Phase 0 · Fundamentals · L4

The calibration mindset

One variable, in a fixed order, written down. The three habits that separate tuning from fiddling.

⏱ ~12 minutes• 🧠 Closes Phase 0• Fundamentals

⏩ Already know this?

This lesson covers the one-variable rule, the fixed calibration order and why it's a dependency chain rather than a preference, and setting up a tuning log. Even if the ideas are familiar, the task at the end creates the log every later lesson writes into — so do that part at minimum. Otherwise, skip straight to Phase 1 →

◎ Why this lesson

You already own a written tuning guide with a full set of numbers in it. Not one of those numbers was measured on your machines. That's the exact gap this course exists to close, and the thing that closes it isn't more knowledge — it's a method strict enough that the numbers you end up with are attributable. Three habits do the work: change one thing, follow the dependency order, and write it down. This lesson installs all three and sets up the log.

Calibration has a precise meaning here, and it isn't "making the printer better". It is:

Measuring a property you cannot look up, by constructing a print whose failure mode isolates it.

Every step in Phases 1 and 2 fits that sentence. A temperature tower isolates layer bonding by varying only temperature. A single-wall box isolates flow by removing every feature except one wall. The design of each test is the argument that the number you read off it means what you think it means.

1Change one thing

The one-variable rule: exactly one difference between two test prints. Never two.

The reason isn't tidiness. Change temperature and retraction together, get a better print, and you now have a result you cannot attribute. Worse — you'll believe you learned something, and carry the belief into the next decision. A confused result that feels like data is worse than no result, because no result at least prompts another test.

Fiddling

# evening one
temp   215 → 210
retract 0.8 → 1.0
fan     50% → 70%

print looks better ✓

Which change did it?
Unknown. Does it work
on PETG? Unknown.

Calibrating

# evening one
temp tower: 215/210/205/200
→ 205 is lowest that bonds

# evening two, temp fixed
retract 0.8 / 0.9 / 1.0
→ 0.9 kills strings

Two numbers, each
attributable, each
reusable.

The practical form of this rule in printing is the banded test print: one object where the variable steps at known heights, so a single print gives you five or six comparisons under otherwise identical conditions. Same nozzle, same spool, same hour, same ambient humidity. You'll build these with After layer change G-code in Phase 2 — it's the one-variable rule made physical.

2The order is a dependency chain, not a preference

Here is the fixed calibration order for this course:

0 · The machine Selftest, belts, first layer, thermal and motion. Phase 1. Everything below measures filament plus machine, so the machine must stop being a variable first.
1 · Temperature Sets viscosity, which changes how much comes out at a given push, how much pressure builds in the nozzle, and how much the print oozes. It moves every number below it.
2 · Extrusion multiplier How much plastic per commanded volume. Measured at the temperature you just fixed — at a different temperature you'd measure a different value.
3 · Pressure / Linear Advance Compensates for pressure lag. Pressure depends on viscosity and on flow, so it depends on both steps above.
4 · Retraction How much to pull back to stop ooze. Ooze depends on residual pressure — which advance has just changed.
5 · Cooling Fan changes the effective temperature of the plastic after it leaves the nozzle. Tuned after temperature, because it modifies temperature's effect rather than replacing it.
6 · Max volumetric speed The hotend's real ceiling, which depends on the temperature you chose in step 1. Measured before speeds, because it silently clamps them.
7 · Speeds and accelerations Last, because everything above bounds them — and because advance must be tuned at the speeds you'll actually use, so this step can send you back to step 3.

▲ The invalidation rule

Changing any step voids every measurement below it. Raise your print temperature by 10 °C and your extrusion multiplier, advance value, retraction length and MVS are all now guesses. This isn't pedantry — it's the reason a profile that "was fine last month" can quietly stop being fine after one innocent tweak. When you change a step, either re-run what's below it or write down that you didn't.

One asymmetry worth flagging, because it catches everyone: Pressure/Linear Advance must be tuned at the accelerations and speeds you'll actually print at. A value calibrated on a slow test print over-compensates and starves corners at production speed. But speed is step 7 and advance is step 3. The resolution: print the advance tower with your real quality profile loaded, not a slow one. The order is a dependency chain, not a promise that you'll never loop back.

3Write it down, or it didn't happen

Three printers × three materials × several spools is more state than anyone holds in their head. The tuning log exists so that in six months you can answer one question: is this number something I measured, or something I copied?

