TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Sensor: The ratio (a/b) is 1-2. The sensor further comprises a reference electrode on the proton conductive layer. The concentration of the hydrogen gas component is measured on the basis of the limiting current flowing across the first electrode and the second electrode by maintaining constant an electrical potential difference between the first electrode and the reference electrode and by varying the voltage applied across the first electrode and the second electrode.The diffusion resistance ratio (a/b) falls in the range of 1 to 2 when a dimensional ratio (de/cf) is set to fall in the range of 1 to 2. (c) is a cross sectional area of a first through-hole formed as the diffusion limiting inlet (19), (d) is a length of the first through-hole, (e) is a cross sectional area of a second through-hole formed as the diffusion limiting outlet (21) and (f) is a length of the second through-hole. The diffusion resistance ratio (a/b) is identified by a second limiting current divided by the first limiting current. The second limiting current is determined by reversing a polarity of the voltage applied across the first electrode (3) and second electrode (5) to determine the first limiting current.The gas sensor is preferably a hydrogen sensor for measuring concentration of hydrogen gas contained in a measurement gas. Hydrogen gas contained in the atmosphere is introduced via the diffusion limiting inlet (19) to the first electrode and is dissociated, decomposed or reacted through application of voltage across the first electrode (3) and the second electrode (5) to generate protons at a boundary face between the proton conductive layer (1) and the first electrode (3). Concentration of the hydrogen gas is measured on the basis of a limiting current that is produced by the protons being pumped via the proton conductive layer (1) from the first electrode (3) to the second electrode (5).    TECHNOLOGY FOCUS - POLYMERS - Preferred Sensor: The proton conductive layer (1) is made of a polymer electrolyte that conducts protons at a temperature less than 150 degrees C, preferably a fluorine-containing resin.