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Fail-safe instrument system

  • US 4,573,040 A
  • Filed: 02/11/1985
  • Issued: 02/25/1986
  • Est. Priority Date: 11/30/1979
  • Status: Expired due to Term
First Claim
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1. An admittance-responsive two-wire instrument system comprising a power supply and a receiver in one location, an active transmitter and an admittance-sensitive probe in another location, said probe being operatively connected to said transmitter, and a pair of signal wires connecting the transmitter with the receiver to form a loop, wherein the transmitter comprises means responsive to the admittance sensed by said probe for controlling the current in the loop independent of the resistance of the loop, said means for controlling being adapted to control the current to take values falling within at least one first predetermined current range within a second larger predetermined current range, said second current range not including the short and open circuit currents in said loop, and wherein the receiver comprises means for distinguishing between currents in the loop falling within the at least one first range, currents within the second larger current range but outside of the at least one first range, and the short and open circuit currents in said loop.

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