There is set forth herein an indicia reading apparatus comprising two or more light sensing assemblies. The sensing unit has an electrical switch generating a pulse sensing signal when switching and a signal output terminal electrically connecting to the electrical switch and outputting said pulse sensing signal. The mobile device then converts the audio instruction into a command script and associates the command script with a document identifier. A blending ratio of the process gas and a natural gas is selected based on the estimate of the composition of the process gas.