drange27 - 14-2-2011 at 12:18
Hey everyone, new to this forum and had a quick question.
I recently built a small electric furnace using the heating coils out of a furnace. It seems to be working well--after being switched on for 12
minutes, the temperature climbed to 400C, which is when my thermocouple stopped reading, so I'm not sure what the max is until I get a better one.
I'm not entirely familiar with circuits and electronics, so here's my question: is there a cheap, easy way of regulating the temperature inside the
furnace? As it is now, it simply heats up until it can't heat any more. I'm looking for a way to keep it at certain temperatures.
I've heard that it may be possible to wire in a light dimmer switch, which I guess would control the current and ultimately the temperature, but I'm
not sure how to go about doing this.
Any ideas?
Twospoons - 14-2-2011 at 13:36
What you really want is a proper PID controller, made by the likes of Omron. These take almost any form of temp sensor ( thermocouple, thermistor,
platinum resistor ), have nice digital displays for setpoint and current temp and are capable of very accurate control. You can also do fancy stuff
with some, such as a controlled rate of temp change. A worthwhile investment.
You can use a light dimmer, but you will be limited to less than 500 watts. A variac would be better (a variable transformer)
Xenoid - 14-2-2011 at 13:44
Yes, get a PID controller. There are plenty for sale on ebay, get one that comes with a thermocouple and SSR (solid state relay). I bought one a while
back, and the seller posts internationally. Very good value.
peach - 14-2-2011 at 14:34
It depends what level of control you need.
PIDs are used when you need a high level of 'intelligence' in the temperature correction. PID is the algorithm the controller uses to make changes;
what it did last, what change that produced and, with that in mind, what it should do to make the temperature it's reading match the set temperature.
It is a closed loop feedback system that applies 'slightly more, lots more' rather than 'leave element on until ....C'. PID is not limited to
temperature, the algorithm (controller) can collect an input form anything and feed back out the correction value to something else (like a valve);
but they're almost always found with degrees C or F on the front.
Alternatively, you can use one of these super powerful solid state dimmers, which only require that you twirl a potentiometer to set the power level.
18 continuous amps of heating pleasure, 4000VA peak. <--- link numbero uno
These will work on 110V as well, if you're state side or using a site transformer for isolation. They sell others. I seem to recall MetalResearcher on
here has one of these and is using it to control his electric furnace. <--- link part deux
If you don't want to fork out more cash on a thermocouple straight away, you can determine temperatures in furnaces by putting little slithers of
known metals in there. When they melt, you will know the temperature has passed that point. It doesn't necessarily have to be metal or melting, or
could be when something fuses.
There are crayons designed for foundry use that can write on metal and then melt as it reaches a specific temperature, which is a useful way of
tagging each item to ensure it hasn't been sat in a cold or hot spot in the oven or furnace, or not been through at all. <--- Three!
And, lastly, there are pyrometric cones, which topple at specific temperatures <--- MULTI-PASS!
[Edited on 15-2-2011 by peach]
Sedit - 14-2-2011 at 15:54
Kind of like what peach stated about the metal slivers you can go to a ceramic supply house and buy cones. They will melt and bend when the desired
temperature is reached and its how they checked the temperatures of kilns before thermocouples. You can buy a range of them rather cheep and it will
allow you to keep tabs on what the temperature is at assuming you can see inside your furnace. Else you will have to wait till its opened to see what
temperature it reached.
not_important - 14-2-2011 at 16:08
Note that kiln cones are designed for and calibrate under the conditions in a pottery kiln, the gases present cause a bit of change in the slumping
temperature from that in an electric one with access to the atmosphere. May not be important, but should be considered.
The other thing about pyrometric cones is that the common ones go no lower than around 600 C, which likely limits their usefulness in this
application.
Regolith - 15-2-2011 at 05:32
Lots of cheap multimeters these days come with a type K thermocouple just make sure it's capable of displaying 4 digits on the multimeter(if you need
that many). The type K are good to over a thousand Celsius. Be aware they have type K made specifically FOR furnaces that have ceramic covering on the
wire leads rather than fibreglass. The fibreglass that covers the wires will get eaten depending on heat and the gases in the furnace. Should you just
need a quick temp check fibreglass works rather well.
I may make my next new probe, there can be pricey. The raw wire that makes type K can be bought in bulk. Thats not for the faint of heart though you
need a furnace large enough to bake the ceramic probe FOR the furnace. (hint you use a pyrometric cone and fire otherwise blind(this is all many hobby
potters use), then success pending you have a new proper shield for your new furnace grade type K.
http://www.bigceramicstore.com/supplies/kilns/Kiln-Spare-Par...
Both ceramic and metal covered type K probes as well as type S. It's easy to tell the difference as there is a table to bend over and straps next to
the type S. There made of platinum, as opposed to nickel and aluminum.
Peach as usual has my answer almost utterly tied up. An infinite switch (can be swiped off the burner controls for a regular oven. High rating, same
design) is cheap and easy. Warning though your heating wire will change resistance over time and or the voltage to your house swaying by a volt or 2
will cause mad temperature swings in your furnace from an infinite switch. So don't write numbers on the switch and assume that the next time it will
be exactly the same temp at that setting. Use it like a ballpark estimate. There not based on a massive resistance element (okay they are but there's
more). It has a bimetallic strip in the switch and cycles the power on and off based on its internal temp and resistance/voltage. A digital control
interestingly uses exactly the same idea but uses a solid state relay controlled by a microcomputer with sometimes many thermocouples.
OH and for the love of god don't spill metal or anything really, on the exposed coils. Any metal will flux the wire (cause it to form a different
metal with a lower melting point) and destroy it instantly.
Melgar - 18-2-2011 at 06:55
I have an infrared thermometer that can measure up to 500C. Beyond that, you can estimate the temperature by the glow color. I just adjust the
dimmer switch to keep my (admittedly small) tube furnace whatever temperature I need.
