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.