thermocouple project -- help, please

[Matt Bower]

I know almost nothing about electronics; I'm basically hoping some of you kind folks will take pity on me and walk me through my proposed project. Please expect me to ask stupid questions.

My problem is this. I have an oil burner that I salvaged from a residential furnace. I want to use it to heat a small casting/heat treating furnace. I want to be able to select a furnace temperature, turn on the burner, and have it heat the furnace to the selected temperature and maintain it there by cycling on and off as necessary.

The temperature range that's of most interest to me is around 1200 F - 2000 F. I plan to use a Type K thermocouple as my sensor. The idea is to use the potential across the thermocouple terminals to trip a relay that shuts down the burner once it reaches the selected temperature, then cycles it back on again as the furnace begins to cool. (At least that's what someone suggested to me. I didn't know what a relay was until I looked it up.) Since I want the burner to come on and remain on until the selected temperature is reached, then come back on once the temperature begins to fall, it'll need to be a normally closed relay.

In the temperature range I'm most interested in a Type K thermocouple should produce roughly 25 to 45 millivolts. I'm not sure whether I need a relay that will trip in that range, or whether I should get a relay that trips at some higher voltage and then somehow multiply the voltage across the thermcouple. (I know that can be done, but I have no idea how to do it.)

My main question is how to make furnace temperature selectable. I know very little about relays, but I gather they're built to open or close at a single, constant voltage. I want my relay to trip at any point along a wide range of voltages, depending on what I've selected. How do I do that? The same guy who suggested the relay also suggested a potentiometer, which I could calibrate by hand by measuring the temperature in the furnace at given potentiometer settings. (I have an inexpensive thermocouple thermometer that I could use for calibration.) But I'm not sure I understand exactly how that'd work.

My next questions are what sort of relay I'd need, and where to get one. (The burner runs on 120V/60Hz and draws a maximum of about 6 amperes.) Assuming I do need a potentiometer, the same questions apply.

Thanks in advance for any help.

 

[jpanhalt]

Matt, There clearly are nice ways of doing what you want to do using microcontrollers and such. However, I gather that you are more interested in getting the furnace going than in a new hobby. I don't believe you will find any relay to which you can simply attach the thermocouple and have it work as you describe.

Assuming that is accurate, I would look at the problem as two parts: 1) You need a thermostat to control the furnace; and 2) You need to interface the thermocouple output to what the thermostat needs as input.

For the first item, I would go with something commercial and probably used. You still might be able to salvage enough parts for that function from a household control. The biggest advantage of the commercial route is that it will have relays of the proper ratings and safety controls(e.g., flame out).

What you need for the second thing will depend on what type of control you have. Datasheets or the manufacturer's site for the original sensor used with that control will hopefully tell you what you need in terms of signal. Then your problem will be simplified and better defined as how to convert the TC output to the needed voltage range for the thermostat.

How big are the pieces you need to heat treat? In the early 1990's, I picked up a small electric furnace for about $50. You may want to reconsider whether the bother and risks of a DIY furnace is worth the money saved. Also, if the parts are very small, you may just use a temperature-indicationg crayon and an open flame to heat treat. John

 

[Leftyretro]

I know almost nothing about electronics; I'm basically hoping some of you kind folks will take pity on me and walk me through my proposed project. Please expect me to ask stupid questions.

My problem is this. I have an oil burner that I salvaged from a residential furnace. I want to use it to heat a small casting/heat treating furnace. I want to be able to select a furnace temperature, turn on the burner, and have it heat the furnace to the selected temperature and maintain it there by cycling on and off as necessary.

The temperature range that's of most interest to me is around 1200 F - 2000 F. I plan to use a Type K thermocouple as my sensor. The idea is to use the potential across the thermocouple terminals to trip a relay that shuts down the burner once it reaches the selected temperature, then cycles it back on again as the furnace begins to cool. (At least that's what someone suggested to me. I didn't know what a relay was until I looked it up.) Since I want the burner to come on and remain on until the selected temperature is reached, then come back on once the temperature begins to fall, it'll need to be a normally closed relay.

In the temperature range I'm most interested in a Type K thermocouple should produce roughly 25 to 45 millivolts. I'm not sure whether I need a relay that will trip in that range, or whether I should get a relay that trips at some higher voltage and then somehow multiply the voltage across the thermcouple. (I know that can be done, but I have no idea how to do it.)

My main question is how to make furnace temperature selectable. I know very little about relays, but I gather they're built to open or close at a single, constant voltage. I want my relay to trip at any point along a wide range of voltages, depending on what I've selected. How do I do that? The same guy who suggested the relay also suggested a potentiometer, which I could calibrate by hand by measuring the temperature in the furnace at given potentiometer settings. (I have an inexpensive thermocouple thermometer that I could use for calibration.) But I'm not sure I understand exactly how that'd work.

