# Programming my thermistor

Hey everyone,

I am working with an NTC thermistor to measure the outlet temperature of a diesel engine.

Now my issue is how to wire it and program it to read out the temperature in celsius degrees on my computer arduino monitor. I am using an arduino mega adk.

The values that i found for my thermistor are: at 0°C 100 Kohm 33°C 1.85 Mohm 47°C 1.02 Mohm

I hope somebody can help me out here

Thank you!

There is something wrong with your measurements. NTC thermistors' resistance always increases at colder temperatures. You are showing a lower resistance at 0 than at 33C.

Do you have the part number or datasheet for the thermistor? The Datasheet should have the equations and (probably) a table for the relationship between temperature and resistance. Thermistors are exponential and there are a lot of different types.

If you are measuring temperatures in the normal ambient temperature range you can measure a few points across your range and use a lookup table with linear interpolation.

If you need a broad range or higher accuracy, you can measure the resistance at a few points and solve for the parameters (see the link below). Typically you would use freezing and boiling temperatures (at your altitude) and at least one other point.

Ametherm has a good write up on thermistors. The Wikipedia page is also good. Ametherm Page describing thermstors

The simplest method is to create a Resistor Divider circuit and read the voltage at the mid point. One resistor is your thermistor and the other resistor is a fixed resistor. The fixed resistor will be approximately the same resistance as the thermistor in the center of the expected temperature range. e.g. if you are measuring normal room temperature and your thermistor is 1MOhm at 25C you would use a 1MOhm fixed resitor

If you need a wide range of temperature measurement then it is common to have another fixed resistor and transistor in parallel with the first fixed resistor. By turning the transistor on and off you change the effective resistance and can measure a broader range of temperature

Could it be a thermocouple. Google "diesel exhaust termocouple" (images). Leo..