ....TC is installed very close to element , something like on this drawing. And to me make perfect sense ? TC and PID will react on temperature of coil much faster . By reading ambient temperature in middle of chamber TC will react more slowly , element will literally glow white before PID shoot them off ?
Blue circle is TC ......

Watch this video from 1 min.and 58sec .when he open door ................then skip to 2 min. and 40 sec. Watch color of elements .....God help to blade on left and right side of chamber close to HT elements , don t you think ?? They take much higher temperature the TC is reading !

 

[JTknives]

My elament(s) are installed as close to the blades as possible.

edit- I meant to say thermocouple not elements

 

[Natlek]

My elament(s) are installed as close to the blades as possible.

If you mean on last oven you build , with heat elements only on top ......Yes , but as far as I remember in your furnace you have controls that control how many watts the elements give not just off/on switch
What do you think about this ? It seems to me like excellent solution ?

 

[Drew Riley]

I gotta think that it all evens out after the oven has soaked for 30-45 minutes, no matter where the element or probe is located.
I see what your getting at, but I just wonder if it’s really an issue in actual practice.

 

[Natlek]

I gotta think that it all evens out after the oven has soaked for 30-45 minutes, no matter where the element or probe is located.
I see what your getting at, but I just wonder if it’s really an issue in actual practice.

When oven temperature get to set temperature elements will work on very short cycles on/off to maintain temperature and that is no problem .
Tell me what will happen when we insert say 5 knife blanks in already hot oven ? Almost two pound cold steel ?

 

[Natlek]

My elament(s) are installed as close to the blades as possible.

With Thermocouple right between them

 

[weo]

The concern that I would have (and it might not be a valid one) would be: what would happen as the furnace is heating up from cold? Wouldn't having a TC too close to the elements give a false high reading, causing the elements to cycle on and off too quickly, making it take longer to reach the target temp as well as wear out the components quicker due to the added cycling?
I tend to use limit theory when thinking about these types of problems, and if the TC was touching the element, it would read the target temp immediately, and then shut off, then turn right back on when the element temp drops, etc....

 

[Skiller1nc]

The concern that I would have (and it might not be a valid one) on that would be what would happen as the furnace is heating up from cold? Wouldn't having a TC too close to the elements could give a false high reading, causing the elements to cycle on and off too quickly, making it take longer to reach the target temp as well as wear out the components quicker due to the added cycling?
I tend to use limit theory when thinking about these types of problems, and if the TC was touching the element, it would read the target temp immediately, and then shut off, then turn right back on when the element temp drops, etc....

I agree, I would put the TC as far from the elements and close to the blades as possible, which would be the middle.

 

[weo]

But to add to my comment, I think Natlek has a point about overheating blades if the distance between the TC and elements is greater than the distance between the blades and the elements.

IMO, the solution is to have a baffle between the blades and the elements. This would also add some thermal mass to the interior and help to stabilize the interior temperature when opening the door.

 

[Natlek]

I agree, I would put the TC as far from the elements and close to the blades as possible, which would be the middle.

Then answer this question .................Tell me what will happen AFTER you insert say 5 knife blanks in already hot oven ? Almost two pound cold steel ?

 

[Skiller1nc]

Then answer this question .................Tell me what will happen AFTER you insert say 5 knife blanks in already hot oven ? Almost two-pound cold steel ?

The oven will ramp temp back up to heat the added thermal mass, if the TC is near the cool steel and not the hot element it will force the PID and TC to heat until equilibrium is reached. If your TC is near the elements it will read a false high temp and cause the elements to shut off, and this will in turn slow heating. Overheating shouldn't be an issue in an oven if you have properly tuned your controller, in a 5 knife batch the outer (closer to the element) knife shouldn't be getting any significant degree hotter than the other blades over the time it takes to reach equilibrium. Then once equilibrium is reached the temperatures should be very similar.

Edit: To add to this, the further your TC is away from your knife the less you know about the area around your knife. You can assume at eq that your Tc and knife are the same temps but while heating and cooling you really have no idea what the temperature around the knife is b/c the TC is being artificially manipulated by the coils to a higher degree than the knife.

