Merge pull request #17 from fmatray/master

Correct issue #16

README.md

@@ -215,6 +215,18 @@ The available status/error codes and pre-defined error messages are:
 For easy of use, the error message are pre-defined with `#define`.
 See the [examples](examples/) for error handling implementation.
 
+## Temperature and humidity offset correction (Optional)
+
+Some sensors have a small deviation with temperature and humidity readings.
+The compensate is possible with a correction offset defined by set_temp_offset() and set_rhum_offset().
+The result will be reading + offset. Default offset values are O.
+
+Example:
+```c++
+set_temp_offset(-0.6);
+set_rhum_offset(2);
+```
+
 ## Contribute
 
 If you have read this far, this is the library for your project

src/PMserial.cpp

@@ -221,15 +221,15 @@ void SerialPM::decodeBuffer(bool tsi_mode, bool truncated_num)
     hcho = buff2word(28) * 1e-3;
     break;
   case PMS5003T:
-    temp = int8_t(n5p0) * 1e-1; // cast to signed integer
-    rhum = n10p0 * 1e-1;
+    temp = int16_t(n5p0) * 1e-1 + temp_offset; // cast to signed integer 16bits
+    rhum = n10p0 * 1e-1 + rhum_offset;
     n5p0 = 0;
     n10p0 = 0;
     break;
   case PMS5003ST:
     hcho = buff2word(28) * 1e-3;
-    temp = int8_t(buff2word(30)) * 1e-1; // cast to signed integer
-    rhum = buff2word(32) * 1e-1;
+    temp = int16_t(buff2word(30)) * 1e-1 + temp_offset; // cast to signed integer 16bits
+    rhum = buff2word(32) * 1e-1 + rhum_offset;
     break;
   default:
     break;

src/PMserial.h

@@ -128,6 +128,13 @@ class SerialPM
   inline bool has_number_concentration() { return (status == OK) && (pms != PMS3003); }
   inline bool has_temperature_humidity() { return (status == OK) && ((pms == PMS5003T) || (pms == PMS5003ST)); }
   inline bool has_formaldehyde() { return (status == OK) && ((pms == PMS5003S) || (pms == PMS5003ST)); }
+
+  // adding offsets works well in normal range
+  // might introduce under- or overflow at the ends of the sensor range
+  inline void  set_rhum_offset(float offset) { rhum_offset = offset; };
+  inline void  set_temp_offset(float offset) { temp_offset = offset; };
+  inline float get_rhum_offset() { return rhum_offset; };
+  inline float get_temp_offset() { return temp_offset; };
 #ifdef PMS_DEBUG
 #ifdef HAS_HW_SERIAL
   inline void print_buffer(Stream &term, const char *fmt)
@@ -154,6 +161,10 @@ class SerialPM
   uint8_t rx, tx; // Serial1 pins on ESP32
 #endif
 
+  // Correct Temperature & Humidity
+  float temp_offset = 0.0;
+  float rhum_offset = 0.0;
+
   // utility functions
   STATUS trigRead();
   bool checkBuffer(size_t bufferLen);