A barometer is one of the most rewarding instruments to own, right up until the moment you realise the number it is showing makes no sense. The good news is that a barometer almost never breaks in a dramatic way. When it goes wrong it usually drifts, sticks, or lags, and nearly every cause is something you can identify in a few minutes at the kitchen table. Here are the seven culprits I see again and again.
The first and most common is simply that it was never set for your altitude. Pressure drops by roughly one hectopascal for every eight metres you climb, so a barometer calibrated at the factory near sea level will read stubbornly low if you live anywhere uphill. This is not a fault, it is a setup step that got skipped. Set it once against a known local sea-level pressure and the problem disappears.
The second is temperature. Aneroid barometers contain a metal capsule and a web of levers, and metal expands and contracts with heat. A barometer hanging on a wall in direct afternoon sun, or directly above a radiator, will show changes that have nothing to do with the weather and everything to do with its own temperature. Move it to an interior wall away from heat sources and windows.
Third is the one nobody expects: people forget that the pressure they should compare against is the sea-level adjusted figure, not the raw station pressure. If you check your reading against an airport or weather-service value, make sure you are comparing like with like, because those official numbers are almost always corrected to sea level.
Fourth, mechanical aneroid movements can develop friction or a tiny amount of stiffness over years, which makes the needle stick and then jump. The classic test is to tap the glass gently with a fingernail. If the needle visibly jumps when you tap, it was sticking, and that lag has been quietly throwing off your readings. A barometer that needs tapping to settle is due for recalibration or a service.
Fifth is a sealing or ageing issue in very old instruments. The sealed capsule inside an aneroid is supposed to hold a partial vacuum for decades, but a century-old antique may have lost some of its seal, which makes it sluggish and inaccurate no matter how carefully you set it. There is nothing to adjust here; it is simply an instrument that has aged out of accuracy.
Sixth, on digital barometers and weather stations, a flat or failing battery shows up as frozen or wandering values long before the display dies completely. If a digital unit suddenly disagrees with reality, change the battery before you blame the sensor.
Seventh and last, remember that a barometer is supposed to disagree with your phone sometimes. Your location, your altitude and the exact minute of the reading all differ from the nearest official station, and pressure genuinely changes from street to street as weather moves through. A small, sensible difference is not an error, it is the instrument doing its job.
If you want a reliable reference to check any of this against, the simplest approach is to compare your instrument with a live, location-aware reading for your exact spot rather than a distant city. That removes the guesswork and tells you in seconds whether your barometer is truly off or simply showing you something real.