HOW TO CALIBRATE PRESSURE ADVANCE (OR LINEAR ADVANCE)

When the extruder pushes filament, there's a delay before plastic actually starts coming out of the nozzle - and another delay when it stops. At the start of a perimeter, this means under-extrusion (gap). At the end, over-extrusion (blob). Without compensation, these defects appear at the same spot on every layer (the seam) and accumulate into a visible vertical scar. Pressure Advance (Klipper) and Linear Advance (Marlin) predict this delay and adjust extrusion *ahead of* speed changes. Properly calibrated, lines start and stop with even thickness. The seam becomes nearly invisible, sharp corners stay sharp instead of bulging, and overall surface quality improves.

Calibrate when: you're on Klipper, Bambu, or Marlin 1.1.9+ and have never run the calibration. Or you switched to a new filament type (each filament needs its own K-factor). Or you swapped extruder hardware. Don't bother when: you're on stock Marlin older than 1.1.9 - Linear Advance isn't supported. Or your printer is Bambu - it auto-calibrates per filament profile and you don't have to do anything.

Klipper (Pressure Advance): 1. Add `[input_shaper]` and accelerometer if not done. Then in `printer.cfg`, set initial `pressure_advance: 0.0` under `[extruder]`. 2. Download a tuning tower model (Klipper docs recommend a specific model; search 'Klipper pressure advance tower'). 3. Slice with no Z-hop, no acceleration changes mid-print, single perimeter or 0 infill. 4. Send via console: ``` SET_VELOCITY_LIMIT ACCEL=500 ACCEL_TO_DECEL=500 SQUARE_CORNER_VELOCITY=1 TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005 ``` 5. Print. K increases with Z height (+0.005 per mm). 6. Inspect tower. Find the Z height where corners are sharpest and lines have most uniform width. Calculate: `K = z_height x 0.005`. 7. Set in `printer.cfg`: `pressure_advance: 0.045` (example). Restart Klipper. Bambu (Flow Dynamics Calibration): 1. Bambu Studio - Calibration menu - Flow Dynamics. 2. Select your filament profile. Run the auto-calibration print. 3. The X1C/X1 Carbon scans the print with its built-in camera and saves the K-factor automatically. 4. P1S/P1P: print, then visually pick the cleanest line and enter that K-factor in the filament profile. Marlin (Linear Advance, 1.1.9+): 1. Confirm Linear Advance is enabled: send `M900` over USB. Should return current K. 2. Generate a calibration G-code at marlinfw.org/tools/lin_advance/k-factor.html with your printer's parameters. 3. Print at slow speed (40-60 mm/s). 4. Inspect for the K value where corners are sharpest. Send `M900 K<value>` to set, then `M500` to save. Typical K values: - PLA direct drive: 0.03-0.05 - PLA Bowden: 0.08-0.15 - PETG direct drive: 0.05-0.08 - PETG Bowden: 0.12-0.20 - ABS: 0.04-0.07 - TPU: very low (0.02 or off entirely)

Calibrating one K and using it for all filaments. Each filament needs its own K - PLA's K won't work for PETG. Skipping the velocity limit setup on Klipper. Without `SET_VELOCITY_LIMIT`, the test isn't a clean reading. Picking the visually 'cleanest' line without measuring. The best K produces uniform line width through corners and at perimeter starts/stops, not necessarily the line that looks shiniest. Forgetting to save. Klipper: SAVE_CONFIG. Marlin: M500. Bambu: built-in. Setting K too high. Causes negative effects: gaps at line ends instead of blobs, gouges in walls. If you see this, your K is over-shot - reduce.

Pressure Advance is the next step after e-steps and flow rate calibration. For seam-related issues that PA didn't fully solve, see seam-scars and blobs-zits.