Update Sensirion_SHT3X.md

Author: oscgonfer

docs/knowledge/air/temperature_rel_humidity/Sensirion_SHT3X.md

@@ -26,3 +26,75 @@ excerpt: "A temperature and humidity sensor that we love! Available in SCK2.X."
 
 !!! info "Version"
     This sensor is supported from V2.0 onwards
+
+
+## Modify temperature and relative humidity offset
+
+Starting in firmware `0.9.11`, we have included a command to `control` temperature and relative humidity offsets for [Sensirion SHT3X](/knowledge/air/temperature_rel_humidity/Sensirion_SHT3X/) sensors, present in the [Urban Board](/hardware/boards/urban-board/) or as [an external probe](/knowledge/air/temperature_rel_humidity/Sensirion_SHT3X-weatherproof/), for both the `SHT31` and the `SHT35` type. This feature allows introducing an offset to both variables: temperature and relative humidity. This offset can be used to compensate for heat build-up on the device. Note that this offset is the same for [wi-fi / online](/hardware/kit/features/#wi-fi-mode) or [sd-card / offline](/hardware/kit/features/#sd-card-mode) modes. In addition, a small adjustment is *hardcoded* to compensate for battery charging additional heat up **only on the urban board**. 
+
+To check available commands (note that you can change `temp` or `hum` for the urban board):
+
+```
+SCK > control temp
+Temperature: 
+Available commands:
+* cal: [float - clear] Sets temperature offset with respect to current offset or clears it
+```
+
+To check the offset currently implemented:
+
+```
+SCK > control temp cal
+Current temperature offset: -1.00 (degC)
+```
+
+```
+SCK > control hum cal
+Current humidity offset: 3.50 (rh)
+```
+
+!!! info "Default offsets"
+    These offsets are calibrated based on internal tests on each hardware version. You can always modify them to suit your needs, or revert back to the default with `control temp cal clear` or `control hum cal clear`
+
+To modify, you can issue the `control temp cal` command followed by a the offset to add in float notation (for instance `-10.0`):
+
+Check the calibration and read the current value:
+
+```
+SCK > control temp cal
+Temperature: cal
+Current temperature offset: -1.00 (degC)
+SCK > read temp
+Temperature: 25.95 C
+```
+
+Set the temperature offset to -10.0 degC. Note that for the previous reading, there was already an offset of `-1.00 degC`, so now the offset will be -11.0 degC:
+
+```
+SCK > control temp cal -10.00
+Temperature: cal -10.00
+SCK > control temp cal
+Current temperature offset: -11.00 (degC)
+SCK > read temp
+Temperature: 15.96 C
+```
+
+To revert to the previous value:
+
+```
+SCK > control temp cal clear
+Set default offset: -1.00 (degC)
+SCK > control temp cal
+Current temperature offset: -1.00 (degC)
+SCK > read temp
+Temperature: 26.09 C
+```
+
+As an example, other sensors can be also modified. For instance, for humidity:
+
+```
+control hum cal
+Current humidity offset: 3.50 (rh)
+SCK > read hum
+Humidity: 63.06 %
+```