Thermal model calibration

Starting from firmware 3.12.0, the Original Prusa MK3/S/+ has Thermal model protection. This results in better operation when dealing with thermal errors.

How it works

During the print, the printer is constantly comparing the temperature measured by the thermistors to a thermal model of what is expected from the printer. In case the measured temperature does not follow the model, the printer will beep and show the warning THERMAL ANOMALY, but the print will continue. If the anomaly disappears the warning goes away after 20 seconds. In case the anomaly continues, the printer will turn off the heating and activate an error. If you choose to continue the print after it stops, it is recommended to stay near the printer.

This will help in cases where the temperature issue is temporary, like when a window is opened near the printer. Also, in case the printer has a blob that wraps around the hotend, the thermal model should be able to stop the print before any damage is done to the hotend cables.

If you're interested in further details of how the thermal model works, check the Dev diary blog.

Calibration

When installing the new firmware, the printer will start the calibration. In case it does not go in the printer menu to Calibration -> Temp Model cal. It takes around 12 minutes, after which your printer profile is automatically saved.

The Thermal model also works for third-party components. Note that if you switch components in your printer often, the Thermal Anomaly error might start to appear more often. If this happens, run the calibration again.

Troubleshooting

In case the Thermal anomaly message keeps showing, try the following steps. 

A visual inspection

• Make sure that the hotend is perfectly inserted in the extruder. Make sure the heatsink is slotted into both slots, the short side of the heaterblock is facing forward, with its wires running back alongside the left side of the heaterblock.
  
• Inspect the hotend thermistor and hotend heater for any visible damage, especially on the part closest to the heaterblock. Make sure the hotend thermistor cables are going above the heater cables. 
  
• Inspect the nozzle position compared to the rest of the hotend. There must always be a gap (~0.5 mm) between the nozzle and the heater block. The nozzle must be tightened and secured in the heater block, and is locked against the heatbreak while heated. Among other issues, such as filament leaking, an incorrectly positioned nozzle can cause thermal anomalies. For more details on nozzle installation, please check the dedicated nozzle replacement article. 
• Inspect the heaterblock, especially its lower part. To raise the print head, navigate to LCD Menu -> Settings -> Move axis -> Z. Filament residue on the heaterblock is a cause for thermal anomalies. To clean it, heat the nozzle for 5 minutes, then disable heating by pressing the Reset button. Immediately after that, use a small brass brush to clear away any old plastic stuck on the nozzle and heater block. Be extra careful around the hotend heater and thermistor wires, avoid touching them with metal tools, as there is a risk of damaging them irreversibly. 

PID tuning followed by another temp model calibration

1. Perform a PID tuning, using the temperature you use most often for printing. 
2. In the printer menu, navigate to Calibration -> Temp Model cal., and perform the calibration again. 

Measuring resistance with a multimeter

It is possible to test the hotend thermistor, and hotend heater resistance values using a multimeter. Please check the dedicated guide for multimeter usage. 

At room temperature, the hotend thermistor should have a resistance in the range of 80 kΩ - 125 kΩ.The hotend heater has an expected resistance in the range of 12.3 Ω - 15.1Ω. 

Limitations

Since the thermal model was developed on a model of a factory-assembled extruder, it may not work correctly on some modified or after-market extruders. 

If you experience issues during the calibration process, start the process described in Crash dump (MK3/S/+, MK2.5/S), and after the logging starts, reset the printer and enter the sequence:

In case you run into issues during the printing process, use the process described in Crash dump (MK3/S/+, MK2.5/S), and after the logging starts run the sequence:

Disabling the Thermal model protection

The thermal model protection can be disabled by connecting the printer to your computer (Octoprint, PuTTY...), and sending M310 S0. An additional M500 will save the setting and the Thermal model protection will remain disabled after the next restart.