setTemperatureOffset can only be positive. Adding notes.

Author: nseidle

src/SparkFun_SCD30_Arduino_Library.cpp

@@ -162,17 +162,18 @@ float SCD30::getTemperatureOffset(void)
   return (((float)signedUnsigned.signed16) / 100.0);
 }
 
-//Set the temperature offset. See 1.3.8.
+//Set the temperature offset to remove module heating from temp reading
 bool SCD30::setTemperatureOffset(float tempOffset)
 {
-  union
-  {
-    int16_t signed16;
-    uint16_t unsigned16;
-  } signedUnsigned; // Avoid any ambiguity casting int16_t to uint16_t
-  signedUnsigned.signed16 = tempOffset * 100;
+  //Temp offset is only positive. See: https://github.com/sparkfun/SparkFun_SCD30_Arduino_Library/issues/27#issuecomment-971986826
+  //"The SCD30 offset temperature is obtained by subtracting the reference temperature from the SCD30 output temperature"
+  //https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/9.5_CO2/Sensirion_CO2_Sensors_SCD30_Low_Power_Mode.pdf
+
+  if(tempOffset < 0.0) return(false);
+
+  uint16_t value = tempOffset * 100;
 
-  return sendCommand(COMMAND_SET_TEMPERATURE_OFFSET, signedUnsigned.unsigned16);
+  return sendCommand(COMMAND_SET_TEMPERATURE_OFFSET, value);
 }
 
 //Get the altitude compenstation. See 1.3.9.