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Crucible
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troubleshooting a homemade hotplate
I made my own explosion proof hotplate since manufactured ones are pretty expensive. I made it out of refractory cement so it would withstand high
temp and pressed nichrome wire into the top, which is connected to a variac.
I fired it up for the first time last week, but it didn't get nearly as hot as I was expecting. By my calculations there should be 3.5 Amps running
through the wire which ought to heat up to 1200C at that amperage. Yet after 3 hours continuous use at 100% I couldn't boil water with it.
I covered the wire over with a very thin (.25") layer of cement to seal it in place and distribute the heat more evenly. I can feel a good bit of
heat escaping out the bottom, and the wire itself heats up red hot within moments. Either its losing a lot more heat than I thought through radiation
or the resistance of the wire goes up significantly when it gets hot, which reduces the available amperage.
I'd be interested to hear what a few others think about it and whether I'm doomed to only boiling solvents or if I can tweak it to get more heat out
of her.
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not_important
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Some refractory cements are fair insulators. The ones that are decent thermal conductors do so in all directions, so so far as the heating wire is
concerned it is buried in stuff that it heats to a more-or-less uniform temperature; said stuff then conducts heat away in all directions.
A quick sanity check would to be to put a similar length of nichrome in series and compare its temperature with that of the buried wire - just
radiated heat.
I think you would have done better to make a plate with a wide channel on top to hold the wire, then thinly coat that with low density ceramic, and
finally place the wire using only small tabs of ceramic to hold the wire in place. Use a slab of aluminium for the top, perhaps with a thin ceramic
wash for electrical insulation, or get some ceramic shelf paper.
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JohnWW
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Quote: Originally posted by not_important  | | (cut)I think you would have done better to make a plate with a wide channel on top to hold the wire, then thinly coat that with low density ceramic,
and finally place the wire using only small tabs of ceramic to hold the wire in place. Use a slab of aluminium for the top, perhaps with a thin
ceramic wash for electrical insulation, or get some ceramic shelf paper. |
Aluminium used for such a purpose
is too soft, too liable to oxidize at raised temperatures, and its melting-point (660ºC) is rather on the low side, for an hot-plate surface. Copper
or stainless steel would be a much better top surface.
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Contrabasso
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You will be losing heat through all the surface, and with nichrome the resistance does go up as the temperature rises, so there will be an equilibrium
where decreasing heating due to resistance rise meets increasing heat loss due to temperature rise.
Rebuild it with a bit less wire so you start at a lower resistance, so the temperature can rise further. ALSO make the insulator as a sandwich of
thinner layers that way they make a better insulator. So wind the element into a 5mm slab and place two more 5mm slabs underneath that, then the heat
preferentially leaves via the hot plate surface.
A 5mm thickness of aluminium is a good thermal conductor, but 5mm of multi layered aluminium foil is a good thermal insulator.
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watson.fawkes
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| Quote: Originally posted by Crucible | | By my calculations there should be 3.5 Amps running through the wire which ought to heat up to 1200C at that amperage. |
Measure the current through the resistive element with an ammeter. A cheap ammeter suitable for the purpose will cost you less than
the nichrome wire; it may not last long, but you'll get your reading.
The temperature you achieve at a given wattage thermal output is related to the thermal mass of what's being heated, the thermal resistance from heat
source to heat sink (typically atmosphere), and the temperature of the heat sink. Poor insulation causes high thermal losses and lower ultimate
temperatures.
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peach
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I think you're loosing too much heat through radiation / convection / conduction. And the heat generated depends on things like how thick and long the
wire is.
E.g. if you've used thick bit of wire and not much of it, it won't take very much voltage at all to get 3.5 amps through it.
Changes in temperature are caused by power dissipation, so you're better of thinking of heating power than temperatures.
Power = Current x Voltage
or = I^2R
or = V^2/R
All the same thing said in a different way.
What voltage setting do you have the variac at? My hotplate is about 6" across, as a thin metal top (~a 1/8th" thick) that's supported by insulators
and the element is directly in contact with the metal top. It manages 380C and uses 680W. At 3.5 amps, it'd need 194 volts across the element to
produce that much heat.
I'm guessing you've worked out the voltage setting based on the resistance given for the wire per unit length.
Tell me what voltages you're using, the resistance of the wire and how much you've used.
I think the mistake you've made is to calculate the current you need based on how hot the nichrome wire becomes when it's out in the air or in a
vacuum, on it's own. Wrapped in the cement, it's effectively attached to a gigantic heatsink, that's sucking all the power from the element before it
can accumulate as thermal energy; a high temperature.