My next questions are what sort of relay I'd need, and where to get one. (The burner runs on 120V/60Hz and draws a maximum of about 6 amperes.) Assuming I do need a potentiometer, the same questions apply.

Thanks in advance for any help.

Your application requires a temperature controller but your proposed solution is a temperature switch and these are two different animals. Temperature switches (thermocouple activated relay) are used mostly as high or low temperature shutoffs or alarms, and have a fixed deadband that does not lend it's self to good temperature control. They lack the circuitry needed to keep a constant temperature at a given setpoint temperature. Temperature controllers usually implement a PID control algorithm that allows 'tuning' of the control loop to minimize over and under shooting of temperature as process load changes.

Large industrial furnaces often utilize both a temperature controller and a independent temperature switch such that the furnace will safely shutdown due to over temperature in case of a failure or maladjustment of the temperature controller. Furnace applications must be designed with safety in mind first and foremost.

Here is a link to PID control theory

https://en.wikipedia.org/wiki/PID_control

Lefty

 

[Matt Bower]

Thanks for the replies.

Let me add a detail that I left out of the original post in an effort not to make it too verbose and complicated. My primary intended use for this furnace is to heat a high temperature salt pot for heat treating. In that application the thermocouple would be placed directly in the salt pot, not the furnace atmosphere. It had occurred to me that a straight thermocouple without some sort of dampening or logic circut might not work that well in an open furnace. But the salts will constitute a reasonably large, stationary thermal mass that, I think, won't be prone to sudden temperature swings, so they shouldn't pose as big a problem for a simple controller to manage. (At least that's my theory; tell me if you think I'm out to lunch.) So I hoped I might be able to get away with a simple controller at least for that purpose.

Matt, There clearly are nice ways of doing what you want to do using microcontrollers and such. However, I gather that you are more interested in getting the furnace going than in a new hobby.

You gather correctly. I'm a little ashamed of my ignorance of electronics -- we live in an electronic soceity, after all -- but I'm already up to my ears in hobbies.

For the first item, I would go with something commercial and probably used. You still might be able to salvage enough parts for that function from a household control. The biggest advantage of the commercial route is that it will have relays of the proper ratings and safety controls(e.g., flame out).

So if I get you correctly, you're suggesting that I should use a commercial thermostat to control the burner and a thermocouple (routed through the appropriate device(s)) to control the thermostat. Yes? That makes some sense to me, since the burner's already designed to be controlled by a thermostat. If that's the case, then I suppose my next step would be to start looking for a used thermostat.

What you need for the second thing will depend on what type of control you have. Datasheets or the manufacturer's site for the original sensor used with that control will hopefully tell you what you need in terms of signal.

And by "control" you mean the thermostat, right?

How big are the pieces you need to heat treat? In the early 1990's, I picked up a small electric furnace for about $50. You may want to reconsider whether the bother and risks of a DIY furnace is worth the money saved. Also, if the parts are very small, you may just use a temperature-indicationg crayon and an open flame to heat treat. John

I'm primarily treating knife blades, so it has to be reasonably good sized; the blade I'm working on right now is about 15" long, including the tang.

I already have an oil burning forge and up to now I've been using that for heat treating -- first in an open flame and now, since I've learned more about heat treating and understand the metallurgy better, in a muffle. The muffle helps with obtaining a more even heat, which is important, and by putting wood scraps in the muffle I can get a nice reducing atmosphere to help prevent scaling. But I still have a problem with temp control. My current oil burner is a very simple, crude design, and while it produces plenty of heat for forging it's not easy to tune precisely and doesn't maintain a very consistent temperature. Consistency is important in heat treating, especially when some of the steels I'm using require a 5-20 minute soak at austenitizing temperature prior to quenching. If the furnace is running a hundred or two hundred degrees hot during part of that soak it can cause problems. So I really want to get more precise control over temperature, and I thought starting with a furnace burner that was designed from the get-go to be electronically controlled would be a good start.

Of course if you think I'd be easier to build an induction furnace, I'm all ears.

 

[Roff]

Could you use **broken link removed**?

 

[Matt Bower]

I don't know. Could I? It certainly looks promising.

 

[Matt Bower]

Heh. I just realized that the seller is the same company that sold me my thermocouple thermometer.

 

[jpanhalt]

Yes Matt, you interpreted my comments correctly, including "control" being the commercial thermostat.

To my way of thinking, every furnace I have seen has all sorts of complicated controls and safety features. I would try to retain those features as much as practical and simply jerryrig the thermostat into "thinking" it was at a much lower temperature. The furnace may also have a high-temp cut-off that is separate from the thermostat. You will probably need to disable or re-rig that too. John

 

[Matt Bower]

As I said, the idea makes sense to me. I don't really know how to accomplish the jerryrigging that you're talking about, though.