Electrochemical sensor having improved stability

Claim 1

An electrochemical sensing device comprising an electrolyte container, an aqueous electrolyte in said container, a hollow cylindrical hydrophobic membrane forming a portion of said container and having a first outer surface accessible to a fluid to be monitored and a second inner surface accessible to said electrolyte, a cathode working electrode, a reference electrode and a counter electrode of cylindrical shape and open structure disposed adjacent the inner surface of said membrane and having its entire inner surface area exposed to and contacting said electrolyte in said container, said membrane being permeable to a component to be determined of said fluid, said counter electrode being coaxial with the interior of said membrane between said membrane and said working electrode and being constructed to allow free diffusion through it radially into said electrolyte and to said working electrode of said component to be determined, said working electrode being a rod extending along the axis of said cylindrical hydrophobic membrane and counter electrode along the entire length of said counter electrode and having all portions of it's surface in contact with said electrolyte and at a constant spacing from said counter electrode along the length of said cylindrical hydrophobic membrane to provide a substantially uniform current flux through the electrolyte between all points of said working electrode and opposed points of said counter electrode.