I'm not sure what you're using 1200C for. That is a HUGE temperature to expect from something not inside insulation (e.g. the hotplate + flask would
need insulation). If you want to bake solids and things at those temperatures, you need to build a mini furnace instead. A real world hotplate will
never reach that due to radiation / conduction / convection losses. You realize your borosilicate glass would actually melt at that temperature?
I'd also warn you that air pockets or moisture in the cement could cause it to pop when it gets hot. The cement will start falling apart at those
temperatures and the nichrome should only be around 1050 for continuous use. Again, if you're really need 1000C type temperatures, it'll need to be a
furnace of some description.
I have Dave Gingery's "Build a crucible furnace" on the desk here. He gives a recipe for DIY refractory.
4 gallons silica sand
4 gallons grog (burnt clay / pulverized firebricks)
4 gallons fire clay
3 quarts borax
Dry mix it all well, then sprinkle on water until it just barely clumps as a solid ball when squeezed. If it looks like brick mortar, that's as far as
it needs to go (better dryer). Then you ram it into a form and fire it.
These guys sell some nice high temperature heating elements. And I think their site has some PDFs with information on how to best wire and install
them.
One could easily make a very clean, tidy and possibly cheap high temperature furnace with one or two of these and some insulation. I'd buy it rather
than make it. Refractory bricks for 1200C service are incredibly hard and don't cut particularly fast even with diamond blades or the scratch and snap
method. Wools are easier to handle.
You'll notice they sell some elements with very wide bores down their centers and open ends. They'd be ideal for a tube furnace and insulating it
would be easy. Slide some quartz glass tube down the middle, done.
[Edited on 2-7-2010 by peach]
[Edited on 2-7-2010 by peach]
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tumadre
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your heating element isn't going to last very long.
3.5 amps at what voltage?
verify your power levels are high enough, and there would have been significant water evaporation.
I've tried making open channels in ball clay as well, issue is that any contamination causes the clay to conduct, just enough to corrode the element,
what you really want is a non porous ceramic.
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peach
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Good point tumadre, cement is relatively caustic.
The non-porous stuff you're talking about would be refractory. The higher the temperature it's made to withstand, the more aluminum oxide (ceramic) it
has in it. The really pure stuff is a serious pain in the ass to shape, you can't easily cut grooves out of it.
Besides, I have no idea why you'd be trying to get 1,200C from a hotplate in the first place. It'd never going to happen.
I'd use two aluminum or steel plates, sandwich the element between the two and butter the sandwich with some none conducting insulation on either side
of the element. You can bolt the two together around the edges.
If you rip the drive coil out of a brushless motor, you could spin a stir bar without any sparks from the brushes of an AC drive motor.
[Edited on 2-7-2010 by peach]
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Crucible
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The heating wire is 28 gauge resistance of 4.25 Ohms per foot. I got this to replace the 20 gauge wire I tried first, which didn't work because it
was too springy and wouldn't stay in place. I forgot to adjust the length to accommodate the new higher resistance wire and ended up with more than
I'd planned. I'm using just under 8 feet of wire, it measures 31 Ohms on my meter. Voltage is standard US house wiring, ie 110 + or -.
Using the power equation P=CxV I get 385 Watts of power, which does seem a little low. I could take some wire out to lower resistance, but I'm
wondering if that would help or hurt. Is less wire hotter actually going to produce more absolute heat output? There is less wire after all to
produce the heat. I'm thinking joules per minute here. It may be that the heat losses due to convection, radiation, and evaporation from the bath
are simply greater than the total heat input into the "system" by the heating element.
To answer a few questions, the cement has a high ceramic content and is rated for 2500F. I don't actually need 1200C, I was just throwing that out
there as the theoretical max. At the moment 300C should do just fine. There was a good bit of water (trapped in the cement) that oozed out the top
when I first turned it on, but its been running dry ever since.
I'm not sure how to check the current while its running, so I calculated it instead. When I hooked up my multimeter while it was on, it created a
short circuit and blew the fuse on the variac. Won't be trying that again! 
Not sure what you mean about putting some wire in series to check heat. How would I actually measure the heat? I can see the wire is getting
red/orange hot, so I don't have much concern on that score. How do you mean to hold the wire with ceramic tabs? The weight of a small tab alone
won't hold it, it would have to be embedded into something. When it was wet, I held the wire in place with staples, pushed into the cement and pulled
them out once it had dried. Getting the wire to stay put was the hardest part and required an extra layer on top because some bits were still coming
out.
If I'm reading you right watson.fawkes then if I reduce the thermal mass, ie the water bath I'm trying to heat, then I will get higher temps. Right?
