Desk Top Furnace

I have recently acquired a small furnace that I plan to use for heat
treating. It?s temperature is controlled by a 3 position switch.
The low setting draws 2 amps and heats to 600 degrees F. The medium
setting draws 4 amps, and heats to 1,000. The high setting draws 8
amps and delivers 1,600 degrees. There are no other temperature
adjustments.
I would like to better control the heat in this furnace to allow
settings between the current settings. Not being an electrical type,
I do not know the best way to cheaply control the temperature. My
two thoughts are a resistor device of some kind, or a Variac. Short
of a professional (expensive) temperature controller, what should I
look into?
Thanks
John Normile
Reply to
John Normile
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I rebuilt a thermal chamber a few years ago and used a little Fuji PID controller in bang- bang mode. Worked great.
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You'll need the controller, a solid state relay, and either a thermocouple or RTD. It will take some electrical skill to put it together, but not too much.
The controller is incredibly complex and will take an hour or two to understand and use.
Reply to
Jim Stewart
As Jim mentioned in his reply a temperature controller, typically with PID (proportional, integral, differential), capable of SSR (solid state relay) drive, and a thermocouple would be a good choice. I don't think RTDs can go to 1600F. You can use a controller without SSR drive, these will have a relay output that can be used to drive a contactor to power the heating coils. The contactor and relay being mechanical can't switch as fast as an SSR so are potentially harder on the heating coils. For the temperature range you are looking at a K type thermocouple would be common and cheap. Ebay is a good place to start looking. I have bought a couple of Cal Controls unit from there but there are plenty of other makes about. A simple controller can be set to do a ramp upto a temp and hold it, more complicated ones can hold multiple programs with many ramp hold segments etc.
Reply to
David Billington
Variac, with an ammeter/voltmeter. The standard "student" variacs are good to 7.5 amps or so, AND also go to about 130 V, iirc. I'm sure you can push them to 8 amps or so.
Of course, any variable 120 V powersupply would do, capable of 8 amps.
A digital thermometer would be nice. Omega in NJ has them, to any accuracy, range.
I suspect you can get a variac perty cheap. But if not, measure resistance of the various coils, and just buy power resistors to place in series, for whatever current draws you like. You'll have to fool around with V=IR, and the P=I^2R, to get the right value and power rating of the resistors.
A variac, tho, is the best and simplest way to go, imo. Or other solidstate variable powersupply. You could use the variac in conjunction with each power setting of the furnace, to get very nice control within each range.
Reply to
Proctologically Violated©®
Goddamm, are we heating a filament to X degrees, or are we sending it to the moon?
Reply to
Proctologically Violated©®
Search on eBay for a thermocouple temperature controller. You will also need to get a high-temperature thermocouple, possibly a platinum type. For a little more money, instead of a simple temperature controller, you can get a "ramp and soak" controller. This allows you to set a temperature rate of rise, a "soak" time and a rate of decrease. You put your piece in the oven cold, set the parameters, and it does the entire heat treat program automatically.
You will also need to get a 10 A solid state relay to control the heating elements. Unless you were to get really lucky on a variac on eBay, the controller, thermocouple and SSR could all cost less than a big Variac.
The Variac way would require constant monitoring as the furnace heats up to prevent an overtemp when you reach the desired setting. Any voltage fluctuation in the home (AC, oven, clothes dryer turning on) would affect your temperature.
Jon
Reply to
Jon Elson
OK, it may be more than he needs or thinks he needs at the moment. I use my furnace for heat treatment such as normalising, annealing, and tempering, also some glass work, and drying and firing refractory castables. Some things I can set the controller to go balls out to get to the temperature, some such as the refractories have ramp rates of 20C/hour to 40C/hour and need to be held at temps before ramping up again. The nice thing about the controller is you can set it and it'll do the ramp and hold the set temperature. I have a small furnace which is not currently controlled, except by an energy controller, and it's very easy to overshoot and a pain to have to tend it to hold temps.
Reply to
David Billington
Yeah, I guess for persnickety stuff, it's nice to have a chip in there, thinking away...
How much does something like that cost? And do they come with the thermometer/pyrometer?
Reply to
Proctologically Violated©®
If you want to take the advice others have given to use a real controller, I have a Partlow like this that I'm not using.
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This is *not* a PID controller, and not programmable, so the performance won't be as good as an autotuning controller, but it'll be a heck of a lot better than a variac.
If your furnace elements are 120V, I also have some solid state relays that would work well with the controller and not require a power supply to drive them.
$35 via priority mail in the US if you're interested.
Reply to
Ned Simmons
The answer depends upon: - how accurately, & - how automatically
If you have someway to measure the temp & don't mind manual adjustment, then a "router speed control" will work & cheaply:
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Next up would be a simple on/off thermostat, using a thermocouple. Google "high temp thermostat".
The best & most expensive is a PID controller like John recommended.
