My wife is a studio potter, and mentioned she would like to be able to "keep an eye" on her kiln firings without having to trot to the kiln every hour or so.
Apart from the very expensive industrial-type controllers that include a remote feed, I couldn't find anything either available or even home-built. So I set to the challenge...
The result is a non-invasive log of the electricity use which is stored in the cloud and can be readout from anywhere (with internet access).
 |
| Screen capture from my kilnmonitor remote on ThingSpeak (a glaze firing) |
The latest / live data of our kiln is shown below, with the full thingspeak page here. If the time is 'now' (and the red power line not complete) then a firing is in actual progress. If the curve looks nothing like the reference curves, it is (hopefully!) because we're firing a non-standard profile.
latest firing profile with references bisque (g) and glaze (b)
The build
Early on I decided against trying to measure the kiln temperature direct: you cannot 'piggy-back' on your existing thermocouple (they are far too sensitive for that) and adding a second one, whilst feasible, was considered both too expensive and too intrusive as you need an R or S type to measure up to the 1300ºC required.So instead I am using a non-intrusive indirect measurement: the kiln's electricity use.
Initially I built a prototype that gave real power by measuring both voltage and current, and that worked well. However, I didn't like the requirement to physically wire into the kiln high-voltage wires, so I dropped the voltage measurement and assume it constant (as most home power monitoring gadgets do). And as it turned out, the lack of measuring the exact voltage does not introduce an unacceptable variation.
Measuring only the electricity amperage turns out to give a perfectly suitable 'signature profile' of your firings which you'll become familiar with very quickly. It has been in use here for well over a year now and many firings.
 |
| figure 1: Comparison of measured profiles for three bisque firings (program: 100ºC/hr to 600ºC, then 200ºC/hr to 950ºC, 30min soak). Ramp switch-over, start of soak and finish are clearly identifiable on all, plenty good enough as "fly on the wall" monitor. |
The monitor consists of a single ESP-32 micro-controller which does all the high-speed sampling (over 90kHz!) of a non-invasive AC current sensor and once a minute sends the sum value via the light-weight and very fast MQTT protocol to a ThingSpeak channel which can be viewed from anywhere with web access.
The monitor itself is so small it fits literally inside a business card box.
A red and a green LED light complement the setup for a local visual indication of the system starting up (red steady), connecting to wifi (red blinking), measuring 'zero' (green blinking) and measuring a 'kiln firing' signal (green steady).
 |
| figure 2: the KilnMonitor and AC current measurement sensor. |
Components in this build
- NodeMCU ESP-32 ESP-32S WiFi Development Board
- 100A non-invasive AC current sensor split core transformer (SCT-013-000)
- 3.5mm Mini Stereo Jack Chassis Panel Mount Headphone Input Socket Connector
- 1 m stereo jack extension cord
- USB-mini power source, 0.5A minimum (e.g. a phone charger will do fine)
- 70 x 50 mm PCB prototyping board
- a red and a green LED
- assortment of wire, resistors etc.
- a box to protect it in (mine is a business card box)
- a Thingspeak account (free)
- the software ;-)
Version I was an experimental prototype build on a breadboard, but it had been so popular in the pottery studio that it was in constant use. For longevity, this version II was soldered on prototype board and attached to the case using standoff columns.
