Quick answer: a barometric altimeter turns a pressure reading into an altitude, and its largest error source is usually the reference pressure you compare against, not the sensor. Treat it as an educational barometry tool, not a navigation or safety-critical instrument.
Pressure first, altitude second
The MPL3115A2 provides pressure, altitude and temperature outputs over I2C, and it performs the pressure-to-altitude processing inside the sensor. That is convenient, but it does not remove the underlying dependency: altitude is only meaningful relative to a reference pressure, and that reference changes with weather and location.
Resolution is not accuracy
The datasheet draws a clear line between 0.1 meter effective resolution and other performance characteristics. Resolution describes the smallest step the output can report; it does not promise the number is correct to that step. That distinction is the single most useful thing to carry into any altimeter discussion.
| Concept | What it means | Practical consequence |
|---|---|---|
| Effective resolution | Smallest reported change | Fine steps do not imply fine accuracy |
| Absolute accuracy | Closeness to true altitude | Set by the pressure reference |
| Weather drift | Reference pressure moves over time | Re-set the reference between sessions |
| Interface voltage limits | Separate sensor vs interface limits | Verify both before connecting |
An ordered evaluation process
- Confirm the sensor’s supply and digital-interface limits are separate figures, and check both for your board.
- Set a local reference pressure for the session instead of relying on a default sea-level value.
- Re-establish the reference when conditions change; a stale reference shifts every later reading.
- Use the altitude output as a relative indicator between nearby points rather than an absolute height.
- Decide whether you actually need altitude or just pressure – the raw pressure may be the more honest output.
Reader questions
Is 0.1 meter resolution the same as 0.1 meter accuracy?
No. The datasheet lists effective resolution separately from other performance figures; they answer different questions.
Why does my altitude drift when the weather changes?
Because altitude depends on the reference pressure, which tracks the weather. Re-set the reference for the session.
Can I use this for aviation or hiking safety?
No. It is an educational barometry reference, not a navigation or safety-critical device.
What about powering it from a LiPo cell?
This guide does not cover cell charging or protection; follow the cell manufacturer’s own guidance.
Documentation
Documentation note: the datasheet supports the sensor-function and resolution facts only. Absolute altitude accuracy depends on your reference pressure and local conditions, so calibrate locally and treat the output as relative.
