Using a second thermistor for an extra safety?
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We use a thermistor in a channel on the top of our beds usually. The keenovo heaters have thermistor as well, but the temp difference between the heater and the print surface is enough we don't rely on that one. What would be nice though is to be able to use it for a safety check. Is something like this possible currently?
Some way where if this second thermistor was more than, say, 15c hotter than the first, it could trigger the bed to shut down?
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That would certainly provide the illusion of increased safety. It would work as long as the controller is working properly, but what if the controller isn't working the way it should?
If you want actual safety, put a TCO in series with the power to the bed heater. If the bed gets too hot, it will kill power to the heater.
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You can use the M143 command to assign an additional thermistor to cut off the heater if a predefined temperature limit is exceeded. But as @mrehorstdmd says, a TCO is a better option.
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Yep, planning a TCO, too. But since the thermistor is there, figured why not use it for something.
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As mentioned all I think its going to do is make you feel better with no physical benefit.
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Though not a safety device per se, if I had a thermistor on the heater itself, I might be tempted to just connect it and set a maximum temperature for it, if only to have more instrumentation. (I'm kind of a fan of having instrumentation, as anyone who has seen the dashboard of my car will tell you.) Having more data available about what's going on is useful when something starts not doing what you think that it should.
As a shutdown trigger, I don't think that it can be compared to another one with the firmware, but it can have an upper limit for itself. If it were (As an example) 150°C, that's probably fine for anything up to a 110°C bed, but wouldn't give you nearly the warning for a 60°C target bed temperature, though a human watching the temperature graphs might notice a discrepancy if the heater was starting to lose good contact to the bed surface.
Human attention is one of the best (and worst) safety mechanisms.