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DJF90
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To measure current connect the multimeter in series, and for voltage, in parallel. I do hope you know what these terms mean. From the power equation
P=V^2/R, recreasing the resistance at a constant voltage ought to increase your power output. Of course if you want to make calculations, you need to
measure the voltage across your coil. I'm no electrical engineer; someone like woelen would be much better at helping you, but I know enough to get me
by
[Edited on 2-7-2010 by DJF90]
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Nicodem
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| Quote: Originally posted by Crucible | | I'm not sure how to check the current while its running, so I calculated it instead. When I hooked up my multimeter while it was on, it created a
short circuit and blew the fuse on the variac. Won't be trying that again!
|
Current is measured in series, not in paralel! 
I thought that is obvious, after all it is not called electrical current for nothing.
It is the potential that is measured in paralel. Obviously, as it is an electrical potential between two different points in the circuit.
Anyway, 385W would be too low for much use already if it was that much, but that is based on the initial resistance. Most metals have a notable rise
in resistance at higher temperatures (http://en.wikipedia.org/wiki/Electrical_resistance).
For example, check the resistance of an incandescent bulb, calculate what power would that give if the resistance would be the same when also when in
operation (P=U^2/R) and compare that to the nominal power on the bulb. You'll see there is a huge difference, and that comes from the rise in
resistivity at the couple of thousand celsiuses at which the filament heats.
For such a massive construction, with such a high area of uninsulated surface, you would need at least 1kW, as you would have to use a current
regulator for most lab uses anyway, so it does not really hurt to have the max power higher than needed.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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densest
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Nichrome is unusual in that its temperature coefficient of resistance is quite low - much lower than most metals. Many nickel-containing alloys are
similar.
One way to salvage the hotplate would be to dig down to the heating element at its midpoint. Connect to it there and join the original ends together.
This will give you an element with 1/4 the resistance of the original. It will draw 4x the current giving 4x the heat output. 4 * 385 = 1540W which is
more useful.
Single-element electric stoves are usually 600-1200W, so that's the right order of magnitude.
The points of connection between nichrome and copper are where it will be most likely to burn out. The copper will oxidize and the nichrome will get
especially hot because there's nothing draining heat away from that point. Doubling the nichrome there would have been a good idea. i.e. wind an extra
strand over the joint and a couple of cm into the heating section. McMaster-Carr (and other suppliers) sell nickel wire covered in glass fiber and
nickel clamp connectors for this sort of application. Join the copper to the nickel at least 10cm away from the hot area.
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Contrabasso
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When making an element without access to all the nickel connector blocks and wire tails, double the element wire over for say 6" and fold the end of
that double wire over again then twist it all together carefully.
This means that the free end is four thicknesses of resistance wire so doesn't get as hot, so ordinary connectors work more safely.
Get some clues about the usual resistance for heating elements by measuring the cold resistance with a meter, and also by calculating the hot
resistance by power and volts relationships
P=EI
E=IR
To have some control you need an element more powerful that you think so that you can turn it down by a switching controller -which is a whole degree
topic!
Think about a soldering iron! A small 15w iron will melt solder but it's SMALL, to melt lots of solder you need a bigger iron and more power simply to
keep the temperature up as the surface area gets bigger.
[Edited on 3-7-2010 by Contrabasso]
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peach
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Right, that's nowhere near enough watts. My hotplate is double that and it's element is attached to a thin plate of metal, which is thermally
insulated from the unit.
Think about the equation P = I^2R. The power you're producing will be exponentially more influenced by the current as opposed to the resistance, so
you want to increase the current.
V = IR, so I = V/R. As you have R fixed at a certain value, you can increase I by increasing V. You'd double the current by running the element from a
220-240V supply. If you're in the US, you can get that by twinning together the two lives that are coming into your home. But I would advise extreme
caution doing that if you're not used to the mains. A transformer would be a safer idea.
A shorter element will get hotter with the same voltage across it, but it won't spread the heat out very well. You could also use a thicker element.
With regards to water, you have to be concerned about water trapped inside the cement. The safest way to get rid of that is to run the plate at a low
temperature for a long time to bake any moisture out.
In terms of getting this to work, I believe you need to rethink the design. Make the platter around 6" and make it thinner. Stand it on thermal
insulating block from the building yard, the kind that are porous, very light and can be cut with a saw. That'll support the thinner platter and
provide good thermal insulation.
A better option, in my opinion, is the one I suggested earlier. Simply get two pieces of aluminum plate (real cheap, onlinemetals.com) and sandwich
the element between the two with some insulation. Drill a few holes round the edges, bolt them together. Stand that on some of those thermally
insulating blocks.
