The eight sensors
What each sensor measures and why it is on the wrist.
Last updated: 2026-04-07
Why eight
A consumer wearable usually has two or three sensors. Wellex has eight because no single signal is enough to model wellness — sleep, stress and recovery only become meaningful when several signals are read together.
The list
| # | Sensor | What it measures |
|---|---|---|
| 1 | PPG (green + red + IR) | Heart rate, HRV, blood oxygen |
| 2 | 3-axis accelerometer | Motion, posture, step count |
| 3 | 3-axis gyroscope | Wrist orientation |
| 4 | Skin temperature | Thermoregulation, fever, ovulation |
| 5 | Galvanic skin response | Sympathetic activation (stress) |
| 6 | Bioimpedance | Hydration trend |
| 7 | Ambient light | Sleep environment quality |
| 8 | Barometric pressure | Elevation, weather adjustment |
Sampling
The PPG sensor samples at 100 Hz during exercise and 25 Hz at rest. Motion sensors at 50 Hz. Temperature at 1 Hz. The on-band ML chip downsamples to a single set of features per minute, which is what gets sent to the phone.
Accuracy
PPG-derived heart rate sits within ±2 bpm of an ECG chest strap during steady-state activity, ±5 bpm during sprints. HRV (rMSSD) tracks within 8% of a Polar H10 reference at rest. Sleep staging matches PSG at ~83% epoch agreement.
See Privacy & security: Audits for the third-party validation reports.