Desk Top Furnace

I have recently acquired a small furnace that I plan to use for heat treating. Its 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
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John Normile wrote:

I rebuilt a thermal chamber a few years ago and used a little Fuji PID controller in bang- bang mode. Worked great.
http://www.instrumart.com/Product.aspx?ProductIDG14
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.
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John Normile wrote:

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.
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Goddamm, are we heating a filament to X degrees, or are we sending it to the moon?
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Proctologically Violated wrote:

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.
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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?
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Proctologically Violated wrote:

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.
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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....
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Proctologically Violated wrote:

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.
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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. :(
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Proctologically Violated wrote:

I think the glass flowing myth is just that but it's still perpetuated. See http://www.glassnotes.com/WindowPanes.html . I expect the patterns are not due to sagging but just due to the way the glass is hand made. IIRC the glass can be spun out into a large disc or blown into a large cylinder which is then slit open and flattened, either way produces glass which is not truely flat and you get the distortions you see. Some of the process is shown here
http://www.youtube.com/watch?v=gsOUyqGa9FM
starting about 6:50 and continues here
http://www.youtube.com/watch?v=u4gy7l6vp5I

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Excellent stuff. I stand corrected!
Next time I want the sagging/distorted effect, I'll just take a torch to new glass!
Excellent video. Def'ly start at 6:50, tho. Fascinating from there on.
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On Fri, 6 Feb 2009 13:50:10 -0500, "Proctologically Violated"

Not if it's Tempered Glass, you won't... (It'll just shatter.)
The very old glass was hand blown and then hand formed into small panes, and even when they were trying for flat glass it was all distorted from the hand work involved. "Picture Windows" were way off on the horizon, the best you got were multiple mostly clear panes in a larger window unit.
Or they didn't even try for clear and molded it with obscuring ridges and patterns, and a variety of colorants added. Then some bright person got the idea for Stained Glass.
They didn't get truly flat glass or very big sheets till they came up with the idea of using a bed of molten Tin as a casting surface, still cool enough that the glass woiuld solidify on the Tin. And the results were... Float Glass. As big as you want it, they just need to build a wider floating tank and a wider finishing line.
--<< Bruce >>--
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David Billington wrote:

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
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Have you found any deals on RTD's on eBay? I see that some RTD's go up to 900 Deg F and they seem more accurate and stable than thermocouples. At work we have machines that use a dual RTD, if one of the sensors goes bad, it can detect it. Maintenance changes out the RTD's and throws the old one away, the old one usually has 1 working and 1 bad RTD in it. I need to have some curing techs stash me some back and get a scrap pass to take them home.
RogerN
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RogerN wrote:

I'll sell you mine, but I don't know what temp it is good for. I'll take another look at it tonight. It is brand-new, with manufacturer's tag still on it.
Jon
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Well, I kind of have about 15 temperature controls right now. I have 3 Cal 9900's, and 11 Fuji controls with ramp and soak, plus another non-display thermocouple in, triac out control. But someone here might want to give it a try. You can use a 100+ ohm resistor instead of an RTD to test them out. I made some test plugs at work using 110 ohm resistors to test between bad RTD's or bad cables.
RogerN
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Proctologically Violated wrote:

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
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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.
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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.
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