PrusaSlicer gives you somewhere to put a short version — Filament Settings → Notes — and that field travels with the profile when you export a config bundle, which makes it the right home for the summary. But a per-printer Markdown log is easier to scan and to diff, so this course keeps both: the fuller record in the repo, the essentials in the profile notes.

Colour matters more than people expect. Pigment loading changes flow behaviour within the same brand and material, so a black and a white spool of the same eSUN PETG can legitimately want different extrusion multipliers. Log the colour.

✓ A calibration session is only valid if all of these are true

The filament was dried, and I know when
The machine passed its health check (Phase 1) since the last hardware change
Exactly one variable differs between the things I'm comparing
Everything above this step in the order is already fixed and logged
I'm testing at the speeds and accelerations I'll actually print at
The result went into the log with printer, spool, colour and date
Any test G-code I added to a printer profile has been deleted again

Tick as you go

⚠ That last item has bitten people badly

The banded test blocks in Phase 2 go into Printer Settings → Custom G-code → After layer change. They live in the printer profile, not the filament profile — so they will keep firing on every unrelated print you slice on that machine until you delete them. A forgotten temperature tower block means every print silently drops 20 °C at layer 250. Delete the block the moment the test is read.

4What does this invalidate?

Click each scenario. Before you read the answer, decide which measurements just became guesses.

Someone changed something. What has to be re-run?

Bumped the nozzle temperature up 10 °C for better bondingStep 1
Swapped in a new 0.4 mm nozzle mid-projectMachine
Raised the fan from 40 % to 70 % on PETGStep 5
Doubled infill speed after everything else was signed offStep 7
Opened a new spool — same brand and material, different colourSpool

5Check yourself

From memory. These interleave all of Phase 0.

Question 1 of 3

Why is max volumetric speed measured before print speeds rather than after?

Question 2 of 3

A profile worked perfectly. You raised the temperature 10 °C. What is now true?

Question 3 of 3

Why is a banded test print better than four separate prints of the same object?

✎ Your task · do this with your tutor

Start the tuning log

Create tuning-log.md in this folder. One section per printer, one table row per calibration result. Leave it mostly empty — Phase 1 starts filling it next lesson.

# Tuning log

## MK3S
### Machine (Phase 1)
| Date | Check | Value measured | Target | Pass? |
|---|---|---|---|---|
|  | belt status X |  | 235–265 |  |
|  | belt status Y |  | 260–290 |  |

### Filament (Phase 2)
| Date | Spool (brand/material/colour) | Dried | Temp | EM | K/S | Retract | MVS |
|---|---|---|---|---|---|---|---|
|  |  |  |  |  |  |  |  |

Then, in your own words and without looking back at the lesson, write two things underneath it:

The calibration order, from memory, with a one-clause reason for each step's position.
One number in your existing esun-starter-profiles-bundle.ini that you now believe you cannot trust, and exactly which measurement would earn it.

→ Paste both back to me. I'll check the order against the dependency chain and tell you whether your reasons are the real ones or plausible-sounding substitutes.

What you can now say

Calibration is measuring a property by constructing a print that isolates it — not "making the printer better".
One variable at a time, and a banded test print is that rule made physical.
The order is a dependency chain. Changing a step voids every measurement below it.
Advance is the one legitimate loop backwards, because it must be tuned at production speeds.
An unlogged measurement is indistinguishable from a guess six months later — and test G-code left in a printer profile keeps firing forever.

📖

Primary source · read this next

Advanced calibration — Prusa Knowledge Base

The index page for every calibration Phases 1 and 2 will use. Skim the titles now so you recognise them later, and notice how many are split per printer model — that split is the reason this course keeps its per-machine panels.

💬 I'm your tutor — ask me anything. If a step's position in the order doesn't feel obviously right to you, say so — the reasoning matters more than the sequence, and arguing with me about why retraction comes after advance will fix it in your head better than reading it again. Confused beats stuck.

Linked: Glossary → calibration order, one-variable rule, tuning log, extrusion multiplier, max volumetric speed, Pressure Advance, retraction, drying. Keep it close with → The Calibration Card.

§References

Prusa Research. Advanced calibration. The index of Prusa's own calibration procedures, split per printer model. Link.
This course. Master tuning guide, §2 — the fixed calibration order, and the caveat that advance must be tuned at production accelerations and speeds. Link.

Lesson L4 of the prusa-profile-tuning course · all lessons · The Calibration Card · glossary