Si7021 wrap around bug now has a partial workaround

CHANGELOG.md

@@ -78,7 +78,7 @@ All notable changes to this project will be documented in this file.
 
 - Humidity threshold heater is now a define instead of hard coded numbers in the source.
 
-## [0.2.5] - 2024-03-13
+## [0.2.6] - 2024-03-13
 
 ### Fixed
 

firmware/weather_station.ino

@@ -243,7 +243,13 @@ void ReadSi7021 (void)
     si7021.triggerMeasurement();
     si7021.getHumidity(humidity);
 
-    if (humidity>100 || humidity<0)
+    // 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 of 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 nor 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.
+    if (humidity>100 || humidity<15)
       humidity = 100;
  
     //If humidity is larger than HUMIDITY_THRESHOLD switch on heater to get more acurate measurement and prevent memory offset
@@ -259,7 +265,8 @@ void ReadSi7021 (void)
       //Serial.print(F("Valid temp"));
 
       si7021.getHumidity(MeasuredData.Humidity);
-      if (MeasuredData.Humidity>100 || MeasuredData.Humidity<0)
+      // See humidity bug workaround above.
+      if (MeasuredData.Humidity>100 || MeasuredData.Humidity<15)
         MeasuredData.Humidity = 100;
       // Scale for more decimal positions when converted to integer value for ModBus
       MeasuredData.Humidity *= 100;      
@@ -460,7 +467,7 @@ void setup() {
   mb.Ireg (SensorRainSinceMidnightIreg, 0);
   mb.Ireg (SensorSnowFallIreg, 0);
  
-  Serial.println(F("Weather station v0.2.5"));
+  Serial.println(F("Weather station v0.2.6"));
   Serial.println(F("(C)2024 M.T. Konstapel"));
   Serial.println(F("This project is free and open source"));
   Serial.println(F("More details: https://meezenest.nl/mees/"));