Interfacing to PT1000 w/ limited range of interest

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hi all,
Im not sure if this is the right forum, but it seemed to be the closest match.

I want to interface an Arduino with a PT1000 RTD temperature sensor. My range of interest is 85C to 95C which corresponds to 1328 Ohms to 1366 Ohms.
I have breadboarded a circuit and am using a 25 ohm variable resistor to roughly simulate the RTD sensor.

So, I am reading the "sensor" value, performing an operation on it to get the range of values I am interested in and serially outputting to my pc. And its working.....

But I am getting only integer values, whereas I really want 0.1 resolution.

Is it possible to do this with the serial output?

Later I will be also interfacing with a LCD, so maybe I can only do it on the LCD?

Here is my code....

/*
  RTD Analog Input
 This will read the value of RTD/POT, save it as 
 a variable, then calculate the temp
 
 The circuit:
 * Wheatstone bridge feeds diff amp attached to analog input 0
 
 
  */

int sensorValue = 0;  // variable to store the value coming from the sensor
int tempValue = 0; // variable to store calculated temp

void setup() {
  Serial.begin(9600);
}

void loop() {
  // read the value from the sensor:
  int sensorValue = analogRead(0);
  tempValue = ((sensorValue-225)/54)+85;
  Serial.println(tempValue);
  delay(10);

}

Any advice would be greatly appreciated! :slight_smile:

2

You're being bitten by "integer division". In C++ 150/54 (for example) is 2. Not 2.777 or 2.8 but just 2. So your expression ((sensorValue-225)/54) loses a lot of precision.

You can try using floats or doubles to work with real numbers prior to converting back to an integer:

tempValue = ((sensorValue-225.0)/54)+85;

Adding a real number forces real-number computation, and the final result is converted back to an integer when you write the result to tempValue.

--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons

3

So, by using floats my calculation is more precise, due to not getting anything after whole numbers chopped off until it gets written to tempValue?

Later, when I interface the lcd with the arduino, would I perhaps send a larger number to the lcd then divide it to get 0.1 resolution?

Sorry, Im still really new to arduino and programming :frowning:

4

Yes, it's best to stick to integer numbers whenever possible as floating-point computation takes quite a bit more work (time). Just computing 10 times the needed quantity and displaying it on an LCD with the decimal point in the right place is the way to go.

--
The Quick Shield: breakout all 28 pins to quick-connect terminals

5

Thank you very much for clearing this up for me! :smiley: