Gas sensor

A gas sensor for detecting a measurement target gas in a measurement gas atmosphere, comprising:

• first and second sensor elements, each of the first and second sensor elements being in the form of a thermal conductivity sensor element having a heating resistor whose resistance value varies with change in temperature thereof, the heating resistors of the first and second sensor elements being connected in series to each other;
• a first installation part defining a first inner space in which the first sensor element is installed;
• a second installation part defining a second inner space in which the second sensor element is installed;
• a casing accommodating therein the first and second installation parts;
• a circuit board configured to apply a voltage to the heating resistors of the first and second sensor elements;
• and
• a calculation unit configured to calculate a concentration of the measurement target gas in the measurement gas atmosphere according to a potential between the heating resistors of the first and second sensor elements under the application of a constant voltage to the heating resistors by the circuit board,
• the casing having a casing body formed with an opening such that the opening is open to the measurement gas atmosphere so as to introduce the measurement target gas into an inside of the casing,
• the first installation part having:
• a first gas introduction hole formed to provide communication between the first inner space and the inside of the casing;
• and a membrane member arranged to cover the first gas introduction hole and having permeability to water vapor and substantially no permeability to the measurement target gas,
• the second installation part having a second gas introduction hole formed to directly introduce the measurement target gas from the inside of the casing into the second inner space,
• the first and second gas introduction holes being respectively formed in the first and second installation parts at positions opposed to the opening of the casing,
• wherein the first and second installation parts are located at such positions that allow a maximum temperature difference of 0.4° C. or less between the first and second inner spaces during change in atmosphere temperature from 0° C. to 80° C., and
• a filter is arranged in the opening of the casing such that the measurement target gas introduced into the inside of the casing passes through the filter, the filter having no permeability to liquid water.