Measuring Temperature With Flowstone

A major component of the automation system I'm building is the ability to measure temperature, specifically the temperature of blower air in an air conditioning system. I use a touchscreen netbook and a Labjack U3-HV to create this windows based automation system.

Here is how I achieved the software reading of real world temperature values.

Step 1: The Hardware Setup.

I used a standard NTC thermistor and 36kohm resistor to form a voltage divider. The free end of the resistor is connected to 0Volt and the free end of the thermistor is tied to 5Volt. Across the resistor the voltage is proportional to the temperature reading. i.e. Temperature rises and Voltage across the resistor rises. The junction of the resistor and thermistor I connected to analog input AIN1.

Step 2: Getting Sample Data.

Using a digital thermometer (its probe and the thermistor I wrapped together), I took temperature and voltage readings from ambient down to that of melting ice. These values I put into excel and made an XY scatter plot. From this I was able to get the equation of the linear relationship between temperature and voltage. Therefore for my hardware, the Temperature is;

T = (Vr - 2.921) / 0.038

Step 3: Implementing in Flowstone.

Using the formula from the previous step, here is how I implemented it in Flowstone. I have attached the schematic to demonstrate how it works.

The last pic is a screenshot of the prototype software I'm developing for the embedded system.

Now with any thermistor and fixed resistor, reliable temperature measurements can be made. I hope you found this instructable useful.