Sensor calibration

OpenSense does not require calibrated sensors. The default Shelly / Aqara / Efento ship calibrated by the chip vendor (Sensirion, typically ±0.4 °C ±2 %RH out of the box) and that is sufficient for most HACCP and climate use cases.

Calibration becomes relevant when:

• A pharmacy or hospital pharmacy auditor asks for traceability.
• A sensor has been in service long enough to drift (typically

  2 years).

• A discrepancy between two adjacent sensors makes you suspect one is wrong.
• You replaced a sensor and the new one reads systematically differently than the chart you have on file.

What "calibration" actually means

Three different things use the word:

1. Factory calibration — done in production, baked into the sensor's internal offset registers. Sensirion publishes the datasheet accuracy.
2. Field check — comparing the installed sensor against a portable reference for 5–10 min, recording the delta. No adjustment is made.
3. Field calibration with offset — same as field check, plus applying the measured delta as a configured offset in OpenSense.

In OpenSense we support all three; the first is the chip's responsibility, the second is operator hygiene, the third is the "channel offset" field.

What you need for a field check

• A reference probe: a calibrated thermistor or PT100 with a recent (< 12 months) calibration certificate. Examples:
  • Fluke 1551A "Stik" — €700, ISO 17025 cert from Fluke.
  • OMEGA HH377 + Type-T probe — €350.
  • For non-pharma use: a Sous Vide-grade ThermoWorks ThermaPen — €100; not certified but reliable to ±0.2 °C against a Fluke.
• A stable environment: not in a fridge being opened, not in direct sunlight, not next to a heat source. A still-air room at 20–22 °C is ideal; an ice bath at 0 °C is the second reference point (single-point calibration on ice + a second at room temperature gives you a two-point line).

The procedure

1. Note the installed sensor's reading right now.
2. Place the reference probe next to it (within 5 cm). Insulate both from local air movement if you can.
3. Wait 10 minutes. Sensors have thermal mass; the reading you see in minute 1 is not the steady-state reading.
4. Record both readings simultaneously. Compute delta.
5. Repeat at a different temperature if practical (ice bath; a freezer at −18 °C; a fridge at +4 °C).
6. Compute the offset and the slope (if you took two points).

Apply the offset

In OpenSense:

PATCH /v1/sensors/snr_a1b2
{
  "offset": -0.3,
  "offset_recorded_at": "2026-05-17T10:00:00+02:00",
  "offset_recorded_by": "ops@cafe-bratislava.sk",
  "offset_recorded_against": "Fluke 1551A s/n 12345 cert 2026-02"
}

The offset is applied post-ingest — the raw reading is stored unchanged; the displayed and rule-evaluated value is raw + offset. This is intentional: if you ever doubt the offset is correct, you can recompute history with a different one without losing data.

The audit trail records the offset change as a configuration event.

How often

If you cannot afford a calibrated reference probe but want a zero-cost sanity check, every 6 months put all your sensors in the same room for 60 minutes and compare their readings. If one drifts

1 °C from the median, it is the suspect; field-check it against a borrowed thermometer or buy a replacement.

What the auditor wants to see

For HACCP / Slovak ŠVPS in our experience:

• A note that "calibration was performed on YYYY-MM-DD with reference X, against probe Y, delta was Z °C". The dashboard's calibration field accepts free-text; auditors read it.
• A copy of the reference probe's certificate (or a serial number pointing at one). The certificate need not be in OpenSense; we have a per-account "documents" attachment area you can put it in.

For pharma / GxP — which we do not service today — the auditor will want a full traceability chain back to a national metrology institute. Buy Eupry or Vaisala for that case.

When in doubt

If a sensor reading does not match reality and you cannot calibrate it, replace it. A Shelly H&T is €30. It is cheaper than an hour of your time arguing with chip drift. Keep the old sensor in a drawer labeled "for the office plant" so you do not throw away a working device — but do not have it on the food.