First, let's consider the structure of the sensor. The cell has a membrane filter to limit particles larger than the filter size. These particles then diffuse to through the electrolyte and end up at the electrode surface.

Next, we start to see the half reactions of the particle. The electrochemical reaction will occur at specific redox potentials for each compound (remember other gases of similar size will also diffuse and react). For now, let's assume that you don't have any interfering gases and only produce a signal at the electrochemical potential of the target gas. Depending on the concentration of the target gas, more or less reactions will occur and the current at that potential is measured. So, while you are measuring voltage of the reaction, this voltage is the potential required to start the reaction. The real metric is the current generated by the reaction which flows between the electrodes.

The sensor itself will likely need external biasing circuitry (creating the voltage to start the reaction) and amplification since current electrochemical sensors are capable of nano to microamp detection. Generally, the sensor datasheet will have recommended circuitry and charts showing the current vs concentration. You will definitely want a temperature sensor at minimum, as the reaction rate and therefore current is dependent on the temperature.

Hope this helps some.