The GA3KPLUS3 measures CH4, CO2, and O2 as standard with a choice of up to two additional gases (one internal to the GA and one external) from three sample points.
It has user selectable sample and air purge intervals which are exhausted back to the process and atmosphere respectively.
The 4-20mA and Modbus outputs are updated at the end of each sample.
Following is a simple block diagram of the system including the optional external gas sensor: