File content as of revision 6:3c9cb2bd08c6:

/* thermister_ert-j0eg1.cpp
  By Joseph Ellsworth CTO of A2WH
  Take a look at A2WH.com Producing Water from Air using Solar Energy
  March-2016 License: https://developer.mbed.org/handbook/MIT-Licence 
*/

#include "mbed.h"
#include "ohms.h"  // from library ohms_law
#include "thermister_ert-j0eg1.h"
// Set of resistance values from data sheet used to lookup temperature to find
// location in a 5V band. 
const  long thermisterResistance[] = {205200,154800,117900,90690,70370,50570,43440,34530,27640,22270,18060,14740,12110,
                  10000,8309,6941,5828,4916,4165,3542,3027,2595,2233,1929,1672,1451,1261,1097,956,836,732,642,565,499};                  
const  int thermisterBase = -40.0f;
const  int thermisterStep = 5.0f;
const  int thermisterMaxTemp = 125.0f;

float conv_c_to_f(float tempC) {  
  return (tempC * 1.8f) + 32.0f;
}

float conv_f_to_c(float tempF) {
    return (tempF - 32.0f) * 0.55555555555f;
}

float ERTThermisterRead(PinName apin) {
  #ifdef DEBUG3
    printf("ERTThermisterRead start\r\n");
  #endif
  AnalogIn adcPin(apin);
  float tmp = ERTThermisterReadTemp(adcPin);
  #ifdef DEBUG3
    printf("ERTThermisterRead temp=%f done\r\n", tmp);
  #endif
  return tmp;
}

// read the temp from thermister and print out
// both the reading and intermediate values.
float ERTThermisterReadPrint(Serial log, char *label, PinName apin) {
   AnalogIn adcpin(apin);
   float sensorValue = adcpin.read() * ERTThermisterAdcRef;
   long sensorResist = calcResistV(22000, ERTThermisterAdcRef, sensorValue);
   float t1C = ERTThermisterReadTemp(adcpin);
   float t1F = conv_c_to_f(t1C);                
   log.printf("%s\tvin=%4.2f\tresist=%ld\tT1C=%4.1f\tT1F=%4.2f\r\n", 
      label, sensorValue, sensorResist, t1C, t1F);
   return t1C;        
}


/* Read Thermister Temperature using ADC and voltage dividor 
 Lookup the range of resistance by finding the first resistance
 lower than our specified value which gives us temp with 5C spread
 then adjust by how far our sensor reading is above the lower
 bound of the range.*/
float ERTThermisterReadTemp(AnalogIn adcpin) {    
    //sensorValue = avgRead(adcpin,3,2);
    float sensorValue = adcpin.read() * ERTThermisterAdcRef;    
    long sensorResist =  calcResistV(22000, ERTThermisterAdcRef, sensorValue);
    int tempCnt = thermisterBase;   
    int tempndx = 0;
    #ifdef DEBUG3
       printf("start ERTThermisterReadTemp adc=%4.2f resist=%ld\r\n", sensorValue, sensorResist);
    #endif
    if  (sensorResist >= thermisterResistance[0]) {
      #ifdef DEBUG3
        printf("resist too high\r\n");
      #endif
      return ERTThermisterErrorUnder;
    }
    
    #ifdef DEBUG3
      printf("entering while loop sensorResist=%ld thermisterResistance[0]=%ld\r\n",sensorResist, thermisterResistance[0]);
    #endif
    while (sensorResist < thermisterResistance[tempndx]) {     
       tempndx++;
       tempCnt += thermisterStep;
       if (tempCnt >= thermisterMaxTemp) {
         return ERTThermisterErrorOver;
       }
    }
    tempCnt -= thermisterStep; // backup to begin of range 
    #ifdef DEBUG3   
    printf("tempCnt=%d\r\n", tempCnt);
    #endif
    int lowTempResist = thermisterResistance[tempndx -1];
    int highTempResist = thermisterResistance[tempndx];
    int resistRangeDelta = lowTempResist - highTempResist;
    float degreesPerUnit = (float) thermisterStep / (float) (resistRangeDelta);
    int unitOverlow = lowTempResist - sensorResist;
    float degreeAdj = (float) unitOverlow * degreesPerUnit;
    #ifdef DEBUG3
    printf("degreeAdj=%f\r\n", degreeAdj);
    #endif
    return (float) tempCnt + degreeAdj;
}

float ERTThermisterAvgReadTemp(AnalogIn adcpin, int numRead) {
  float sum = 0;
  int ndx; 
  for (ndx = 0; ndx < numRead; ndx++) {
    wait(0.005);
    sum += ERTThermisterReadTemp(adcpin);
  }
  return (float) ((float) sum / (float) ndx);
}