I got a Licor CO2-Logger (Li-820) for a scientific experiment. The Li-820 outputs an analog signal (voltage) and also offers a serial communication via a RS-232 port. I would like to sniff the serial communication with an Arduino Uno and display the CO2 values on a LCD display such that I can control the analog signal that is logged in a computer system. I want to display both the analog and the digital signal.

I use a RS-232 level shifter to connect the Licor CO2 analyzer to the Arduino Uno, and I can sniff the COM port successfully both with the Arduino serial monitor and a sniffer program. Over the RS-232 port the Li-820 device outputs an XML-like line that is as follows (Line breaks and indentation added for readability):


I would like to parse that information for the relevant part with the Arduino Uno which is the <co2>5.2527637e2</co2> ("CO2" value) and first output it to the serial monitor. Next, I will display that value on a LCD display. This last step should be a minor problem.

So, how can I parse the information for the relevant bits and then display it to the serial monitor?

I looked into many examples on the Internet. A modified version of the working code from here was the closest I got.

  • 2
    \$\begingroup\$ This is a software problem and does not belong in this forum. I suspect stackover flow might be a better place to ask the question but am not sure... \$\endgroup\$ Commented Jul 19, 2012 at 16:46
  • \$\begingroup\$ I think you find more subject matter experts at stackoverflow, maybe migrate the question to it. \$\endgroup\$
    – jippie
    Commented Jul 19, 2012 at 17:56
  • \$\begingroup\$ This question is being worked with on an arduino, this seems like a boundary question but one that fits on either site. If it continues to not get attention I am willing to migrate. \$\endgroup\$
    – Kortuk
    Commented Jul 20, 2012 at 4:04
  • \$\begingroup\$ See if arduino gives you a sscanf() which works with floating point types or even scientific notation. If not you will have to use an integer solution with a denominator you can grow. Break the problem down into pieces - finding the right value, then reading it in. \$\endgroup\$ Commented Jul 20, 2012 at 17:14

2 Answers 2


I'm assuming that you can already read the string into a buffer so I will start at the buffer. Your code should go something like this (you will need to import the string library):

String wholeString = String(yourLoadedCharBuffer);
int startTag = wholeString.indexOf("<co2>");
int endTag = wholeString.indexOf("</co2>");

String co2FullString = wholeString.substring(startTag+5,endTag-1);

This gives us "5.2527637e2", you add 5 to the start tag because it is 4 characters long and you want to start at the "5", and you subtract 1 from the endtag, because we don't want the first character of it.

int eStart = co2FullString.indexOf("e");
String decimalString = co2FullString.substring(0,eStart-1);
String powerString = co2FullString.substring(estart+1, co2FullString.length-1);

char decimalCharArray[] = decimalString.toCharArray();
char powerCharArray[] = powerString.toCharArray();

float decimal = atof(&decimalCharArray[0]);
float power = atof(&powerCharArray[0]);

float finalCO2Value = pow(decimal,power);

This may be obvious to you, but the output is being sent in XML format. Getting an XML parsing library for arduino will go a long way to meeting your needs.

  • 1
    \$\begingroup\$ Technically correct, but probably a bit overweight for the problem at hand where likely only one value is needed. \$\endgroup\$ Commented Jul 20, 2012 at 17:11
  • \$\begingroup\$ Thanks to all for your answers. It is kind of working now. However, I only get half of the string via the soft serial port. The part with the CO2 concentrations is missing. Any ideas why not the whole string is read in? \$\endgroup\$
    – Strohmi
    Commented Aug 15, 2012 at 8:10

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