# reading temp from an unknown temp sensor

Hi,

I have a temperature probe I bought a while ago for use with a fridge controller (for beer brewing). I never ended up using it, as the controller came with a different more suitable sensor.

I was thinking about trying to see if i could get it working with an arduino and maybe use it for something. I’ve managed to hook it up with a voltage divider (I’m pretty new to electronics, is that the right term to use here?) and i’m able to get an analog reading which appears to rise and fall when the probe is heated and cooled, so I assume its working.

The probe has no brand/model markings whatsoever anywhere on it, so I cannot look up a datasheet to see how to convert it to a temperature.

So I was thinking, it is possible to take a reading at 2 known temperatures (ie, 0 degrees C and 100C) and then work it out based on the difference? Do temperature sensors normally have linear readings based on temperature?

Thanks.

It is probably just a thermistor and what you propose is a pretty sensible way to get an idea of what is going on. Thermistors are not linear but work well for their intended purpose, like trigger points or working over a narrow range. I recall they come in standard types, and Google should come forth with that.

Measure the sensor, at 25 degrees C, with a DMM . The resistance could be 10k, or some other standard value. Then use a thermistor library, and enter that value. The library takes care of the non linearity. Leo..

Hi,

Thanks for the info.

I measured the resistance and I got a reading of 14.81k Ohms at room temperature, however I don't have a thermometer to test what the exact room temperature is. I'll have to pick one up on the weekend and test it again when I know the temp.

I tried heating it up a bit, and the resistance decreased when it was hotter, so I'm guessing its an NTC type sensor, correct?

I've found a few thermistor libraries on these pages:

http://playground.arduino.cc/ComponentLib/Thermistor2

http://garagelab.com/profiles/blogs/tutorial-using-ntc-thermistors-with-arduino

I'll give them a try on the weekend when I have a thermometer to verify the temperature readings.

Thanks again for the help.