# Arduino Forum

## Forum 2005-2010 (read only) => Hardware => Interfacing => Topic started by: ikawa on Mar 20, 2010, 11:12 am

Title: temperature sensing with pt1000 sensor
Post by: ikawa on Mar 20, 2010, 11:12 am
I am hoping somebody can help me with a temp sesning problem..

I need to measure the temperature of air between about 150 deg C and 250 deg C using my arduino board. To do this I have bought a PT1000 platinum temp sensor. The resistance of the sensor varies between 1570 ome at 150 degC and 1940 ome at 250 deg C.

I have connected the sensor up in parallel with a 2.2kome resistance and i am measuring the voltage at the point between them using the analog input on the arduino. Using this set up the voltage varies from 1.31V at 150 deg C to 1.53V at 250 deg C. The problem is that this only represents about 44 steps on the arduino board so i get a 2.3 deg error before i even start.

Is there a way to design a circuit that gives me 0 to 5V across the temperature range i want to measure????

Andrew
Title: Re: temperature sensing with pt1000 sensor
Post by: ArduinoM on Mar 20, 2010, 04:39 pm
The simple hack solution is to use the 3.3V out pin and connect it to the REF pin, this will make the analog in range 0-3.3V when you set the ref range to exernal
http://www.arduino.cc/en/Reference/AnalogReference

if you on the other hand use a pot to tune the ext ref from a 5 or 3.3 V pin to 2V or so, you will get more steps.

The harder or more expensive way is to use opamps diodes and resistors

David
Title: Re: temperature sensing with pt1000 sensor
Post by: pluggy on Mar 20, 2010, 04:53 pm
Voltage divider feeding the Aref pin with say around 1.66 volts and use the External setting.  That should treble the number of steps between your two voltsges. Simple way is 3 1k resistors (common value in microcontroller work)in series between 5v and Gnd and feed the Aref pin from the point between resistors 1 & 2 from the Gnd side.

5v - 1k - 1k - aref - 1k - gnd

Title: Re: temperature sensing with pt1000 sensor
Post by: retrolefty on Mar 20, 2010, 05:19 pm
Normally RTD sensors are wired into a resistive Wheatstone bridge configuration, driven by either a constant current or constant voltage source, followed by a differential instrumentation op-amp with a amp gain set such the desired temperature measurement range falls within the voltage range of the A/D stage reading the output, in your case the Arduino 0-5vdc measurement range.

If you Goggle RTD you should get a lot of information on RTD interfacing circuits. One generally wishes to keep the current flowing into a RTD as low as possible to offset the 'self-heating' created by the current which can effect the measurement accuracy. RTD are among the best temperature sensors available, but do require careful interface design.

Lefty
Title: Re: temperature sensing with pt1000 sensor
Post by: ikawa on Mar 20, 2010, 11:21 pm
thanks for your help!! i think i will go for the simple hack solution unless it is possible to buy a ready made RTD interfacing circuit??
Title: Re: temperature sensing with pt1000 sensor
Post by: ArduinoM on Mar 20, 2010, 11:32 pm
Instrumental amplifiers come in all flavours and prices
Google around. 12bit i2c specific for pt RTD exists, easy to read, no analog and greater range (12bit)

david