jbruce

@liambesaw The Picker SSR I chose to use does not have mean time between failure (mtbf) information on their spec sheet. They mention UL testing at 100k cycles, but have no details. If you want to believe Crydom, the largest SSR manufacturer, they estimate 2 million hours as their MTBF. This would be for an SSR under consistent use and switching often enough to minimize expansion and contraction of components inside the package. Two million seems a little high to me, haha, 228 years. So, worry about other things, like safety.

I'm not really worried about the number of SSR switching cycles. SSRs use a few really large transistors to switch the current on & off, so as long as you keep them cool, that is what they are designed to do. My particular SSR is zero crossing, so this also reduces wear and tear. The cycle time I use is two seconds and can be changed in the config file. Every 2s, a decision is made about how long to turn the elements on for that interval. Sometimes the elements are not on at all, sometimes they are on for the whole two seconds. If you were genuinely worried about switching cycles, sensor_time_wait is the number to change. Not sure about the elements being "Cranked too high". The elements are all set on high using existing switches on the kiln and controlled by software / SSR. If the set value is 250F, the elements are turned on every 20s or so. If the set value is 1888, the elements are on a little more than half the time. Here are the things I bought: rpi3 with power supply = $48.99 (you could get an rpi zero with a header here for much less) max31855 thermocouple amp = 15.69*2 = $32 (blew one up) gpio breakout board = $8.00 k type thermocouple = $30.00 50 amp ssr = $23.50 leviton 50amp nema 6/50 female connector = $13.00 eaton nema 6/50 male connector = $12.00 aluminum bud box = $35 (you could use something smaller here, but this acts as a partial heatsink for my project. big aluminum boxes love to absorb heat!) package of 300 transistors = $8 (only 299 left to use) package of random resistors = $7 (only 996 left to use) for a total of $217 if my math is right. Hi Bill, yes, the average error in degrees F listed in the stats is the average difference between the PV and SV recorded every 2s during the schedule. I'm not using PWM, just simple on/off for 0 to 100% of every two seconds. It is a zero crossing relay which helps reduce noise caused by switching high currents. I think if you really squeezed the budget and bought just what was necessary from the cheapest sources, you could make this for $100, but some of these components offer convenience or peace of mind or extras that I will use later.

Firing went well. It was a slow bisque fire. Here are the stats... schedule name cone-05-long-bisque-no-evap schedule date 12/17/2018 average error in degrees F 0.74 solid state relay cycles 11249 schedule length in hours 13.35 elements on (s) 22197.33 element percent on 46.19 element watts 9640 cost per kwh 0.126 schedule total cost $7.49 I'm really happy with the PID accuracy. The stats above include the thermocouple spikes, so the actual average error was less. Here is a screenshot after the schedule ended. I stopped it 12 minutes into a soak at 1888F. This is when the cone 05 cone was almost touching the shelf. In the screenshot, the kiln has already cooled a few hundred degrees. I had some problems with thermocouple noise as you can see in the downward spikes shown in the graph. I need to work on fixing that. Also had some strange thermocouple errors in the last 100 degrees that I need to investigate. The software is dependable. I connected three computers, my phone, and my tablet to the rpi during the day and no glitches or problems with it. I did realize I'm eventually gonna need to add a new feature to restart at a specific point in a firing. I need to think about how I want to code that for the next couple days. I need this in case I accidently hit the STOP button, or if there is a power outage.

I test fired my kiln with the new controller to 1000F last night. It included some fast ramps of 1200F/hour to 800F and from there 600F/hour to 1000F plus a soak of a few minutes. This was just to work out any glitches before I do a bisque fire. The software is configured to run every 2s and I grabbed some stats from that... average error in degrees F 0.7665486726 solid state relay cycles 977 schedule length in hours 1.00 elements on (s) 1934.91 element percent on 53.75 element watts 9640 cost per kwh 0.126 schedule total cost $0.65 I'll pack the kiln today and bisque fire tomorrow. I'll post details after that.

I added a parts list and schematic. I'll post my stats on a firing next week... things like number of times the ssr switched, the average error across a firing etc. https://github.com/jbruce12000/kiln-controller

Thanks with regards to safety. I'll continue to use both the kiln sitter (with one cone above intended firing temp), and safety timer so that I don't burn down the house. When solid state relays fail, they usually fail with a short circuit... so the kiln get's full power. The safety I have in place today is... 1. warning if temp is N degrees outside of schedule 2. software shutdown if max temp limit is reached 3. kiln sitter 4. kiln timer and of course walking down to the basement to check near end of schedule

Hi Bill, Cool project! and great to meet you. That looks like a lot of work too! So, you're regulating a gas kiln! What kind of regulator/actuator do you use to control the gas pressure? My kiln is an old Skutt kiln sitter so it is electric. I hope to have the kiln firing in the next two weeks. We'll see how busy things get with the holidays coming up. Thanks for the suggestion on Bartlett schedules. I'll check them out. I've found a couple schedules and they all advocate a slow approach to the final temperature. I plan to soak for as long as it takes for the witness cone to reach 90. Everything in the project is released under the gnu gpl license, so liability will not be an issue for me (as far as others using the code or ideas in github). If I was to implement this for say... a school... especially if I charged for my work, I could be held liable... but this is my personal kiln.

I got tired with going down to the basement to make sure my kiln was working properly and running on schedule so I started (forked) a project to control my KS-1018 using a raspberry pi. With this I can: monitor my kiln from anywhere using any device that has a web browser (phone/tablet/computer) easily create new schedules, edit existing schedules, including infinite ramps & soaks accurate PID control get schedule cost estimates and actuals If anyone is interested, here is the github link... https://github.com/jbruce12000/kiln-controller The cost for everything I bought was under $200.00, but my time investment has been high. Again, this is for geeks that were born with a soldering iron in one hand.