 

[Drew Riley]

When oven temperature get to set temperature elements will work on very short cycles on/off to maintain temperature and that is no problem .
Tell me what will happen when we insert say 5 knife blanks in already hot oven ? Almost two pound cold steel ?

I think my oven would recover back to short cycles before my knives reach temp.

 

[Natlek]

The oven will ramp temp back up to heat the added thermal mass, if the TC is near the cool steel and not the hot element it will force the PID and TC to heat until equilibrium is reached. If your TC is near the elements it will read a false high temp and cause the elements to shut off, and this will in turn slow heating. Overheating shouldn't be an issue in an oven if you have properly tuned your controller, in a 5 knife batch the outer (closer to the element) knife shouldn't be getting any significant degree hotter than the other blades over the time it takes to reach equilibrium. Then once equilibrium is reached the temperatures should be very similar.

Edit: To add to this, the further your TC is away from your knife the less you know about the area around your knife. You can assume at eq that your Tc and knife are the same temps but while heating and cooling you really have no idea what the temperature around the knife is b/c the TC is being artificially manipulated by the coils to a higher degree than the knife.

Inside oven we have two kind or heat transfer .One is direct radiation from elements and other is convection .Blade close to elements will receive radiation and convection heat , next blade will receive almost no radiation heat because of first blade ?
So you think that both blade will receive same amount of heat and same heat intensity over the time it takes to reach equilibrium again ?

 

[Skiller1nc]

Inside oven we have two kind or heat transfer .On is direct radiation from elements and other is convection .Blade close to elements will receive radiation and convection heat , next blade will receive almost no radiation heat because of first blade ?
So you think that both blade will receive same amount of heat and same heat intensity over the time it takes to reach equilibrium again ?

I don't have the required heat transfer equations near me and I don't feel like doing this by hand, however, feel free to. I am not saying they "receive same amount of heat and same heat intensity over the time it takes to reach equilibrium," what I am saying is that the difference in heat experienced by the outer and innermost blades on the way to eq is minimal and in a good oven won't effect heat treatment. The low convection heat transfer coefficient of air and the properties of radiation won't make a huge difference and the blades will more or less undergo identical processes.

 

[Natlek]

I don't have the required heat transfer equations near me and I don't feel like doing this by hand, however, feel free to. I am not saying they "receive same amount of heat and same heat intensity over the time it takes to reach equilibrium," what I am saying is that the difference in heat experienced by the outer and innermost blades on the way to eq is minimal and in a good oven won't effect heat treatment. The low convection heat transfer coefficient of air and the properties of radiation won't make a huge difference and the blades will more or less undergo identical processes.

I don't know, my friend, I don't know ... somehow I like that idea . Element s would NEVER overshoot set temperature in that case .I m not sure about other way....blade one inch to glowing element ??
Look at this picture , that oven probably costs like twenty of ours oven , I don't think the TC was exactly there by accident ?
One more thing.......TC should be in horizontal position in ovens for accurate reading .

The one common feature of all furnaces is the fact that within their insulated chambers there are horizontal planes of even heat flow (i.e. areas of equal temperature called isotherms). Factors that cause these temperature planes or gradients to occur include uneven heating, inadequate circulation, uneven distribution of the workload within the furnace, improper location of the heating source and the like. In order for a thermocouple to properly sense furnace temperature it must be oriented parallel to these isotherms and installed into the furnace a minimum of 20 thermocouple diameters. For example, a 3-mm (1/8-inch) diameter thermocouple will need to be inserted 63.5 mm (2.5 inches) into the chamber furnace.

 

[weo]

I don't think the TC was exactly there by accident ?

Perhaps not. But (not talking about this case specifically) I'll caution everyone to be careful about assuming that businesses do things because it's the best thing for the consumer or the product. Often times decisions are made because it's the best thing for the bottom line. And depending on the company, what's best for the bottom line might not be the best thing for the consumer. Just look at Boeing...(I apologize if my example brings up painful memories for anyone. If so, let me know and I'll delete this comment.)

 

[Backyard]

Thermocouple locations like that are often times used for opening up more floor space. Place it in the middle lower portion between knives will limit someone wanting to place crucibles in the oven. It's not always about the most accurate temp location, even on an expensive model