thermister_ert-j0eg1.cpp

00001 /* thermister_ert-j0eg1.cpp
00002   By Joseph Ellsworth CTO of A2WH
00003   Take a look at A2WH.com Producing Water from Air using Solar Energy
00004   March-2016 License: https://developer.mbed.org/handbook/MIT-Licence 
00005 */
00006 
00007 #include "mbed.h"
00008 #include "ohms.h"  // from library ohms_law
00009 #include "thermister_ert-j0eg1.h"
00010 // Set of resistance values from data sheet used to lookup temperature to find
00011 // location in a 5V band. 
00012 const  long thermisterResistance[] = {205200,154800,117900,90690,70370,50570,43440,34530,27640,22270,18060,14740,12110,
00013                   10000,8309,6941,5828,4916,4165,3542,3027,2595,2233,1929,1672,1451,1261,1097,956,836,732,642,565,499};                  
00014 const  int thermisterBase = -40.0f;
00015 const  int thermisterStep = 5.0f;
00016 const  int thermisterMaxTemp = 125.0f;
00017 
00018 float conv_c_to_f(float tempC) {  
00019   return (tempC * 1.8f) + 32.0f;
00020 }
00021 
00022 float conv_f_to_c(float tempF) {
00023     return (tempF - 32.0f) * 0.55555555555f;
00024 }
00025 
00026 float ERTThermisterRead(PinName apin) {
00027   #ifdef DEBUG3
00028     printf("ERTThermisterRead start\r\n");
00029   #endif
00030   AnalogIn adcPin(apin);
00031   float tmp = ERTThermisterReadTemp(adcPin);
00032   #ifdef DEBUG3
00033     printf("ERTThermisterRead temp=%f done\r\n", tmp);
00034   #endif
00035   return tmp;
00036 }
00037 
00038 // read the temp from thermister and print out
00039 // both the reading and intermediate values.
00040 float ERTThermisterReadPrint(Serial log, char *label, PinName apin) {
00041    AnalogIn adcpin(apin);
00042    float sensorValue = adcpin.read() * ERTThermisterAdcRef;
00043    long sensorResist = calcResistV(22000, ERTThermisterAdcRef, sensorValue);
00044    float t1C = ERTThermisterReadTemp(adcpin);
00045    float t1F = conv_c_to_f(t1C);                
00046    log.printf("%s\tvin=%4.2f\tresist=%ld\tT1C=%4.1f\tT1F=%4.2f\r\n", 
00047       label, sensorValue, sensorResist, t1C, t1F);
00048    return t1C;        
00049 }
00050 
00051 
00052 /* Read Thermister Temperature using ADC and voltage dividor 
00053  Lookup the range of resistance by finding the first resistance
00054  lower than our specified value which gives us temp with 5C spread
00055  then adjust by how far our sensor reading is above the lower
00056  bound of the range.*/
00057 float ERTThermisterReadTemp(AnalogIn adcpin) {    
00058     //sensorValue = avgRead(adcpin,3,2);
00059     float sensorValue = adcpin.read() * ERTThermisterAdcRef;    
00060     long sensorResist =  calcResistV(22000, ERTThermisterAdcRef, sensorValue);
00061     int tempCnt = thermisterBase;   
00062     int tempndx = 0;
00063     #ifdef DEBUG3
00064        printf("start ERTThermisterReadTemp adc=%4.2f resist=%ld\r\n", sensorValue, sensorResist);
00065     #endif
00066     if  (sensorResist >= thermisterResistance[0]) {
00067       #ifdef DEBUG3
00068         printf("resist too high\r\n");
00069       #endif
00070       return ERTThermisterErrorUnder;
00071     }
00072     
00073     #ifdef DEBUG3
00074       printf("entering while loop sensorResist=%ld thermisterResistance[0]=%ld\r\n",sensorResist, thermisterResistance[0]);
00075     #endif
00076     while (sensorResist < thermisterResistance[tempndx]) {     
00077        tempndx++;
00078        tempCnt += thermisterStep;
00079        if (tempCnt >= thermisterMaxTemp) {
00080          return ERTThermisterErrorOver;
00081        }
00082     }
00083     tempCnt -= thermisterStep; // backup to begin of range 
00084     #ifdef DEBUG3   
00085     printf("tempCnt=%d\r\n", tempCnt);
00086     #endif
00087     int lowTempResist = thermisterResistance[tempndx -1];
00088     int highTempResist = thermisterResistance[tempndx];
00089     int resistRangeDelta = lowTempResist - highTempResist;
00090     float degreesPerUnit = (float) thermisterStep / (float) (resistRangeDelta);
00091     int unitOverlow = lowTempResist - sensorResist;
00092     float degreeAdj = (float) unitOverlow * degreesPerUnit;
00093     #ifdef DEBUG3
00094     printf("degreeAdj=%f\r\n", degreeAdj);
00095     #endif
00096     return (float) tempCnt + degreeAdj;
00097 }
00098 
00099 float ERTThermisterAvgReadTemp(AnalogIn adcpin, int numRead) {
00100   float sum = 0;
00101   int ndx; 
00102   for (ndx = 0; ndx < numRead; ndx++) {
00103     wait(0.005);
00104     sum += ERTThermisterReadTemp(adcpin);
00105   }
00106   return (float) ((float) sum / (float) ndx);
00107 }