What is gap sensing?
A label printer needs to know where each label begins so it prints in the right spot and stops with the gap lined up at the tear bar or cutter. With gap sensing (sometimes called web sensing), a sensor in the media path looks for the space between die-cut labels on their backing liner. Notched tags work the same way, with the sensor looking for the notch or hole. Most gap sensors are transmissive: light shines through the media to a receiver on the other side, and more light gets through at the gap, where there's only liner, than through a label stuck to the liner.
Gap sensing vs. black-mark sensing
| Gap sensing | Black-mark sensing | |
|---|---|---|
| What it detects | The gap, notch or hole between labels | A printed black mark on the back of the liner or media |
| Sensor type | Usually transmissive (light passes through) | Reflective (light bounces off the media) |
| Typical media | Die-cut labels on a liner, notched tags | Tags, tickets and labels with marks printed on the back |
A third option is continuous media, with no gaps or marks. The printer doesn't sense label boundaries at all; the label length comes from the driver or the labeling software.
When gap sensing goes wrong
- Wrong sensor mode: the printer is set for gap sensing but black-mark media is loaded, or the reverse. It may feed blank labels, stop early, print across label edges or show a media or paper-out error.
- Not calibrated: after a new size or type of labels is loaded, the printer still uses the old label length.
- Sensor out of position: on printers with a movable sensor, it has to sit where the gap or notch passes.
- Dirty sensor: adhesive or paper dust on the sensor makes labels and gaps look alike.
- Driver mismatch: the media type in the driver (gap, mark or continuous) doesn't match the loaded roll.
Calibrating after loading media
Run a media calibration whenever you load a different label size or type. The printer feeds a few labels while it measures the label length and the sensor readings for label and gap. Depending on the model, you start it from a button sequence on the printer, the printer's menu or the driver's utility. Then print a test label to confirm the image lands inside the label and the gap stops at the tear bar or cutter.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.8 covers thermal printer maintenance, including the feed assembly, special thermal paper and removing debris. Gap sensing isn't named, but skipped labels or media errors right after a roll change usually trace back to the wrong sensing mode or a missing calibration, not a failed printer.