I think you may struggle with the cement mix and making it thinner risks it snapping with hot chemicals on the platter.
I've often heard people going on about using low wattage soldering irons for 'careful work'. That's complete balls. If I use a lower wattage iron
(15W), it cools down too much when it touches the component and takes time to heat up. During which, the heat is flowing down the leads and into the
parts. I do far less damage to parts with a 30W+ iron as I can melt the solder as soon as I touch the lead, so the heat has less time to transmit down
the lead. Even with some small components, I've barely been able to get them hot enough to solder with 15W irons, but the component has become
roasting hot in the mean time. A lower wattage iron means more energy ends up in the component than it would with a higher wattage iron, since it
needs such a long contact time, allowing conduction to take place. I've also had one of Hakko's new soldering stations, the really expensive ones.
They're incredibly nice, but I can do 99% of the work with a $5, unregulated, no name iron.
[Edited on 3-7-2010 by peach]
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Crucible
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| Quote: Originally posted by densest |
One way to salvage the hotplate would be to dig down to the heating element at its midpoint. Connect to it there and join the original ends together.
This will give you an element with 1/4 the resistance of the original. It will draw 4x the current giving 4x the heat output. 4 * 385 = 1540W which is
more useful.
|
That's a great idea! There are several points I could bridge in the wire to shorten the circuit. The middle will go dead but the edges will heat up.
I think I won't cut quite so much as you suggest out because the variac has a minimum load of 10Ohms. Still I think that basic concept should work.
| Quote: Originally posted by densest |
The points of connection between nichrome and copper are where it will be most likely to burn out. The copper will oxidize and the nichrome will get
especially hot because there's nothing draining heat away from that point. Doubling the nichrome there would have been a good idea. i.e. wind an extra
strand over the joint and a couple of cm into the heating section. McMaster-Carr (and other suppliers) sell nickel wire covered in glass fiber and
nickel clamp connectors for this sort of application. Join the copper to the nickel at least 10cm away from the hot area.
|
I'll keep that in mind, but there's not any copper in this at all. The nichrome is connected to a ceramic terminal connector with I'm guessing nickel
or steel terminals. I'll check the resistance while its hot and see how much the temp changes it.
@peach - I thought about using a step up transformer to increase the voltage but the variac is only rated for 120 V, so that won't work. When I make
my next heater I will have all these lessons under my belt and make something really good!
Something else I was wondering was since I wanted it to be explosion proof, whether the exposed heating element could trigger an explosion, since it
gets quite hot. Well above the flash point of some of the more flammable things I may be heating. This is why I buried it under a layer of cement,
but it pokes up where it connects to the terminals. Is this a serious risk or am I being paranoid?
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peach
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[edit: I'm glad to see you're using a transformer; providing an isolated floating voltage. I expect someone may be tempted to try it directly from the
mains. That will end in pain]
You can still use a step up transformer after the variac, because it'll only see 120V on it's own coil. Just be sure to put it AFTER the variac.
The flash point of things gets confused with another term, the auto-ignition temperature.
The first is the temperature at which the chemical has a vapor pressure high enough that'll ignite if subjected to a source of ignition in the air.
Auto-ignition is when something spontaneously ignites without a spark, and it's often at a much higher temperature.
For your plate, you'd want to be more worried about sparks coming from the contacts on the variac as you dial the voltages around. Or from the light
switch.
But having white hot wires poking through the cement might be an issue too. I'm also wondering how long those bits will last with the temperature
difference, alkaline nature of the cement and all that jazz.
Check the auto-ignition temperatures of the things you're dealing with prior to heating them up.
Remember, industry only uses explosion proof plates in case there's an accident, not as a form of protection. The first thing they do is try to stop
that flammable / explosive thing getting into the atmosphere in the first place. This is analogous to my ideas on fume hoods versus wash bottles. One
stops a problem from becoming a problem (wash head), the other attempts to fix it once it is a problem (fume hood).
Now you've given me a burning desire to make some fireworks. Maybe I could use one to distract this damn moth that keeps harassing me.
[Edited on 6-7-2010 by peach]
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densest
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@crucible - it's easy to lower the resistance without making any part go dead.
Right now you have mains1 --/\/\/\/\/\/\/-- mains2
If you tap into the middle and join the ends so end1 & end2 go to mains1 & tap goes to mains2,
end1--/\/\/\/-tap-\/\/\/\---end2
then the resistance is 1/4 the original.
You can tap in 2 places and get 1/9 the resistance by joining end1 & tap2, end2 & tap1.
Good luck!
[Edited on 6-7-2010 by densest]
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cnidocyte
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This is honestly one of the coolest improvs I've ever seen. Electronics is one of my many hobbies so I may build one of these myself some day.
