- add NIST-linearized thermocouple conversion for accurate thermocouple readings

lib/max31855.py

@@ -39,7 +39,8 @@ class MAX31855(object):
         '''Reads SPI bus and returns current value of thermocouple.'''
         self.read()
         self.checkErrors()
-        return getattr(self, "to_" + self.units)(self.data_to_tc_temperature())
+        #return getattr(self, "to_" + self.units)(self.data_to_tc_temperature())
+        return getattr(self, "to_" + self.units)(self.data_to_LinearizedTempC())
 
     def get_rj(self):
         '''Reads SPI bus and returns current value of reference junction.'''
@@ -135,6 +136,105 @@ class MAX31855(object):
         GPIO.setup(self.cs_pin, GPIO.IN)
         GPIO.setup(self.clock_pin, GPIO.IN)
 
+    def data_to_LinearizedTempC(self, data_32 = None):
+        '''Return the NIST-linearized thermocouple temperature value in degrees 
+        celsius. See https://learn.adafruit.com/calibrating-sensors/maxim-31855-linearization for more infoo.
+        This code came from https://github.com/nightmechanic/FuzzypicoReflow/blob/master/lib/max31855.py
+'''
+        if data_32 is None:
+            data_32 = self.data
+        #       extract TC temp
+        #       Check if signed bit is set.
+        if data_32 & 0x80000000:
+            # Negative value, take 2's compliment. Compute this with subtraction
+            # because python is a little odd about handling signed/unsigned.
+            data_32 >>= 18
+            data_32 -= 16384
+        else:
+            # Positive value, just shift the bits to get the value.
+            data_32 >>= 18
+        # Scale by 0.25 degrees C per bit and return value.
+        TC_temp =  data_32 * 0.25
+        # Extract Internal Temp
+        data_32 = self.data
+        # Ignore bottom 4 bits of thermocouple data.
+        data_32 >>= 4
+        # Grab bottom 11 bits as internal temperature data.
+        Internal_Temp= data_32 & 0x7FF
+        if data_32 & 0x800:
+            # Negative value, take 2's compliment. Compute this with subtraction
+            # because python is a little odd about handling signed/unsigned.
+            Internal_Temp -= 4096
+        # Scale by 0.0625 degrees C per bit and return value.
+        Internal_Temp = Internal_Temp * 0.0625
+
+        # MAX31855 thermocouple voltage reading in mV
+        thermocoupleVoltage = (TC_temp - Internal_Temp) * 0.041276
+        # MAX31855 cold junction voltage reading in mV
+        coldJunctionTemperature = Internal_Temp
+        coldJunctionVoltage = (-0.176004136860E-01 +
+            0.389212049750E-01  * coldJunctionTemperature +
+            0.185587700320E-04  * math.pow(coldJunctionTemperature, 2.0) +
+            -0.994575928740E-07 * math.pow(coldJunctionTemperature, 3.0) +
+            0.318409457190E-09  * math.pow(coldJunctionTemperature, 4.0) +
+            -0.560728448890E-12 * math.pow(coldJunctionTemperature, 5.0) +
+            0.560750590590E-15  * math.pow(coldJunctionTemperature, 6.0) +
+            -0.320207200030E-18 * math.pow(coldJunctionTemperature, 7.0) +
+            0.971511471520E-22  * math.pow(coldJunctionTemperature, 8.0) +
+            -0.121047212750E-25 * math.pow(coldJunctionTemperature, 9.0) +
+            0.118597600000E+00  * math.exp(-0.118343200000E-03 * math.pow((coldJunctionTemperature-0.126968600000E+03), 2.0)))
+        # cold junction voltage + thermocouple voltage
+        voltageSum = thermocoupleVoltage + coldJunctionVoltage
+        # calculate corrected temperature reading based on coefficients for 3 different ranges
+        # float b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, b10;
+        if voltageSum < 0:
+            b0 = 0.0000000E+00
+            b1 = 2.5173462E+01
+            b2 = -1.1662878E+00
+            b3 = -1.0833638E+00
+            b4 = -8.9773540E-01
+            b5 = -3.7342377E-01
+            b6 = -8.6632643E-02
+            b7 = -1.0450598E-02
+            b8 = -5.1920577E-04
+            b9 = 0.0000000E+00
+        elif voltageSum < 20.644:
+            b0 = 0.000000E+00
+            b1 = 2.508355E+01
+            b2 = 7.860106E-02
+            b3 = -2.503131E-01
+            b4 = 8.315270E-02
+            b5 = -1.228034E-02
+            b6 = 9.804036E-04
+            b7 = -4.413030E-05
+            b8 = 1.057734E-06
+            b9 = -1.052755E-08
+        elif voltageSum < 54.886:
+            b0 = -1.318058E+02
+            b1 = 4.830222E+01
+            b2 = -1.646031E+00
+            b3 = 5.464731E-02
+            b4 = -9.650715E-04
+            b5 = 8.802193E-06
+            b6 = -3.110810E-08
+            b7 = 0.000000E+00
+            b8 = 0.000000E+00
+            b9 = 0.000000E+00
+        else:
+            # TODO: handle error - out of range
+            return 0
+        return (b0 +
+            b1 * voltageSum +
+            b2 * pow(voltageSum, 2.0) +
+            b3 * pow(voltageSum, 3.0) +
+            b4 * pow(voltageSum, 4.0) +
+            b5 * pow(voltageSum, 5.0) +
+            b6 * pow(voltageSum, 6.0) +
+            b7 * pow(voltageSum, 7.0) +
+            b8 * pow(voltageSum, 8.0) +
+            b9 * pow(voltageSum, 9.0))
+
+
 class MAX31855Error(Exception):
      def __init__(self, value):
          self.value = value