 Can't get correct value from thermistor

Hi I have a 50K thermistor (this one), which I cannot get to work correctly.

My set up is the following: 5V -- T -- A -- 10K -- GND Where T is the thermistor, A is the analog PIN.

The problem is that I got weird temperatures like 100 celsius degrees, even if I am sure that the SH equation is correct, since I read the same values as in the datasheet table.

Thanks

Have you tried manually measuring the resistance of the thermistor and verifying the temperature from the datasheet?

Can you post your code? and/or the SH equation?

Such a table is work for - http://arduino.cc/playground/Main/MultiMap -

Think 11 - 15 entries should be enough to get an acceptable approximation although I do not know your requirements

Since you have a 50K@room_temp thermistor, is it possible your curve is expecting to see
R = 50K rather than 10K?

My code is:

#include <math.h>
#define SERIESRESISTOR 10000

double R1 = 10000.0; //resistance put in parallel
double V_IN = 5.0;

double A = 9.6564E-04;
double B = 2.1069E-04;
double C = 8.5826E-8;
double K = 9.5; // mW/dec C – dissipation factor

double SteinhartHart(double R) {
// calculate temperature
double logR  = log(R);
double logR3 = logR * logR * logR;

return 1.0 / (A + B * logR + C * logR3);
}

void setup() {
Serial.begin(9600);
}

void loop() {

double V =  (adc_raw / 1024) * V_IN;
Serial.print("V =");
Serial.print(V);

//calculate resistance
double R_th = (R1 * V ) / (V_IN - V);
Serial.print("V   R_th = ");
Serial.print(R_th);

double kelvin = SteinhartHart(R_th) -  V * V / (K * R_th);
double celsius = kelvin - 273.15;
Serial.print(" Ohm  -  T = ");
Serial.print(celsius);
Serial.println("C");
delay(1000);
}

For the map function, I think that is not the problem. I can correctly map my resistance value to temperature values, but is my resistance value which is wrong, not its mapping.

I also tried with 5 10KOhm resistors in series, but this hasn’t work either.

According to your datasheet the Thermistor has 50k @25°C , which should result in V =0.83, if I understand your ascii wiring scheme correctly.

At lower temperatures you get a higher resistance, e.g. 100k @10°C, which would result in V = 0.45.
Higher temperatures provide a higher voltage, e.g. 10k @62.5°C yields V =2.50

Is that in line with your first debug output, [edit:]and what you read with a voltage meter between A5 and GND ?
If no, check your wiring (or correct my understanding)
If yes, check the maths.

Your equation for Rth assumes a set up like so:

5V -- 10K -- A -- T -- GND

That deserves a karma brownie, PapaG.

And it provides a much bigger voltage swing that way.

Thank you, michael_x, my first!

I got weird temperatures like 100 celsius degrees

Did you get the right V?

Did you get the right Rth?

Did you get the right T?

Using PapaG help, I got it working! Thanks!

@ dhenry... did you read the whole post... you know, from the first posting??

Bob