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
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
I rebuilt a thermal chamber a few years ago
and used a little Fuji PID controller in bang-
bang mode. Worked great.
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.
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.
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,
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
You could use the variac in conjunction with each power setting of the
furnace, to get very nice control within each range.
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.
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.
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.
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.
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
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
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
Rambling on various trains of thought but hope that info is of use.
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
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....
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.
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
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
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. :(
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.
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.
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
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
and get my email - the reply to
address I use for USENET is invalid
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.
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