# temperature sensing with pt1000 sensor

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

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

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

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

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??

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