# Using a 2 wire RTD Temp Sensor

Hello all,

So after doing some reading, I believe I'm using my reclaimed RTD temp sensor incorrectly. I have been measuring it via ADC 0 using a voltage divider circuit. The thermistor or Temp probe is 1k and my resistor is 1k as well for the circuit.

When I measure it via the ADC the scale is very low, approx 2ohms / 5deg.

for example;
55 deg F = 532 ohms
100 deg F = 554 ohms
150 deg F = 576 ohms
212 deg F = 601 ohms

as you can see it makes it difficult to get an accurate reading. So what I think I need to do is make a bridge amp for the Temp Probe. Unfortunately I need some guidance from the Arduino community on if my suspicions are right, and what my solves can be for a bridge Amp.

Just noticed i screwed up on the data i provided. i gave the readings in ohms, when they are ADC numbers 0-1024.

[Corrected]
For example;
55 deg F = 532 ADC
100 deg F = 554 ADC
150 deg F = 576 ADC
212 deg F = 601 ADC

The relation between resistance and temperature is not linear. You can feed any three of your sample points into an [online calculator](http://"http://www.thinksrs.com/downloads/programs/Therm Calc/NTCCalibrator/NTCcalculator.htm") to get the Steinhart-Hart parameters of the thermistor. Those parameter can then be inserted into the Steinhart-Hart equation to convert resistances to temperatures.

Understood, i have used both the steinhart and a lookup table, But feel that the resolution is too low and would be "filling in the gaps. i need to inscrease the scale somehow for better reliability and accuracy. Right now im at +- 5 deg F.

johnwasser:
The relation between resistance and temperature is not linear. You can feed any three of your sample points into an [online calculator](http://"http://www.thinksrs.com/downloads/programs/Therm Calc/NTCCalibrator/NTCcalculator.htm") to get the Steinhart-Hart parameters of the thermistor. Those parameter can then be inserted into the Steinhart-Hart equation to convert resistances to temperatures.

Hi, in the top right hand corner of this screen is a window that says, search the arduino forum.
try arduino rtd as your search parameters.
It has been done before.

Tom......