Homebuilt heatbed/heating issues

Nvigr8

Hello all. I've built myself a new heatbed for my Frankenprinter and having some issues heating it.
Issue: The temp reading from the top of the bed is about 30 degrees less than what is reading on the screen. Top reads 78C with IR thermometer and the controller is commanding 110C.
Bed stats:
16" x 16" x 0.25" aluminum plate with spring steel print surface. .25x.25 magnets imbedded in the plate - no magnetic sheet.
I recycled the heat pad from the original bed which was one of the type that plug into the wall and have an inline temperature controller. I assume it is a 120v heater.
It is now mains powered with a SSR.
The powersupply is MEAN WELL SE-600-24 AC to DC Power Supply, Single Output, 24V, 25 Amp, 600W.
I'm using the code below for the bed heater

M308 S0 P"bedtemp" Y"thermistor" T100000 B3950         
M950 H0 C"bedheat" T0                                  
M307 H0 B0 R0.225 C668.2 D8.04 S1.00            	  
M140 H0                                               
M143 H0 S120

I am wondering if this is just how its going to be with the aluminum plate bed...
or...
Am I using the wrong thermistor code? I don't know what thermistor is imbedded in there. Any suggestions for a different one that would compensate for the 30 degree difference?
Should I switch to a 24v 600W heat mat or something similar?
Should I move the thermistor head elsewhere to get a better reading?
Any help or suggestions will be greatly appreciated.

Stephen6309

@nvigr8 IR thermometer will not show the correct temp on shiny aluminum. Put a piece of black tape on the bed to read from. Or, use a thermal couple.

Insulating the bed on the bottom will help.

Nvigr8

@stephen6309 Thanks for the reply. I have a flexible build plate on top of the aluminum. It is textured but a little shiny. I'll take the temp again with it off.

I do have the underside fully insulated. Forgot to put that in the specs.

infiniteloop

@nvigr8 You say the heat pad has an inline temp. controller: how is this specified? In your gCode snippet, you declare it as a thermistor with a resistance of 100k at 25°C (T parameter) and a beta value of 3950 (B parameter). This will eventually be correct, but to be sure, check the specs.

Assuming that the thermistor reports proper values, these are the readings from the heater itself. It takes some time until the heat propagates through your aluminium plate, it is an inertial system. As you have an insulated underside, the heater will reach the required temperatures very fast - and stop there, feeding no more heat to the aluminium plate.

It takes a long time until the bed’s surface will reach the requested temperature - 10-20 minutes can be realistic, assuming your thermistor’s parameters are correct. Even then, the thermistor can be up to 10°C off.

You can improve the situation by adding a second thermistor: drill a hole into the side of your bed plate and glue the thermistor in there - its readings should give you a better idea of the temperatures at the surface.

Nvigr8

@infiniteloop Thank you for the reply. My apologies, I should have clarified. I removed the plug and controller from the heatpad and connected it to the mains on the powersupply and use the Duet board as the controller. Not sure if that makes a difference in regard to your comments.

I agree, moving the thermistor would be much more accurate. I see your point about having it next to the insulated pad. I will do that next. Is there preferred thermistor type here for this situation/?

infiniteloop

@nvigr8 said in Homebuilt heatbed/heating issues:

I removed the plug and controller from the heatpad and connected it to the mains on the powersupply and use the Duet board as the controller. Not sure if that makes a difference in regard to your comments.

If you've done it as described in your OP: no.

Is there preferred thermistor type here for this situation?

Take a bead type thermistor with 100k and a known beta value.

Nvigr8

@infiniteloop Thanks. I'll give that a try.

Nvigr8

@infiniteloop I have a Semitec 104GT-2 from an E3D hotend on hand. Any reason that won't work?

infiniteloop

@nvigr8 said in Homebuilt heatbed/heating issues:

I have a Semitec 104GT-2 from an E3D hotend on hand. Any reason that won't work?

AFAIK that's no bead type, so you must look how to apply it to your bed plate (with good thermal contact). From the data sheet, I read it to have a beta value of 4267 - that would be the B parameter of M308.

Nvigr8

@infiniteloop If can find a drillbit that's close to the diameter of the cylinder that the thermistor head is in, it should fit nicely in with a little thermal paste and a screw placed near the hole so the head overlaps slightly to keep it from coming out. I think...

infiniteloop

@nvigr8 Sounds reasonable. Good luck!