Bob
Reply to
Bob Engelhardt
Well for the original unit I bought all the parts new so probably about £200 or a bit more, that was maybe 7 years ago. That's a Cal 9400, 50A SSR, heatsink, high speed semiconductor protection fuse (for the SSR), 230V 30A plug and socket, strain reliefs, box, thermocouple connectors, and some cabling. The parts all fitted in a box with the plug and socket on tails out the back so I can use it on various equipment although until recently I only used it on a 4.5kW 18" top loading ceramic kiln. The kiln has the thermocouple and socket wired by me, and the controller has a similar socket so an appropriate thermocouple lead is used to connect the two.
I recently used it to bake some refractory in situ so rigged up a heater for that using metal sheathed elements like those used in an oven and poked a thermocouple in the heated chamber. I use a type N thermocouple due to the main furnace being capable of 1300C (in theory), as it's old and the elements have aged I've only achieved 1200C. As the controller box I built has the socket on the front I can connect any N type with a suitable plug.
Subsequently I have acquired a new Cal 9500P, which can store multiple programs and multiple ramp/hold sections, and a new Cal 3200 from ebay for about 1/4 the new price. Also a new SSR for about 50% new off ebay. The 9500P is destined for a glass melting furnace and the 3200 is likely to end up in my little desktop furnace as it's 1/32 DIN IIRC (about 2" wide x 1" deep). The 9500P could replace the 9400 in it's box as they are the same format 1/16 DIN IIRC ( 2" x 2"). The 9500P is better than the 9400 for the glass melting because it can handle the multistage temperature regimes used for melting glass prior to blowing, and it can store more than one.
To answer the thermometer/pyrometer question, no not that I have seen. The microprocessor controllers I have looked at all could cope with at least maybe 6 thermocouple types and Pt100 so you have the range of capability from the common K type to the more expensive S and R type Pt/PtRh so you chose the thermocouple for the application.
You can buy very basic, these days anyway, controllers which are tailored to a particular thermocouple but are adjustable for temperature, these might suit the OP if basic control only is required. Not looked into these myslef.
So I guess the cost depends on what you can get, get a good deal on ebay or elsewhere and the control comes much cheaper than buying new. I have a number of large heat sinks I can cut up now from a local scrap dealer. looking at the heatsink though on the back of my box at about 3.5" x 3.5" and it only gets too warm to touch when the furnace is running around 1200C.
I have standardised currently on the Cal units as I am familiar with them but they're many other makes out there. On ebay in the UK there is a seller selling out of Ireland that does PID controllers for about £35 delivered IIRC, seems cheap,and can't comment on quality having not bought one. I expect that similar items would be available in the US as well.
Rambling on various trains of thought but hope that info is of use.
Reply to
David Billington
Excellent info. But ahm thinkin of the poor OP, who's proly sayin, Goddamm, all I axed about was a variac.... :)
You mention melting glass for blowing. Why all the complexity for just melting glass? I could see some complexity for tempered glass, as I presume this is a kind of heat treating....
But, from what I lernt, glass is technically a fluid (albeit a highly viscous one), devoid of crystalline structure, so you wouldn't expect so much req'd programmable thermal drama.
Are these type of setups required for good heat-treating of knife blades? Or are knife blades pretty much "heat'em red hot/dump'em in oil" type deal? I should be so lucky....
Reply to
Proctologically Violated©®
Well isn't that the fun of the newsgroup that people with different experiences can give feedback and the OP can learn and hopefully make more informed decisions based on what they intend to do.
While glass for blowing is often held at about 1050C - 1100C, depending on the glass composition, the temperature profile before blowing can effect the quality of the glass. As I intend to use what is called cullet, pre-melted glass, the demands are not as demanding, but basically prior to blowing the glass it is beneficial to raise its temperature to around 1250C - 1300C then reduce it as this helps remove bubbles which are detrimental if the intention is a high quality piece. In my case using wire elements the upper limit is near 1250C but the temp profile would ramp upto the max then run, hold, then down to the blowing temp, not something I can do on the simpler 9400 controller without intervention, although saying that I forgot to mention in the previous post that I have a comms module in the 9400 and can monitor it from my PC, in my case using code I wrote that runs under Linux but IIRC Cal do free software that runs under Windows. IIRC parameters in the controller can be set remotely so a basic controller might be enhanced with outside software.
BTW the other option is what is called batch. Cullet is pre-melted glass but batch is all the constitiuents that make up the given glass, typically in a pellet form, and requires higher temperatures to achieve complete melting but the result is better quality glass from the virgin materials. Often some cullet that is naturally produced in the blowing process as waste is included with the batch to reduce costs.
Yes my understanding as well although the idea that glass still flows at room temp, albeit extremely slowly, has I think been discarded as incorrect as it is so viscous as to be effectively solid in the lifetime of the planet. The main issue I think is the temp profile to get rid of bubbles trapped in the glass to provide a glass as free from defects as possible. The other thermal drama happens when the glass is getting down towards the strain point, where it becomes solid enough to suffer damaging internal stresses, at this point the temperature gradients within the glass have to be managed to prevent destructive stresses. This is the annealing stage and requires slow cooling based on the material and thickness.
