Oxygen sensor

An oxygen sensor comprising:

• a solid electrolyte;
• a first active electrode provided on one of a pair of main surfaces of said solid electrolyte and having gas permeability;
• enclosing means for forming a reference chamber, said reference chamber being airtight to be isolated from a surrounding space, said reference chamber including therein at least a portion of said first electrode;
• a second active electrode and a third active electrode each provided on the other main surface of said solid electrolyte at a place opposite that portion of said first electrode which is included in said reference chamber, each of said second and third electrodes having gas permeability;
• a current path formed between said first electrode and said third electrode for continually supplying oxygen to said reference chamber in such a manner that oxygen in said surrounding space is ionized and then passes through said solid electrolyte;
• power supply means for continually supplying a current between said first electrode and said second electrode to continually pump out oxygen from said reference chamber in such a manner that oxygen in said reference chamber is ionized and then passes through said solid electrolyte, said power supply means and said current path being connected to the electrodes such that oxygen supplied from said surrounding space to said chamber is continually pumped out from said chamber, simultaneously with the supply of oxygen from the surrounding space, to maintain the oxygen partial pressure to be always far smaller in the chamber than the oxygen partial pressure in a space existing on the other main surface side of said solid electrolyte;
• and
• means for detecting the oxygen concentration in the space existing on the other main surface side of said solid electrolyte, on the basis of said current supplied by said power supply means.