Chapter 5

Sensor Calibration

Offset adjustment via web interface and calibration button

1Overview

The SM5000 applies per-sensor temperature and humidity offsets in firmware before any data is published, logged, or displayed. Offsets are stored in NVS and survive power cycles and OTA updates. There are two calibration methods:

Via the web interface - for entering known offset values directly. No additional hardware required.

Via the calibration button - for automatic field calibration against a physical reference sensor on Port 3. The device computes and stores offsets automatically.

2Calibration via Web Interface

Refer to §2.2.2 - Sensor Readings & Calibration for the full procedure and web interface screenshot. This method is used when the operator already has known offset values to apply - for example, from a prior calibration run or a measurement taken with a separate reference instrument.

3Calibration via Button (Reference Sensor Method)

This method uses a calibrated reference sensor plugged into Port 3 (Modbus ID 3). The device averages 10 samples from the target sensor and the reference, then stores the difference as the offset automatically.

Critical - Factory Reset Risk: If no calibration sensor is connected to Port 3 and the button is long-pressed (≥1 second), the device will perform a factory reset - not exit calibration mode. Always ensure the reference sensor is plugged into Port 3 before pressing the button if a factory reset is not intended.

Button Logic Reference

Action	Condition	Result
Single press	Cal sensor connected, idle	Enter Cal Mode - Sensor 1 selected
Single press	In Cal Mode, S1 selected	Switch to Sensor 2
Single press	In Cal Mode, S2 selected	Switch back to Sensor 1
Double-click	Sensor selected in Cal Mode	Start calibration immediately
3-second inactivity	Sensor selected in Cal Mode	Start calibration automatically
Long press (≥1s)	In Cal Mode, cal sensor connected	Exit calibration mode
Long press (≥1s)	No cal sensor on Port 3	⚠ Triggers factory reset

Full Calibration Button LCD Sequence

① Normal (idle)

② Single press → S1 selected

③ Single press → S2 selected

④ Calibration in progress

⑤ Calibration complete

⑥ Sensor disconnected mid-cal

⑦ Long press - exit

⑧ No cal sensor - button press

State ⑤ displays for 3 seconds then automatically returns to normal. The offset values shown (e.g. T:+0.30 H:-1.20) are the computed averages applied to that sensor.

5.3.1 - Calibration Procedure

Connect the reference sensor to Port 3 and allow at least 2-3 minutes for both sensors to stabilise in the environment.
Press the calibration button once. The LCD shows Cal Mode: S1 / Double=Start.
To calibrate Sensor Rear instead, press once more to switch to Cal Mode: S2.
Either double-click the button or wait 3 seconds without pressing - calibration begins automatically.
The device takes 10 samples. The LCD updates with each sample count. Do not disturb the sensors during this period.
On completion the LCD shows the computed offset values for 3 seconds, then returns to normal. Offsets are saved to NVS automatically.
Repeat from step 2 for the second sensor if required.
Long-press to exit calibration mode at any time and return to the normal display.

Verifying calibration: After calibration, check the Calibration Information table in the web interface (§2.2.2) to confirm the stored offset values are plausible. If an offset appears unreasonably large (e.g. ±5°C or more), verify the reference sensor is functioning correctly and that both sensors were measuring the same environment during sampling.