Coaxial differential PH system

Claim 1

A circuit for measuring the pH of a fluid comprising: a pH sensing probe consisting of a pH sensitive glass chamber, a conductive fluid contained in said glass chamber, and a pH conductor extending from a point in contact with said conductive fluid inside said glass chamber to the outside thereof, whereby contact between said fluid and said pH sensitive glass chamber produces a pH signal on said pH conductor; a reference probe surrounding said pH sensing probe and coaxial and symmetrical thereto, said reference probe consisting of a cylindrical tube aligned coaxial with said pH sensing probe; first and second gaskets spaced apart and extending between said tube and said glass chamber, whereby a duct surrounding a part of said glass chamber is formed by said gaskets, said tube and said glass chamber; a second conductive fluid contained in said duct; and a reference conductor extending from a point in contact with said second conductive fluid inside said duct to the outside thereof, whereby contact between said fluid and said pH sensitive glass chamber produces a reference signal on said reference conductor; a differential amplifier having first and second input terminals and an output terminal, said first input terminal being connected to said pH conductor to receive said pH signal, said second input terminal being connected to said reference conductor to receive said reference signal, whereby said amplifier produces at said output terminal a voltage indicative of said fluid pH; a cable shield surrounding said pH conductor and extending between said pH sensing probe and said amplifier, said cable shield being electrically connected to said amplifier output terminal whereby said cable shield has applied thereto a voltage equal in magnitude to the output voltage of said amplifier; and indicating means connected to receive said amplifier output for indicating the magnitude of said pH signal.