Thermometry : Automated data acquisition

Digital thermometer using a Wheatstone bridge and a DMM. This is calibrated by varying the $200\Omega$ resistance at low temperatures and the $10k\Omega$ resistance at high temperatures such that 1mV corresponds to 1^oC.

The Arduino board has analog inputs which have a voltage range of 0V to 5V, to make full use of this range it is necessary to amplify the signal as the values measured across the Wheatstone bridge is in the mV range. To accomplish this we use a pre-built buffered amplifier based on three operational amplifiers, or op-amps. This circuit diagram is shown below.

For the connections see the Thermometry script.

The important part here is that all devices (the Wheatstone bridge, the amplifier, the Arduino and the power supply) share the same ground. To ensure this, the Arduino Uno needs to have one of its ground (Gnd) pins connected to the common ground. Without this connection there will be significant errors in the readouts on pins A0 and A1.

OpAmp amplifier for thermometer

/*
  OpAmp Thermometer
  written by: Navot Arad, Queen Mary University of London
  Modified: AJM 23 Oct 2015 to use single input from Peter's new amplifier.
*/
 
int InputPin = 0;          // A0 is the amplified signal pin
 
float temp = 0;            // Variable to store temperature reading
float Voltage10 = 0;       // Voltage read using 10 bit ADC

float AmpVoltage = 0;
float G = 50.0;            // Amplification factor
float scaleG = 0;
float conversion = 0;      // Conversion rate to degrees
float average = 0;
 
void setup() // Only runs once after board is reset 
{
  Serial.begin(9600); //Rate at which data is sent to serial monitor
  Serial.println("Time    Temperature"); // Sends everything inside " " as a string to the serial monitor
}
 
void loop() // Runs continuously on repeat
{
  // average temperature over first second
  for (int i = 0; i < 100; i++) {
    Voltage10 = analogRead(InputPin);   // Read data from input pin (A0) on a 10 bit scale
    /*
       Navot used an empirical correction to account for amplification changes.
       We will not use it with Peter's new amplifiers
       scaleG = G + ((temp - 1023) * 1.5 * 0.00147);
       conversion = (1023 * scaleG)/50; // conversion factor  
       AmpVoltage = temp *(100.0/conversion); // Converts signal to temperature
    */ 
    /*
       simple voltage conversion:
       Voltage10 is a 10bit number from 0 to 1023 (=2^10-1)
       Voltage10 = 0 corresponds to 0mV
       and Voltage10 = 1023 corresponds to 100 mV, so the conversion from Voltage10 to 
       a reading in mV is:
       V = Voltage10 * 100 / 1023
       This gives us V in mV. As this is also the temperature in Centigrade, we have T = V.
    */
    temp = Voltage10*(100.0/1023.0); // Converts signal to the amp voltage in mV == temperature    
    average = average + temp;
    delay(10);
  }
 
  Serial.print(millis()/1000); // Sends value stored on AmpVoltage
                               // to serial monitor and starts new line
  Serial.print("    ");
  average = average/100;
  Serial.println(average,1);
  average = 0;
  delay(29000); // 29 second delay before loop runs again
}

OpAmp amplifier for thermometer : One input code : old version

/*
OpAmp Thermometer
written by: Navot Arad, Queen Mary University of London
*/
 
int opAmp  = 0;  // A0 is the amplified signal pin
int ground = 1;  // A1 is the ground pin
 
float temp0 = 0, temp1 = 0;// Variable to store data from pin A0
float temp = 0;
float AmpVoltage = 0;      // Use float to convert data from A0 to voltage
float G = 50.0;            // Amplification factor
float scaleG = 0;
float conversion = 0;      // Conversion rate to degrees
float average = 0;
 
void setup() // Only runs once after board is reset 
{
  Serial.begin(9600); //Rate at which data is sent to serial monitor
  Serial.println("Time    Temperature"); // Sends everything inside " " as a string to the serial monitor
}
 
void loop() // Runs continuously on repeat
{
  // average temperature over first second
  for (int i = 0; i < 100; i++) {
    temp0 = analogRead(opAmp);   // Read data from pin A0 on a 10 bit scale
    //temp1 = analogRead(ground);  // Same for pin A1
    temp = abs(temp0 - temp1);
    scaleG = G + ((temp - 1023) * 1.5 * 0.00147);
    conversion = (1023 * scaleG)/50; // conversion factor  
    AmpVoltage = temp *(100.0/conversion); // Converts signal to temperature    
    average = average + AmpVoltage;
    delay(10);
  }
 
