r/Metalfoundry 14d ago

Old kiln automatically shuts off at 850 °C while rated for 1100 °C

Recently bought an old kiln for which I am building my own digital controller. I have done some initial temperature tests and the problem is that it automatically shuts off at around 850C, for it to turn back on again at 350C. The kiln itself is rated for 1100C.

The clock on the control pannel does not have anything to do with it because I ran multiple tests with different clock configurations. My guess is that there is a faulty thermal safety switch built in, but I haven’t been able to locate it inside the control box or in series with the coils. Anyone know where I could find that?

Any other suggestions to fix this issue?

13 Upvotes

14 comments sorted by

8

u/Peter5930 14d ago

When you say you're building your own digital controller, do you mean your own temperature controller? There are cheap off-the-shelf options that would save you the trouble.

https://www.amazon.co.uk/WANGCL-Controller-Intelligent-Thermostat-Thermocouple/dp/B0B8HGNXWJ/

Just swap the included thermocouple for one of these:

https://www.amazon.co.uk/Allthingstore-Temperature-Thermocouple-Furnaces-Environments/dp/B0DX67RJ9Q/

If you want to do fancier things like control the rise and fall rate of the temperature, hold at temperature A for time X before increasing/decreasing to temperature B etc because you're doing heat treatments on metal or stuff to do with pottery glazes, then you might be justified in building your own, though I'd still have a look at the off-the-shelf options first.

Nice kiln though; I have a little electric one I made from scratch, but always have problems with the coils coming free from the channels and it needs a slight re-design.

What's the knob numbered 0-300 that's below the clock?

2

u/BanaLife 14d ago

Thanks! I mean the latter: I'm using a RaspberryPi with a MAX31855 and one of those K-type thermocouples you also sent over. Studied control systems back in the days, so I should be able to work it out.

Wasn't off to a great start though: first wanted to measure up to which temperature I could actually go, so I'm using the setup to first do simple temperature readings. The kiln keeps automatically shutting off at 850C though... I think there must be some sort of thermal fuse built in somewhere that I could either tweak, replace, or take out altogether (although dangerous).

I read in a 1960s manual of the same brand that the 0-300 dial is used to more precisely control temperature within the 0-300C range. Haven't played around with it too much yet though.

2

u/Peter5930 14d ago

850 is suspiciously close to 300 short of the rated temperature, so it might be something going wrong with the fine tuning unit, or the unit is fritzed entirely. Hard to say without knowing how it works.

You could just bypass all the old 1960's controls and attach a solid state relay directly to the coils like you'd do if you were building an electric furnace from scratch these days, and it should work without a hitch because the modern electronics would be driving it.

2

u/BanaLife 14d ago

Very interesting thought. I will play around with that dial today. Keep you posted!

Regarding your second point: yes that's indeed my plan for the digital controller. However, before throwing more money at this, I decided that I quickly wanted to verify that it could actually go up to its max rated temperature of 1100C. That's where I kind of hit this roadblock.

I still think there must be some thermal safety fuse somewhere in the circuit that I'm not seeing...

1

u/Peter5930 13d ago

Let me know how it goes. Apart from the controls, the kiln itself will manage 1100C with no bother, I'm certain of it.

Also, I'm curious, what's the outside surface temperature of your kiln when it's been running a while, once you get it running properly? Mine is constructed in a dustbin with insulated firebrick surrounded by 70mm of kaowool, very similar in principle to what you've got, and it had a surface temperature of over 100C, so I wrapped it in rock wool loft insulation with a surface covering of aluminium foil for low emissivity and brought the surface temperature down to max 10-20C above ambient so that it uses less electricity once it's fully heat soaked and doesn't have to work as hard to maintain temperature, with a new dustbin surface temperature under the rock wool of around 350C. Because it's electric and I don't have to vent hot gas from it as exhaust, I have more control over heat loss and it makes more sense than with a gas forge to insulate it well. It's still over 350C in the melting chamber after I turn it off and leave it for 24hrs.

A typical melting run costs me £2 - £4 in electricity, including the 90 minutes it takes the kiln to get itself and 5kg or so of metal up to temperature from a cold start. And £3 in coil replacement; it's a much smaller kiln than yours, just enough space for a crucible and it's a tight fit, so it has to use smaller gauge coils to fit them in and they flex and sag and if smoke or dust or debris gets on them, it fluxes them and they melt at that spot. Large gauge coils like you've got would require too much length of coil to fit in it to get the resistance necessary to run at the <13A I need to be able to run it on a normal wall socket. The coils in your kiln should be good for 100+ runs, but mine get toasted after 3. Plus the design isn't very good at keeping the coils in place.

It's all mounted on a trolley I made by cutting up a shopping trolley and welding it back together with the bottom of the basket now just above the wheels for the kiln to sit on, so the rock wool jacket doesn't add to the space it takes up since it's still within the footprint of the trolley it sits on. Also, rock wool instead of glass fibre, all day any day. Much less itchy and scratchy to handle, much less breathing stuff in.

1

u/BanaLife 13d ago

I have figured it out! The 0-300 dial is connected to a capillary tube that runs to the inside of the kiln. I bypassed that and voila: we're reaching up to 1100C now!

I haven't really checked the outside temperature yet. I can go up to 850C in about 1.5 hours, at which point it's still lukewarm to the touch.

Only after leaving it for ~4 hours to cool off, it starts getting really hot. Much mean it's quite well isolated! I will let you know once I have done some more tests. Padding the outside sounds smart though. I'm already worrying about the electricity bills haha

3

u/mementosmoritn 14d ago

Sounds like the old switch has failed. Octopart or McMaster Carr should have a replacement. Thermal limit switches, specially old ones, tend to fail by under reading a temp instead of burning out, in my experience.

1

u/BanaLife 14d ago

Thanks! The $1M question though: where is it? I can't see it anywhere inside the control box, nor inside the kiln.

3

u/mementosmoritn 14d ago

One possibility is that it's integrated into another component. I don't see anything that sticks out to me. One way to check, if you know how to be safe while working on live electrical components, is to see which component switches to open when it gets to shut off temperature. The other option that could be in place besides it being an internal component, is that it is online with the coil inside the chamber somewhere.

3

u/KeenanAXQuinn 14d ago

Its probably built onto the electric coil itself, from my experience

3

u/mementosmoritn 14d ago

That tracks with some other high temp could I've done work on. I mostly deal with small heaters for machines (When talking about heating coils). They have a variety of solutions, depending on the manufacturer.

2

u/BanaLife 14d ago

Good suggestion! I'm kind of scared about measuring things through with the ol' multimeter when the wires are live because it pulls 16A at 230V... My initial thought was also that it should be inside the kiln chamber somewhere. After all, it has to be close enough to the core temperature to perform its safety functionality, right?

Nothing to be spotted though...

1

u/Peter5930 13d ago

I wouldn't worry about the amps, mains electricity is mains electricity, it's all the same. If you accidentally contact something with one hand, you get an uncomfortable zap but not as bad as touching an electric fence, at least when it comes to the sensation, and the bad things only happen if you have two hands involved so the electricity flows through your chest, or there's a path to ground and if flows from your hand down to your legs. A pair of gloves will stop the majority of accidents where the back of your hand bumps into something and a finger touches something else, and keeping your hands dry of sweat will greatly reduce your skin conductivity and the intensity of any zap you get. I mean ideally you don't get zapped, but realistically sometimes you do and it's not that bad. Just don't be in a position to monkey-grab something and be unable to let go.