Displayed heatbed temperature differs from reality  

RE: Displayed heatbed temperature differs from reality

Hi zerwas,

Everyone lives in his own reality 😉 .

Most likely your IR sensor readings are not accurate. Heat up the bed, remove the sheet and try to read it from different spots on the blank black bed. You should have a wide range of values because there are wire traces embeded.

I think you just need to clean your sheet properly and it will solve your adhesion issues.  Powder-coated spring steel sheets are known to require a perfect cleaning and perfect first layer.

RE: Displayed heatbed temperature differs from reality

Hi Nikolai,

thanks for your response. The problem isn't actually the adhesion in general when the I set the temperature to measured temperature rather then to the displayed temperature which is too low. 

RE: Displayed heatbed temperature differs from reality

Have you tried what I've recommended to do?

If you think your bed is not heating up to right temperature, then replace the thermistor. It's dangerous to try to compensate a sensor value which is essential for safety purpose.

RE: Displayed heatbed temperature differs from reality

It's a measurement issue.  Wherever the thermistor is is probably what is showing on the display (within a few percent).  Since the thermistor is on the bottom of the bed, I doubt the top is the same temperature the thermistor indicates.

If you want traceable metrological accuracy in bed measurements, you will need to instrument/fixture the bed using embedded thermocouples or thermistors.  It will be a fun engineering project for you.  

I set my bed temp to 60c; the actual print surface of the PEI sheet measures around 54c.  There are warm spots, there are cool spots.  But 10% of set temp is close enough for a bed that is fixtured the way it is.   If I were concerned, I'd raise my base bed temp in my slicer to 65 or even 67c and been happy.  I could even test 60, 80, and 100c and makes needed corrections in my slicer settings... but again, 10% is close enough. And I know the back side temp is whatever the probe is telling me.

ps: if you have to set the programmed temp to 85 to get 60, you have an assembly problem and should get it fixed.

And what emissivity is your IR thermometer set to? And what is the emissivity of the surface you are measuring?

RE: Displayed heatbed temperature differs from reality

I'd agree that the IR thermometers could be inaccurate, and you can receive the wrong measurement. But nowadays there are some models that have pretty decent accuracy (at least +-1%, I'm not sure about 0,5%). If I were you I'd measure the temperature with at least 2 different thermometers and try different spots. Interesting to know which unit did you use. Many articles recommend Fluke models but not everyone is ready to spend their money on it because it is a bit pricey (for this type of product).

RE: Displayed heatbed temperature differs from reality

Posted by: @john-vanderboom48

I'd agree that the IR thermometers could be inaccurate, and you can receive the wrong measurement. But nowadays there are some models that have pretty decent accuracy (at least +-1%, I'm not sure about 0,5%). If I were you I'd measure the temperature with at least 2 different thermometers and try different spots. Interesting to know which unit did you use. Many articles recommend Fluke models but not everyone is ready to spend their money on it because it is a bit pricey (for this type of product).

I've done some additional research and found out that ennoLogic brand also has the IR thermometers with +-1% accuracy. I thought this information would also be useful to someone. 😉 

RE: Displayed heatbed temperature differs from reality

I have the same issue.. recently I discovered that, printing at 60°C of bed temp, I was able to keep the hand on the bed (previously it was hurting!)

So I interposed a thermocouple between the bed and the sheet (obviously not printing, due to the increased thickness, and I made some tests:

• heating at 60°C --> thermocouple reads 49
• heating at 85°C --> thermocouple reads 70

apparently the thermistor is fixed in place.. could it be possible that it is shifting? kind of ageing?

RE: Displayed heatbed temperature differs from reality

Posted by: @vnirnn66

I have the same issue.. recently I discovered that, printing at 60°C of bed temp, I was able to keep the hand on the bed (previously it was hurting!)

 

So I interposed a thermocouple between the bed and the sheet (obviously not printing, due to the increased thickness, and I made some tests:

• heating at 60°C --> thermocouple reads 49
• heating at 85°C --> thermocouple reads 70

 

apparently the thermistor is fixed in place.. could it be possible that it is shifting? kind of ageing?

 

did you ever figure out what was happening here?  i have noticed similar issues on my plate / sheet.

RE: Displayed heatbed temperature differs from reality

There's an explicit bed temperature correction factor in the firmware that cases this discrepancy. 

see https://github.com/prusa3d/Prusa-Firmware/issues/2642

RE: Displayed heatbed temperature differs from reality

My cheap IR thermometer is convinced my body temperature is several degrees cooler than my medical thermometer, so I have to add correction there. The on-board firmware can compensate for "normal" bed conditions, but if I'm using a different print surface (textured, smooth, brand X, cold room), it can't know the actual print surface temp. I just assume it's going to be a bit low. I get good results with 68C for PLA.