Temperature safety monitor

[sverreb]

I have a relatively thick print bed which means there is a high time constant between heat input at the bottom of the bed until the top responds. I want to regulate the heat to the topside temperature using a probe inserted into the bed near the top for better accuracy, but then I risk having the heater overshoot the temperature by so much it becomes a safety risk and/or a regulating problem.

Is there any way I can regulate a heater to two temperature sensors. Where one is the setpoint and the other is for safety? (I.e. state that the setpoint is to get the top sensor to 120C, but never exceed 150C on the heater element)

I guess as a primitive solution I could set a thermostatic fan to the safety sensor then wire the output from the fanheader to a spare endstop and use that as an e-stop or emergency heater-off event, but this seems a bit clunky and limited, so I am wondering if I am missing some better way?

[Dougal1957]

how thick is the plate?

[sverreb]

The plate plus various things on top (magbase, spring-steel plate, and build plate coating) is approximately 8mm

[Dougal1957]

ah right so your useing steel plate then ally is absolutely fine upto about 10mm thick nevr heard of anyone using steel before as it isn't a particularly good conductor of heat also be aware that magnets when exposed to heat degrade rapidly

[sverreb]

The steel plate is quite thin, so it does not add that much thermal impedance. The magbase is likely the worst offender. The topside including magnets are good to 125C (limited by the polymer coating on top). The bottom side is good to 150C (When the glue starts to break down).
I tend to see 10-20C difference between the topside and bottom during heating so this is a bit too close for comfort if I want to reach 120C bed temperature. I also haven't insulated the bottom yet which may momentarily increase this difference. This is why I'd ideally would want to have the bottom sensor in the regulating loop as well to I could ensure that the heater never exceeds 150C at the price of slowing down the heating a little.

[deckingman]

If you size the heater to suit the build plate then I doubt you'll have any problem with the underside getting so hot that it becomes a fire risk. People regularly use temperatures in the order of 240 deg C plus on their hot ends and I doubt very much if the underside of your build plate would get anywhere close to that (assuming your heater isn't massively over powered).

For info, my bed is 10mm thick aluminium with the sensor fitted into a hole about 40mm deep in one edge but close to the upper surface and I use a 350Watt 240v mains heater and over temperature to the extent that it might be hazardous is not an issue.

If the bed isn't insulated, you could adding insulation which will help from a safety point of view but also speed up the heating time. You could also consider fitting a thermal fuse which would cut the power to the heater.
HTH

Edit. I wrote the above while you were typing so didn't see the bit about the glue breaking down at 150 deg C. That could be an issue.

[sverreb]

I am reasonably certain I will not have a fire hazard regardless. The heating element have a 185C thermal fuse built in.
The 3m 200MP adhesive (which seems to be the ubiquitous choice) is rated to 149C long term.

I do intend to add insulation. I am planning to tape some superwool HT (refractory insulation, non flammable) to the backside using high-temperature aluminum tape.

[fulg]

I believe you could do this with a virtual heater on firmware 1.19. If you use a silicone heater with an integrated thermistor and a separate thermistor at the top (or the side or whatever), then you calibrate the PID loop to the first thermistor (showing the temp of the heater) but in your prints wait for the 2nd thermistor (the virtual heater) to be at the desired temp.

This way the bottom will never exceed 150C even if the top hasn't reached that temperature yet.

I have not done this personally but I plan to do this if I ever rebuild my silicone heater bed, for exactly the same reason.

Cheers,
Ben.

EDIT: Sorry, I meant to link to the relevant parts of the documentation: https://duet3d.com/wiki/G-code#M305:_Set_temperature_sensor_parameters