Off-hours hobby question from an IT person. My partner has an electric kiln in the garage that fires to cone 6 (about 1220 °C) and occasionally cone 10 (around 1300). The kiln’s own controller is old and twice it has “stuck” a relay and overfired, melting a shelf of work. I want an independent temperature reading in Hermetry and an alert if it goes past a limit or stays hot far too long.
I have an ESP32 already sending garage temperature to Hermetry. I tried a K-type probe with a MAX6675 and it reads 1023.75 and stays there once the kiln is past about 1000 °C. Reading online about R and S type and platinum and “sheaths” made me realise I don’t know what I’m buying.
What hardware for the measurement, and how should the alarm be set up so I don’t get woken at 2 am every normal firing?
MAX6675 tops out at 1024 °C by design, that’s your 1023.75.
Type K goes higher than that though, right? Just the chip is the limit?
K is rated to about 1260 °C but drifts and ages fast near the top, especially in a kiln atmosphere. For cone 10 it’s on the edge.
I fuse glass and have a ceramics friend I set up exactly this for, so I’ve been through the shopping list.
Measurement: use a MAX31856 breakout instead of the MAX6675. It reads K, but also R and S, and handles the full range. For a kiln going to cone 10, an S or R type probe in a ceramic (alumina) protection tube is what kiln makers fit themselves. It’s pricier than K but doesn’t drift the way K does at 1250+. If you stay at cone 6 most of the time, a heavy-gauge K in a ceramic sheath is acceptable, just plan to replace it. Mount the probe through the wall at the same depth as the kiln’s own one so the two readings compare.
Send it as kiln_c every 30 s or so, plus kiln_fault from the MAX31856’s fault register.
Alarm rule on the profile, written in JSON mode because I wanted dampening defaults kept and two severities:
{
"type": "kiln_overfire",
"title": "{{device.name}}: {{value}} °C",
"severities": [
{ "severity": "CRITICAL", "condition": { "key": "kiln_c", "op": ">", "value": 1290 } },
{ "severity": "MAJOR", "condition": { "key": "kiln_c", "op": ">", "value": { "attribute": "max_c" } } }
],
"debounce": { "mode": "count", "raise": 3, "clear": 3 },
"hysteresis": 15,
"enabled": true
}max_c is a device attribute you change per firing (1240 for cone 6), so a normal firing never touches it. The CRITICAL line is the “something is very wrong” ceiling. Three readings in a row means a noisy spike won’t wake you. See Debounce, hysteresis and dampening.
“Stays hot too long” can’t be done in a profile alarm rule since it can’t track time since the firing began. I used a Rules card with a value trigger “rises above 600 and stays so for” 720 minutes, then notify.
And keep a real over-temperature cutout in the kiln’s power circuit. The cloud tells you, it can’t open the contactor if the Wi-Fi is down.
why S type rather than K at that temperature? is it just drift?
Ordered an S type in an alumina tube and a MAX31856. Rule is in, the attribute trick works nicely, set max_c to 1240 tonight and the firing peaked at 1224 with no alarm. A hardware over-temp relay is going in next weekend, fair point. @Aoife mostly drift and the probe’s life, from what I’ve read since.
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