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Crucible
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@densest - Ah, I get you now. But that would require a more thorough rewiring than a simple bridge and that's going to be hard to do with the cement
already set.
@peach - I hadn't thought of that! Yeah, I could step up after the variac. I'd need the right kind of transformer though, but I could easily double
the temps by doubling voltage. Since both 110 and 220 are very common voltages, there must be a simple transformer I could use.
That could end up being the best of both worlds, because one of the things I was concerned with about having a super high temp hotplate was losing
granularity of control. But I can take a transformer out of circuit for low temp stuff and only use it when I need more heat. Do you have any
suggestions for what kind of transformer I would want specifically?
Yeah, auto-ignition is what I was thinking of. So far I've only used it to distill acetone, and I sealed up the joints but I could still smell it a
little. At some point I plan to fraction some pentane which is quite a lot more flammable. Seeing those wires glowing red hot made me a little
concerned. I've already eliminated pretty much every other source of ignition. I turned the breakers for the room off and run an extension cord from
outside to the plate. And the variac itself is solid state and a good 6 feet away from anything combustible. The water pump is, well, under water.
So my biggest worry is those wires.
At some point I want to slide a brushless fan underneath it with rare earth magnets glued to the blades so I can use a magnetic stir bar. I'm just
not sure if the fan I had in mind is going to take the heat coming off the bottom of the hot plate, and if its too far down, the magnets lose strength
to "reach" the top of the plate.
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Crucible
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| Quote: Originally posted by cnidocyte | | This is honestly one of the coolest improvs I've ever seen. Electronics is one of my many hobbies so I may build one of these myself some day.
|
Thanks! I put a lot of work into it. The weakest link temp-wise is the ceramic terminal connector which is still rated at 800C, so once I get the
electrical stuff worked out I should be able to get it quite hot. It has definitely been a learning experience though, all the materials are quite
"finicky" to work with and take a special manner to get them to take on the shape I had in mind. A lot of mistakes were made and redone! I'm going
to make a heating mantle next, using the same type of materials. If you are looking for something a little easier to make, I'd suggest taking a
cooking hotplate and take the temp control knob out and rewire the heating element to a variac.
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Contrabasso
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When I had a hotplate stirrer, the element was a ceramic ring under an alloy plate, and the spinning magnet was in the centre of that toroid.
You could increase the power and temperature of your hotplate by adding a 120 - 180 transformer if you can locate one. but that would also change the
life expectancy too. Having the connector block on the top seems strange as that's where you want to put the beaker or flask. Also it means that you
have mains live contacts easily within your touch range when operation the hotplate, Again better if the contacts were under the hearing surface.
Distilling volatile and flammable organics in any quantity is better done very carefully and possible with steam heating -even raise steam in another
room and pipe it to where you need it. I've seen one lab fire through a vapour flash over, one is quite enough.
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JohnWW
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Has anyone bothered to look up the patent literature on hotplates?
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zed
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Umm. I've generally found that standard heating elements, similar to those from an electric stove top, work pretty well. I used to just take a
portable hot plate unit, and remove the temp control, replacing it with a soldered (hard wired) connection.
Thereafter, I would control the temperature remotely. Don't remember what kind of voltage controller I was using, but it wasn't anything as fancy as
a variac. Worked OK I guess. I'm still alive, and I'm not hard to look at.
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densest
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I'll vote with zed - $10 to $15 at several local "remaindered general merchandise" stores. Spilling aqua regia on one usually means replacing it,
though, since the cheap ones have metal covers over the resistance wire. Next time I'll look for a ceramic top one.
I recently got a very inexpensive stirring hot plate on EBay. They're around if you scrounge.
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zed
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I actually got TWO very fancy name-brand stirring hot-plates for under a hundred bucks total....including shipping. After a few minor adjustments,
both worked perfectly.
Now, neither one is explosion proof. For explosion proof, you pay a big premium.
For a hundred bucks or so, I could probably build an explosion proof unit.... inside the shell of a standard stir-plate unit.. Probably use a Gast
2AM airmotor for the stirring component. They usually run at 1000 to 3000 RPM and they don't require a huge flow of compressed air to operate. 1000
RPM is the "sweet spot" for entraining gas.
Once again, I'd remove the temp controller, and control the heat remotely. Then, I'd use a special grounding wire for the whole unit . Static
build-up being a possibility. A spark from any conceivable source, is still a spark.
Naturally, using such an explosion proof unit requires compressed air. So..... you need a pretty good compressor to produce it. Another expense.
Still, you need that compressor anyway.....to power your explosion-proof overhead stirrer.
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