Temperature

These are the most involved sensors to calibrate, primarily due to the response of the thermistors being non-linear. A typical 10kΩ thermistor resistance vs temperature graph is shown below:

• V2 Arduino code (code version 2.0 or later): enter the calculated calibration coefficients in the calibration web app at configure.echook.uk.

• Legacy Arduino code (below version 2.0): enter these values in the calibration.h file and re-upload the code.

Note that the 10k refers to the resistance at room temperature (25°), and that the resistance decreases as temperature increases. This is called a negative temperature coefficient, or NTC. The eChook board uses a 10k NTC thermistor.

On the board, the thermistor completes the lower half of a potential divider circuit with a 10kΩ resistor, so the voltage at the output of the potential divider falls as the temperature of the thermistor increases.

To translate this voltage to a temperature, the resistance curve of the specific thermistor being used is needed. Due to tolerances, this is subtly different even between identical thermistors. To do this the eChook uses the Steinhart-Hart equation, which uses three coefficients calculated from the thermistor to convert the resistance seen to an accurate temperature reading.

Use this process:

1. Take three measurements of thermistor resistance at different known temperatures.

2. Open the online calculator here.

3. Enter your three temperature/resistance points.

4. Copy the calculated A, B, and C coefficients.

5. For legacy code versions, enter the coefficients in calibration.h and re-upload.

6. For V2 code versions, enter the same coefficients in the web app.

The calculator looks like this:

For legacy code versions, enter the three temperature and resistance measurements and copy the A, B and C coefficients from the calculator into the calibration.h file.

//Board and Sensor Specific Calibrations
const float CAL_THERM_A = 0.001871300068; //Steinhart-Hart constants - See documentation for calibration method
const float CAL_THERM_B = 0.00009436080271;
const float CAL_THERM_C = 0.0000007954800125;

To get the default calibration we used a pan of water, digital thermometer, and a multimeter. We wrapped the thermistor in cling film to prevent the water conducting across the legs and altering the reading.