  Serial.print(millis()/1000); // Sends value stored on AmpVoltage
                               // to serial monitor and starts new line
  Serial.print("    ");
  average = average/100;
  Serial.println(average,1);
  average = 0;
  delay(2000); // 29 second delay before loop runs again
}

Other Stuff : Do not use

OpAmp amplifier for thermometer : Two input code

/*
  OpAmp Thermometer
  written by: Navot Arad, Queen Mary University of London
*/
 
int opAmp  = 0;  // A0 is the amplified signal pin
int ground = 1;  // A1 is the ground pin
 
float temp0 = 0, temp1 = 0;// Variable to store data from pin A0
float temp = 0;
float AmpVoltage = 0;      // Use float to convert data from A0 to voltage
float G = 50.0;            // Amplification factor
float scaleG = 0;
float conversion = 0;      // Conversion rate to degrees
float average = 0;
 
void setup() // Only runs once after board is reset 
{
  Serial.begin(9600); //Rate at which data is sent to serial monitor
  Serial.println("Time    Temperature"); // Sends everything inside " " as a string to the serial monitor
}
 
void loop() // Runs continuously on repeat
{
  // average temperature over first second
  for (int i = 0; i < 100; i++) {
    temp0 = analogRead(opAmp);   // Read data from pin A0 on a 10 bit scale
    temp1 = analogRead(ground);  // Same for pin A1
    temp = abs(temp0 - temp1);
    scaleG = G + ((temp - 1023) * 1.5 * 0.00147);
    conversion = (1023 * scaleG)/50; // conversion factor  
    AmpVoltage = temp *(100.0/conversion); // Converts signal to temperature    
    average = average + AmpVoltage;
    delay(10);
  }
 
  Serial.print(millis()/1000); // Sends value stored on AmpVoltage
                               // to serial monitor and starts new line
  Serial.print("    ");
  average = average/100;
  Serial.println(average,1);
  average = 0;
  delay(29000); // 29 second delay before loop runs again
}

Older code

/*
OpAmp Thermometer
written by: Navot Arad, Queen Mary University of London
*/

int opAmp = 0; // Set a variable to read the analogue input to pin A0
int diode = 1;
int R10 = 4700, R11 = 10000, R12 = 300;
float lowlim = 0, uplim = 0;

int temp = 0, temp1 = 0; // Variable to store data from pin A0
float AmpVoltage = 0; // Use float to convert data from A0 to voltage
float G = 0, scaleG = 0; // Amplification factor;
float conversion = 0; // Conversion rate to degrees
float average = 0;

void setup() // Only runs once after board is reset 
{
  Serial.begin(9600); //Rate at which data is sent to serial monitor
  G = (R10 + R11 + R12) / R12;
  lowlim = G * 0.98;
  uplim = G * 0.99;
  Serial.println("Time    Temperature"); // Sends everything inside " " as a string to the serial monitor
}

void loop() // Runs continuously on repeat
{
  // average temperature over first second
  for (int i = 0; i < 100; i++) {
    temp = analogRead(opAmp); // Read data from pin A0 on a 10 bit scale
    temp1 = analogRead(diode);
    temp = abs(temp - temp1);
    scaleG = G + ((temp - 1023) * ((uplim - lowlim) * 0.00167));
    constrain(scaleG, lowlim, uplim);
    conversion = (1023 * scaleG)/50; // conversion factor  
    AmpVoltage = temp *(100.0/conversion); // Converts signal to temperature    
    average = average + AmpVoltage;
    delay(10);
  }
  Serial.print(millis()/1000); // Sends value stored on AmpVoltage to serial monitor and starts new line
  Serial.print("    ");
  average = average/100;
  Serial.println(average, 1);
  average = 0;
  delay(29000); // 29 second delay before loop runs again
}

This will read out the temperature to the serial monitor in degrees celsius.

Sample Data

Below is a series of tables comparing the readings between a mercury thermometer, DMM which has been calibrated such that 1mV corresponds to 1^oC, and the arduino.