How come the altitude calculation is wrong? Is my sensor broken?

No, your sensor is likely just fine. The altitude calculation depends on knowing the barometric pressure at sea level

If you do not set the correct sea level pressure for your location FOR THE CURRENT DAY it will not be able to calculate the altitude accurately

Barometric pressure at sea level changes daily based on the weather!

If I have long delays between reads, the first data read seems wrong?

The BMx280 'saves' the last reading in memory for you to query. Just read twice in a row and toss out the first reading!

Last updated on 2017-11-10 at 03.46.29 PM Published on 2015-07-24 at 06.07.39 PM