Well from reading my packets of O1 it should be held at the tempering temperature for a certain time per inch of thickness so the controller would help but in reality for knives I don't know as the section is small or are you doing crocodile dundee knives?. A thought might be that knives might benefit from a harder edge but softer back for durability but that maybe dependant on use, my knives are used on vegetables and meat and don't put up much of a fight. I tend to follow the book regarding some of these things as I do make items out of thicker O1 where 1/2 hour temper times or more are required IIRC. That being said I had a chat with a mate that is a blacksmith about this and he described how items might be heated, quenched partly, and the remaining heat in the part soaked out to the tip to temper it, quite skillful by the sound of it but that's some of what he does.
Reply to
David Billington
Yeah, I saw something where the blade edge (or the back, forgot which), is masked, somehow, so you get the best of both worlds in the heat treating process. I spose programmability would benefit any type of heat treating.
But, about glass being fluid or not, I'm sure you've seen sagging window glass, which is really quite attractive, in its subtle distortion patterns. And can happen in just a couple of decades.
Under high pressure, like in an ocean floor, glass supposedly becomes quite plastic and fluid, with some environmental consequences.
In fact, for the Yupsters and their Old House Restoration obsessions over here, old window glass is actually salvaged whenever possible, and people reglazing old sash windows pay, I kid you not, $12/sq ft for effing 3/32 window glass, and this was a bunch of years ago!!
I thusly occasionally come home with thrown-out sashes, if I detect any sag in it them. :)
The desirability of this sagging glass attribute in windows has inspired a manufactured effect, and this brand of "textured" clear glass is called "Desag". I know, because me being a wannabee Yupster with a previous restoration obsession, I bought and used some. It comes in all kinds of tints/colors as well.
I myself would have gone after this effect differently, perhaps using a flame for sagging effects, but desag I think essentially casts the glass, so the texture is essentially the pattern in a mold.
The restoration obsession has since been beaten out of me. :(
Reply to
Proctologically Violated©®
Years ago I bought some Fuji digital temperature controllers on eBay for $11 each. They have 4 ramp and soak settings, reads many different types of thermocouples plus RTD's. I also bought a couple of solid state relays (600V 150A) and some thermocouple wire to make my own thermocouples. I have a couple of them running oil filled radiator heaters in my house taking care of colder areas. I've also used these kind of controls with my electric smoker, keeps the temperature right on.
I don't remember right off but I believe type J thermocouples are good up to 1490 or so degrees F and type K up to 2192 deg. F.
I think the ramp and soak would be great for annealing and heat treating metals. If you want to cool down from 1600 degrees to 70 degrees in 99 hours, you can. Some heat treating recommends you heat to 1300 or so degrees for an hour (or so) and then heat to a higher temperature.
My recommendations would be a PID temperature controller (ramp and soak would be nice), a solid state relay, and a type K thermocouple.
RogerN
Reply to
RogerN
Close enough, but you shouldn't use J much about 1000°F, and K will go to about 2500°F before it melts (but won't last long).
Best regards, Spehro Pefhany
Reply to
Spehro Pefhany
if this "small furnace" is an old dental kiln like the one I have, it has a nice big temperature gauge and a suitable thermocouple already - you don't need another thermocouple, use the one that is already there. Oh, and the company is still privately owned by very nice people.
Reply to
Bill Noble
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that's an interesting thought - I have a bag of older Gulton West controllers, model TB1516A-3 - they go from 0 to 1000 degrees F and have an analod temperature display and a knob to set the regulation temperature. I haven't found a manual for them, so
1. if anyone has a manual and could scan it and email to me, I'd be really grateful 2. if anyone wants one or two of these for $10 or 15 each (you choose), drop me a note
to email me, please visit
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and get my email - the reply to address I use for USENET is invalid
thanks
Reply to
Bill Noble
If we had to buy these controllers new from Omega at full list price, I wouldn't have even mentioned such stuff. But, there is SO MUCH industrial control stuff on eBay for cents on the $, that it just doesn't make sense to not take advantage of it. Now, I did have a much more critical thermal job here, I needed to reflow solder on circuit boards, and the temperature between the solder just melting and the board "melting" is not very far apart at all. I managed to snag a 1000 foot roll of micro-size thermocouple wire on eBay. The first temp controller I got was kind of a dud, but the second one was a jewel. I already had experience at work with these things, so it was not a big deal for me to set it up.
Heat treating sounds exactly like a job for a ramp-and-soak programmer, although the job would be in hours instead of very few minutes in my case with the PC boards.
I also think that a ramp temperature programmer would be a lot gentler on heating elements.
Jon
Reply to
Jon Elson
You can get plain single-setpoint Chinese controllers on eBay for $60 new, and some recent surplus units probably are going around $25 - 40. (I haven't looked lately, so this could all be different now.) I got 2 different ramp and soak multi-setpoint controllers on eBay for about $35 each. One wouldn't save the settings, and would only handle RTD sensors. So, I got another one, and it is really nice. The sensors are always separate. As I said, one was for RTD (Platinum resistance temp detector) only. It said so in the eBay ad, I was hoping it could be converted - but no go. The second unit would take any thermocouple known. I snagged a 1000' reel of micro-gauge thermocouple wire (in the unusual T type) on eBay for under $100. There are lots of guys selling pre-made thermocouples on eBay for $15 or so each.
Jon
Reply to
Jon Elson

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