Added new temperature lookup table creation tool. This one uses stein hart-hart equations.

You need 3 temperature resistance:combinations. This gives a better result then using beta.

Marlin/createTemperatureLookupMarlin.py

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+#!/usr/bin/python
+#
+# Creates a C code lookup table for doing ADC to temperature conversion
+# on a microcontroller
+# based on: http://hydraraptor.blogspot.com/2007/10/measuring-temperature-easy-way.html
+"""Thermistor Value Lookup Table Generator
+
+Generates lookup to temperature values for use in a microcontroller in C format based on: 
+http://hydraraptor.blogspot.com/2007/10/measuring-temperature-easy-way.html
+
+The main use is for Arduino programs that read data from the circuit board described here:
+http://make.rrrf.org/ts-1.0
+
+Usage: python createTemperatureLookup.py [options]
+
+Options:
+  -h, --help        show this help
+  --rp=...          pull-up resistor
+  --t0=ttt:rrr      low temperature temperature:resistance point (around 25C)
+  --t1=ttt:rrr      middle temperature temperature:resistance point (around 150C)
+  --t2=ttt:rrr      high temperature temperature:resistance point (around 250C)
+  --num-temps=...   the number of temperature points to calculate (default: 20)
+"""
+
+from math import *
+import sys
+import getopt
+
+class Thermistor:
+    "Class to do the thermistor maths"
+    def __init__(self, rp, t1, r1, t2, r2, t3, r3):
+        t1 = t1 + 273.15               # low temperature (25C)
+        r1 = r1                        # resistance at low temperature
+        t2 = t2 + 273.15               # middle temperature (150C)
+        r2 = r2                        # resistance at middle temperature
+        t3 = t3 + 273.15               # high temperature (250C)
+        r3 = r3                        # resistance at high temperature
+        self.rp = rp                   # pull-up resistance
+        self.vadc = 5.0                # ADC reference
+        self.vcc = 5.0                 # supply voltage to potential divider
+        a1 = log(r1)
+        a2 = log(r2)
+        a3 = log(r3)
+        z = a1 - a2
+        y = a1 - a3
+        x = 1/t1 - 1/t2
+        w = 1/t1 - 1/t3
+        v = pow(a1,3) - pow(a2,3)
+        u = pow(a1,3) - pow(a3,3)
+        c3 = (x-z*w/y)/(v-z*u/y)
+        c2 = (x-c3*v)/z
+        c1 = 1/t1-c3*pow(a1,3)-c2*a1
+        self.c1 = c1
+        self.c2 = c2
+        self.c3 = c3
+
+    def temp(self,adc):
+        "Convert ADC reading into a temperature in Celcius"
+        v = adc * self.vadc / (1024 * 16)   # convert the 10 bit ADC value to a voltage
+        r = self.rp * v / (self.vcc - v)    # resistance of thermistor
+        lnr = log(r)
+        Tinv = self.c1 + (self.c2*lnr) + (self.c3*pow(lnr,3))
+        return (1/Tinv) - 273.15        # temperature
+
+    def adc(self,temp):
+        "Convert temperature into a ADC reading"
+        y = (self.c1 - (1/(temp+273.15))) / (2*self.c3)
+	x = sqrt(pow(self.c2 / (3*self.c3),3) + pow(y,2))
+        r = exp(pow(x-y,1.0/3) - pow(x+y,1.0/3)) # resistance of thermistor
+        return (r / (self.rp + r)) * (1024*16)
+
+def main(argv):
+
+    rp = 4700;
+    t1 = 25;
+    r1 = 100000;
+    t2 = 150;
+    r2 = 1641.9;
+    t3 = 250;
+    r3 = 226.15;
+    num_temps = int(36);
+    
+    try:
+        opts, args = getopt.getopt(argv, "h", ["help", "rp=", "t1=", "t2=", "t3=", "num-temps="])
+    except getopt.GetoptError:
+        usage()
+        sys.exit(2)
+        
+    for opt, arg in opts:
+        if opt in ("-h", "--help"):
+            usage()
+            sys.exit()
+        elif opt == "--rp":
+            rp = int(arg)
+        elif opt == "--t1":
+            arg =  arg.split(':')
+            t1 = float( arg[0])
+            r1 = float( arg[1])
+        elif opt == "--t2":
+            arg =  arg.split(':')
+            t2 = float( arg[0])
+            r2 = float( arg[1])
+        elif opt == "--t3":
+            arg =  arg.split(':')
+            t3 = float( arg[0])
+            r3 = float( arg[1])
+        elif opt == "--num-temps":
+            num_temps =  int(arg)
+
+    max_adc = (1024 * 16) - 1
+    min_temp = 0
+    max_temp = 350
+    increment = int(max_adc/(num_temps-1));
+            
+    t = Thermistor(rp, t1, r1, t2, r2, t3, r3)
+    tmp = (min_temp - max_temp) / (num_temps-1)
+    print tmp
+    temps = range(max_temp, min_temp + tmp, tmp);
+
+    print "// Thermistor lookup table for Marlin"
+    print "// ./createTemperatureLookup.py --rp=%s --t1=%s:%s --t2=%s:%s --t3=%s:%s --num-temps=%s" % (rp, t1, r1, t2, r2, t3, r3, num_temps)
+    print "#define NUMTEMPS %s" % (len(temps))
+    print "short temptable[NUMTEMPS][2] = {"
+
+    counter = 0
+    for temp in temps:
+        counter = counter +1
+        if counter == len(temps):
+            print "   {%s, %s}" % (int(t.adc(temp)), temp)
+        else:
+            print "   {%s, %s}," % (int(t.adc(temp)), temp)
+    print "};"
+    
+def usage():
+    print __doc__
+
+if __name__ == "__main__":
+    main(sys.argv[1:])