Si7021 wrap around bug now has a partial workaround

CHANGELOG.md

@@ -77,3 +77,11 @@ All notable changes to this project will be documented in this file.
 ### Changed
 
 - Humidity threshold heater is now a define instead of hard coded numbers in the source.
+
+## [0.2.5] - 2024-03-13
+
+### Fixed
+
+There is a bug in the Si7021 chip: when the sensor is exposed to extreme environmental conditions for a longer period (very high or very low humidity), the humidity register will wrap around and give wrong values. (https://community.silabs.com/s/question/0D51M00007xeOo9SAE/si7021-relative-humidity-reading-underflowing-?language=en_US).
+Workaround: let's assume the humidity will never go below 15%. In the Netherlands a humidity below 20% is rare, but can occur and the minimum value ever measured in De Bilt was 6% (on 1 april 1965). At several other sites in the Netherlands the humidity went as low as 10%. But as this was a one time event and the measurements are doubtful, this event was not registered in the original database. (https://nl.wikipedia.org/wiki/Relatieve_luchtvochtigheid)
+In rare cases, the humidity value could creep up above 15% (when the actual humidity is 100%) and still give wrong values.

README.md

@@ -44,6 +44,12 @@ An apprehensive description of the project can be found here: [https://meezenest
 
 Libraries are included with the source code of this project
 
+## Known bugs
+
+There is a bug in the Si7021 chip: when the sensor is exposed to extreme environmental conditions for a longer period (very high or very low humidity), the humidity register will wrap around and give wrong values. (https://community.silabs.com/s/question/0D51M00007xeOo9SAE/si7021-relative-humidity-reading-underflowing-?language=en_US).
+Workaround: let's assume the humidity will never go below 15%. In the Netherlands a humidity below 20% is rare, but can occur and the minimum value ever measured in De Bilt was 6% (on 1 april 1965). At several other sites in the Netherlands the humidity went as low as 10%. But as this was a one time event and the measurements are doubtful, this event was not registered in the original database. (https://nl.wikipedia.org/wiki/Relatieve_luchtvochtigheid)
+In rare cases, the humidity value could creep up above 15% (when the actual humidity is 100%) and still give wrong values.
+
 ## License
 
 Copyright (C) 2023, 2024  M.T. Konstapel

firmware/weather_station.ino

@@ -105,7 +105,7 @@ const int SensorStatusBitsIreg        = 14;
  * 0 = Heater algorithm (0 = disable, 1 = enable)
  */
 const int HeaterCoil = 0;
-#define HUMIDITY_THRESHOLD 92
+float HUMIDITY_THRESHOLD = 92.0;
 
 // RS-485 serial port 
 #define MySerial Serial // define serial port used, Serial most of the time, or Serial1, Serial2 ... if available