# Calibration

Dallas DS18B20 sensors can show small deviations from true temperature. Typical datasheet accuracy is about +/-0.5°C (from -10°C to 85°C).

Brautomat supports linear 2-point calibration.

## 2-point calibration with reference thermometer

Use a calibrated reference thermometer.

1. Heat a typical kettle volume to 40°C.
2. Enter the difference in `Offset 1 [40°C]`.
3. Heat to 78°C.
4. Enter the difference in `Offset 2 [78°C]`.

Brautomat then applies this correction to future measurements.

During calibration, the sensor runs at 12-bit resolution (0.0625°C). Each calibration run records 60 samples (about 60 seconds), and the average is used.

In many setups, a 1-point calibration in an ice bath is already sufficient because DS18B20 offsets are often close to linear.

## Calibration without reference thermometer

If no reference thermometer is available, use:

* an ice bath (0°C)
* boiling point at your local altitude

For the low point, prepare an ice bath with about 50% ice and 50% cold water and stir continuously.

For the high point, estimate local boiling point via altitude. At sea level (`0 m`, about `1.013 bar`), boiling point is `100.0°C`. Boiling point decreases by about `0.003354°C` per meter altitude.

You can use tools such as: [rechneronline boiling point calculator](https://rechneronline.de/barometer/siedepunkt.php)

For reliable results, keep boiling stable for at least one minute before starting the high-point calibration run.

Offset definitions:

* `Offset #1`: difference between 0.0°C and first-run average
* `Offset #2`: difference between local boiling point and second-run average

## Calibration with clinical thermometer

A clinical thermometer can be used only in the lower range (up to about 40°C).

If using 40°C as upper calibration point:

1. Heat to 40°C.
2. Switch heating off.
3. Wait until temperature is stable for about 60 seconds.
4. Start calibration sample run.

## Sensor calibration log file

A log file is written for each calibration. Example:

```
13:22:37 Sensor calibration started
*** Sensor Name: Sensor IDS2
*** Model: DS18B20
*** Address: 2827c59d0d0000b1
*** Resolution: 12bit
*** Timeout: 750ms
------------------------------------------------------
ID Target Actual Diff Offset
#01 24.6000 24.0000 -0.6000 0.6000

 - Sensor values 2 to 59 accordingly

#60 24.6000 25.1875 0.5875 -0.6083
------------------------------------------------------
Temperature of offset #1: 24.6000
Average after 60 measurements: 25.2083
Offset #1: -0.6083
=========================================
```