With all the virus frenzy behind us now, finally got back into making some progress.
A lot has happened since the last update.
1. Lined the inside of the kiln with 1400C KAO wool and learned a couple of things:
- It is not easy to mount a hot element on KAO wool.
- You have to glue KAO wool to the fire bricks using mortar and the density of high temp wool makes it not want to stick to the sides and top. Eventually used some pins made from Nichrome to pretty much staple it to the bricks.
- Used 25mm wool, reducing the inside dimensions by 50mm on all axis.
- Reached 900 C in about 10 minutes and shortly thereafter reached room temperature as the element burned right where it enters the kiln. Suspect the mounting/"hanging" of the element put too much strain on the single strand entry into the hot face and gravity pretty much did the rest.
- Even though the KAO wool product I used only had a non-carcinogenic irritation risk during handling, after it was heated to over 700 C, apparently it forms structures that are pretty much back to carcinogenic when inhaled again. This prompted me to seal the hotface to prevent those small needles from circulating and venting. Used some water glass; it worked okay but its like glue and sticks to everything. For some reason a yellow deposit formed just behind where the element touched the wool/glass sealed surface after firing. Hoping it was sulfur and not chromium.
- To avoid stuff sticking to the water glass and thus sealing the door shut once fired, had to use some kiln wash on the door to create a barrier.
All up, the KAO wool lining made the internal go hotter faster using less electricity, but the lack of structural integrity on the KAO wool walls caused a element failure and the super fast temperature climb and fall caused the sealed glass structure to crack and decompose even after the second firing. This made the kiln a pretty unsafe tool as it was and decided to abandon the KAO wool hot surface idea.
2. Having removed the KAO wool, the overall structure was back to the original design. Difference now was that the bricks all had mortar between them, sealing the inside more than it was. Cut deeper groves to take the element a bit better and replaced it with some off the shelve 1200W elements (5mm x 480mm).
- Testing revealed similar performance to the original design, suggesting that the sealing did not make that big a difference and convection was not that big an energy drain.
- Cheap hacksaw blades work like a charm to cut the grooves and a lot less dust than power tools. They work for cutting the bricks in half as well but I used 5 blades in total for 4 brick splits and grooves on 3 bricks. Probably because the blades are just carbon steel and the stuff the bricks are made of is the same stuff they put on emery paper which in turn tends to be used to sharpen metal blades (doing the blunting role in this case).
At this point, the only way to get to the ramp rates I was after at higher temperatures was to either go to a 15A power point or start looking at 3 phase/multiple feed points to power the coils. High voltage is scary and adding so much complexity is not something I'd like to troubleshoot and maintain. So major turning point for the kiln design was in order.
3. Pretty much ditched the original brick layout and reduced the overall design to use 7 bricks. Cut all but the base 2 bricks to 50mm thickness and created a 127x127x280 internal size oven. Wrapped the lot in some of that leftover KAO wool from point 1 above and only fitted one coil. Some notes and current performance:
- Front door is a stainless box wrapping some of that 25mm KAO wool.
- Brick structure is now snugly wrapped in a KAO wool blanked, 25mm thick. As a result, the biggest heat leak is the 25mm thick door.
- With less brick mass to heat up and less surface area to conduct heat to the outside, the kiln now goes from room temperature (20 C) to 1000 C in under an hour and up to 700 C in about 20 minutes. Sounds slow, but to speed it up is simple, just throw more watts at it. Currently it pulls a mere 1200W under about 400C and because the coil resistance increases, it looks like it settles at about 1000W over 400/500C.
- Calibrated the PID up to 1100 C and the element did not melt, guess that's a nice bonus and gives me the full range I was aiming for. Just have to be patient with the > 700 C sessions.
- Outside of the kiln was constructed from a mix of galvanized sheet and stainless. The door gets to about 260 C, but the rest of the structure stays under 200 C easily. The bottom half of the structure can be touched and handled; this excludes the floor though as it is in contact with the base bricks and not KAO wool as the rest of the inside.
- Even with the venting hole open, the temperature does not drop by more than a couple of degrees and the PID does a good job to add some more juice when needed.
That wraps up some notes on the enclosure construction and the overall design changes to where it is today. On the software front, some changes include:
- Wifi connectivity to control the system using REST endpoints. Updating the firmware to test small changes to code gets old very quickly and UI is very complex needing lots of changes to tweak and test things. So writing a website on my local PC with the flexibility to change things easily with the aim to use it as a final UI website hosted on the ESP. The touch screen will still contain basic controls and feedback but things like custom schedules etc. will have to be done via the website UI.
- PID now supports calibration with 12 or so preset points. One reader asked how the system would cope with the temperature accuracy across such a wide range; this is how. In general the overshoot is less than 5 degrees on the low end and at the high end (>500C) it seems to be less than 2 degrees. These numbers are not very indicative as it is the temperature measured at a single point in mid air by the thermocouple but I suppose I'm not using it for super accurate lab work and in most use cases some sort of soak is needed to make sure the work piece is a temperature anyway. Suppose in use I'll get to know what the on screen temperature suggests a certain point in the kiln is actually at (like colder at the door and hotter at the top).
- PID also supports ramp rate limiting now. This was implemented using a very basic technique and does the job given the limited resources available on the MCU. So slowing down the heating process on the way to a hotter target temperature is working as well as slowing down the cooling process on the way down. The brick insulation does a pretty good job of slowing down cooling without this feature, but I suspect for glass work it will still be too fast.
Managed to fuse some PMC silver at 900 C for 2 hours. It did a good job as the pieces flex without breaking. Also melted down some silver directly into a graphite ingot mould at 930 C. Guess it was sterling silver. As for PID accuracy, polished up some mild steel and set the target to 290 C. Came out an even, dark blue as the color charts suggest.
That's a lot of writing, but there was even more making to have it all happen.
Next steps include finishing that UI and hunting down some bugs. I think the PID is pretty much done and can be used to control any oven matching the control circuitry it uses. If anyone is interested in building a clone PID component and giving some feedback on notes and code I can get it hosted for so even more people can make their own working builds.
In the meantime, my wife will be using it to fire some of the PMC as the touch screen UI as it stands supports that. I'll use Postman and the REST endpoints to fire some other projects needing custom schedules and temperatures.
In your face CovId.