Temperature fix (Firmware > 5.0)

The temperature of the sensor for firmware versions greater than 5 has received a fix.

This changes have only been tested on the MHZ19C.

Please note:  older versions no longer offer a float argument for the getTemperature() function. Instead,  the value returned is in accordance with the data sheet. This is due to the float version being somewhat unreliable.

As a result, temperature values received in older library versions will see an increase by 2C as the offset used in the datasheet differed.

The firmware version (the major version) is obtained on initiation and used to determine which temperature code to run.

Please report any issues as there are many firmware versions and I have only two sensors.

Author: WifWaf

src/MHZ19.cpp

@@ -39,6 +39,13 @@ void MHZ19::begin(Stream &serial)
         Serial.println("!ERROR: Initial communication errorCode recieved");
         #endif 
     }
+
+    /* What FW version is the sensor running? */
+    char myVersion[4];          
+    this->getVersion(myVersion);
+    
+    /* Store the major version number (assumed to be less than 10) */
+    this->storage.settings.fw_ver = myVersion[1];
 }
 
 /*########################-Set Functions-##########################*/
@@ -197,61 +204,27 @@ float MHZ19::getTransmittance(bool force)
         return 0;
 }
 
-float MHZ19::getTemperature(bool isFloat, bool force)
+float MHZ19::getTemperature(bool force)
 {
-    if(isFloat)
-    {
-        static byte baseTemp = 0;
-        static bool isSet = false;
-
-        if(!isSet)
-        {
-            provisioning(CO2LIM);
-            byte buff = (this->storage.responses.CO2LIM[4] - TEMP_ADJUST);
-
-            baseTemp = buff - (byte)getTemperatureOffset(true);
-            isSet = true;
-        }
-        
-        if(force)
-            provisioning(CO2UNLIM);
-
-        if(this->errorCode == RESULT_OK || force == false)
-        {
-           float buff = baseTemp;
-           buff += getTemperatureOffset(false);
-           return buff;
-        }
-    }
-    
-    else if(!isFloat)
+    if(this->storage.settings.fw_ver < 5)
     {
         if (force == true)
             provisioning(CO2LIM);
 
         if (this->errorCode == RESULT_OK || force == false)
             return (this->storage.responses.CO2LIM[4] - TEMP_ADJUST);
     }
-    
-    return -273.15;    
-}
- 
-float MHZ19::getTemperatureOffset(bool force)
-{
-     if (force == true)
-        provisioning(CO2UNLIM);
+    else
+    {    
+        if (force == true)
+            provisioning(CO2UNLIM);
 
-    if (this->errorCode == RESULT_OK || force == false)
-    {
-        /* Value appears to be for CO2 offset (useful for deriving CO2 from raw?) */
-        /* Adjustments and calculations are based on observations of temp behavour */
-        float calc = (((this->storage.responses.CO2UNLIM[2] - 8) * 1500) + ((this->storage.responses.CO2UNLIM[3] * 100) * 1 / 17));
-        calc /= 100;
-        return calc;
+        if (this->errorCode == RESULT_OK)
+            return (float)(((int)this->storage.responses.CO2UNLIM[2] << 8) | this->storage.responses.CO2UNLIM[3]) / 100;
     }
 
-    return -273.15;
-} 
+    return -273.15;    
+}
 
 int MHZ19::getRange()
 {

src/MHZ19.h

@@ -11,14 +11,10 @@
 #endif
 
 #define MHZ19_ERRORS 1			// Set to 0 to disable error prints
-
-#define TEMP_ADJUST 38			// This is the value used to adjust the temeperature.
-								// Older datsheets use 40, however is likely incorrect.
+#define TEMP_ADJUST 40			// This is the value used to adjust the temeperature.
 #define TIMEOUT_PERIOD 500		// Time out period for response (ms)
-
 #define DEFAULT_RANGE 2000		// For range function (sensor works best in this range)
-
-#define MHZ19_DATA_LEN 9		// Data protocl length
+#define MHZ19_DATA_LEN 9		// Data protocol length
 
 // Command bytes -------------------------- //
 #define MHZ19_ABC_PERIOD_OFF    0x00
@@ -73,8 +69,8 @@ class MHZ19
 	/* returns Raw CO2 value as a % of transmittance */		//<--- needs work to understand
 	float getTransmittance(bool force = true);
 
-	/*  returns temperature using command 134 or 135 if isFloat = true */
-	float getTemperature(bool isFloat = false, bool force = true);
+	/*  returns temperature using command 133 or 134 */
+	float getTemperature(bool force = true);
 
 	/* reads range using command 153 */
 	int getRange();
@@ -155,6 +151,7 @@ class MHZ19
 			bool filterCleared = true;				// Additional flag set by setFiler() to store which mode was selected			
 			bool printcomm = false;					// Communication print options
 			bool _isDec = true;						// Holds preferance for communication printing
+			uint8_t fw_ver = 0;                     // holds the major version of the firmware
 		} settings;
 
 		byte constructedCommand[MHZ19_DATA_LEN];	// holder for new commands which are to be sent
@@ -180,9 +177,6 @@ class MHZ19
 	/* generates a checksum for sending and verifying incoming data */
 	byte getCRC(byte inBytes[]);
 
-	/* Returns what appears to be an offset */ 		//<----- knowledgable people might have success using against the raw value
-	float getTemperatureOffset(bool force = true);
-
 	/* Sends commands to the sensor */
 	void write(byte toSend[]);