@. Software application for temperature measurement

Temperature can only be measured to 0.05°C accuracy, if all the math equations provided by the thermistor manufacturer are included in the software application. The most important is to find the correlation between A/D converter output values and actual thermistor resistance at a given temperature. The small current through the thermistor must be measured precisely. Therefore an ordinary multimeter may interfere with the sensor operation, if one tries to connect it in series with the thermistor. The only option is voltage measurement. It is important to measure two voltages: the voltage on thermistor terminals and the voltage on “3k” resistor on the circuit. It is also important to determine the actual resistance of the “3k” resistor that is connected in series with the thermistor. We can see from the temperature measurement circuit schematic that the electrical current through thermistor is the same as the current through “3k” resistor. If voltage on “3k” resistor is determined accurately it is now possible to accurately calculate the current through the thermistor. Give the voltage and the current through thermistor one can calculate the resistance of the thermistor at a given temperature.

There is a sample MS Windows Thermometer application with all the equations inbuilt. Setting up temperature measurement correctly requires obtaining the following reference data at two different temperatures:

- Thermistor voltage (U_T1 and U_T2)

- “3k” resistor voltage (U_R3kT1 and U_R3kT2)

- Temperature reading from a reference thermometer (T1 and T2)

- A/D output value (A/D value at T1 and A/D value at T2)

Thermometer sample application v1.0 from Downloads section has text boxes ready for all the mentioned reference parameters. You can read the actual room temperature after you enter the data for two reference temperatures. It is important to note, that the reference temperatures should be at least 10°C apart. A good idea is to take ice melting point as one reference temperature and the room temperature as the other. You need to fill-up a bowl with ice cubes from a freezer and then wait for them to melt sufficiently produce some ice cold water with around 0°C. Be careful to protect the sensor terminals from the contact with water. The best way to do this is to incase the sensor into shrinkable plastic tube and soldering a long enough wires to the thermistor terminals. It is wise to do the room reference temperature measurement before measuring the temperature at ice melting point. If you do the opposite, you will have to wait for the sensor to dry completely and warm-up, which may take up to a half an hour…