The trend of sensitivity factors can be explained by total amount of Pt loading in each sensor; the higher the Pt loading in grams, the higher sensitivity. Although, it is reported (and also shown in Table 1) that catalysts made from HiSpec 3000 (20% Pt/C) has higher electrochemical specific surface area (ECSA) than catalyst made from HiSpec 4000 (40% Pt/C) [26] but higher Pt loading in 40% Pt/C than other GDE catalysts led to the highest sensitivity for 40% Pt/C catalyst. Based on the study shown in Table 2, although the ECSA is very important but the key factor effecting on sensor performance and its sensitivity is the sensor total Pt loading; e.g. although the sensor with 30% Pt/C loading has the highest surface area (0.072 m2) but the highest sensitivity goes to 40% Pt/C sensor with highest total Pt loading (1.08 mg).