Constant current loop impedance measuring system that is immune to the effects of parasitic impedances

A constant current loop measuring system for measuring a characteristic of an environment comprising:

• a first impedance, positionable in the environment and having a variable impedance that varies in accordance with the characteristic of the environment;
• a second impedance coupled in series with said first impedance;
• a parasitic impedance electrically coupled to said first and second impedances;
• current means, electrically coupled in series with said first and second impedances, for providing a constant current through said first and second impedances to produce first and second voltages across said first and second impedances, respectively, and a parasitic voltage across said parasitic impedance;
• voltage measuring means for measuring a voltage difference between said first and second voltages independent of said parasitic voltage to produce a characteristic voltage representative of the characteristic of the environment, said voltage measuring means comprising:
• first and second terminals electrically coupled to said first impedance;
• and
• third and fourth terminals electrically coupled to said second impedance;
• calibration means for calibrating said characteristic voltage in accordance with said variable impedance independently of said first, second and parasitic impedances, said calibration means comprising:
• a calibration impedance;
• and
• first switching means for optionally electrically coupling said calibration impedance in parallel with said second impedance;
• first detecting means for detecting at least one of common mode voltages and induced voltages present in the constant current loop measuring system and an operating error of the voltage measuring means, said first detecting means comprising second switching means for electrically coupling said first terminal to said second terminal and said third terminal to said fourth terminal;
• and
• second detecting means for detecting an operating error in said voltage measuring means, said second detecting means comprising third switching means for electrically coupling said first terminal to said third terminal and said second terminal to said fourth terminal.

A constant current loop measuring system for measuring a characteristic of an environment, comprising:

• a constant current loop including
• a constant current source,
• a first impedance operatively coupled to said constant current source,
• a second impedance operatively coupled in series with said first impedance,
• parasitic impedance electrically coupled to said first and second impedances;
• first voltage sensor circuit operatively coupled across said first impedance;
• second voltage sensing circuit operatively connected across said second impedance;
• and
• a voltage subtracter operatively coupled to said first voltage sensing circuit and said second voltage sensing circuit.