How a Gas Pump Knows When Your Tank Is Full

kaifi — Wed, 07 Oct 2026 08:49:26 +0000

When a gas pump clicks off, it is not reading your fuel gauge.

It does not know the size of your tank, how much fuel was already inside, or how many litres have been dispensed.

The shutoff happens because of a small sensing system built into the nozzle.

The small hole near the nozzle tip

Near the end of a fuel nozzle is a tiny opening.

While fuel is flowing normally, air is drawn through that opening.

As the tank fills, the fuel level eventually rises high enough to cover the sensing hole.

Once that happens, airflow through the sensing passage changes.

That pressure change is what tells the nozzle that fuel has reached the correct level.

The pump does not rely on electronics

The automatic shutoff is largely mechanical.

Inside the nozzle, the pressure change affects a diaphragm connected to the shutoff mechanism.

When the sensing hole becomes covered, the pressure difference causes the mechanism to trip.

The main valve closes and fuel flow stops almost immediately.

That familiar click is the result.

No communication with the car is required.

Why the pump sometimes stops too early

The system responds to conditions around the sensing hole, not only to the actual fuel level in the tank.

That means the nozzle can shut off early if:

fuel splashes back toward the nozzle
the nozzle is inserted at an unusual angle
the filler neck causes turbulent fuel flow
the tank is being filled too quickly
vapour or liquid temporarily blocks the sensing path

That is why repositioning the nozzle or reducing the flow rate sometimes fixes repeated early shutoffs.

Why topping off is a bad idea

Once the nozzle clicks off, continuing to add fuel manually can cause problems.

The automatic system is designed to stop when fuel reaches the intended level around the filler neck.

Adding more fuel after that can lead to spills or interfere with systems designed to handle fuel vapour.

The click is there for a reason.

A simple system solving a complex problem

The clever part of the design is that the pump does not need to know anything about the vehicle.

It works with different tank sizes, different cars, and different starting fuel levels.

The nozzle simply reacts to a physical change near its tip.

Airflow changes.

Pressure changes.

The valve trips.

Fuel stops.

That makes the system simple, fast, and reliable without requiring sensors inside the vehicle.

I wrote a deeper explanation of the sensing hole, pressure system, diaphragm, shutoff mechanism, and why pumps sometimes stop early here:

Why QR Codes Can Still Work When Part of Them Is Missing

kaifi — Wed, 07 Oct 2026 08:32:06 +0000

A QR code can be scratched, partly covered, printed badly, or have a logo placed over it and still scan correctly.

That seems strange. If some of the black and white squares are missing, shouldn’t part of the information disappear too?

The reason is simple: a QR code does not store only the message. It also contains structure and extra recovery data designed to tolerate errors.

A QR code is more than a grid

The small black and white squares are called modules, but they do not all have the same job.

The three large square patterns near the corners are finder patterns. They help a scanner locate the QR code, determine its orientation, and understand how the grid is positioned.

Other areas contain timing information, alignment patterns, format information, encoded data, and error-correction codewords.

That means location matters when a QR code is damaged.

Losing a few data modules may be recoverable. Damaging an important structural pattern can stop the scanner from reading the code at all.

Reed-Solomon error correction

QR codes use Reed-Solomon error correction.

Instead of storing only the minimum data required for the message, the QR code includes additional information that can be used to reconstruct some missing or corrupted data.

There are four QR error-correction levels:

L: roughly 7%
M: roughly 15%
Q: roughly 25%
H: roughly 30%

These percentages are useful guidelines, not guarantees.

A QR code set to Level H does not mean you can remove any random 30% of it and expect it to work.

Where the damage occurs matters just as much as how much is damaged.

Why QR codes can contain logos

This is also why many branded QR codes can place a logo in the center.

The logo intentionally covers part of the code, while the remaining data and error-correction information allow the scanner to recover what is missing.

Higher error correction usually gives a designer more room for this, but there are still limits.

A logo that is too large, low contrast, or placed over important patterns can make the QR code unreliable.

Detection and recovery are different problems

A scanner has to do two things:

Find and understand the QR code.
Decode the information inside it.

Error correction mainly helps with the second step.

If the scanner cannot locate the code properly because a finder pattern is damaged, the data may technically still be recoverable, but the scanner may never reach that stage.

This is why two QR codes with similar amounts of visible damage can behave very differently.

One may scan instantly while the other fails completely.

The quiet zone matters too

The blank margin around a QR code is called the quiet zone.

It helps the scanner distinguish the QR code from nearby text, images, or other visual elements.

A QR code can fail even when its actual data is intact if the quiet zone is removed or cluttered.

So readability depends on more than the encoded message itself.

Why badly damaged QR codes sometimes still scan

Whether a damaged QR code works depends on several things:

error-correction level
location of the damage
condition of finder and alignment patterns
QR code size
print quality
contrast
lighting
camera quality
the surrounding quiet zone

The scanner is not simply measuring how much of the image is missing. It is trying to reconstruct a structured code.

That is why a QR code can sometimes look badly damaged and still work perfectly.

The deeper explanation, including masking, codewords, alignment patterns, and why damage percentages can be misleading, is here:

DEV Community: kaifi