초록 ▼

An intrinsically safe system for monitoring the condition of materials includes a low power, stable frequency RF oscillator of the class C type comprising a resonant circuit which is coupled to a bridge network including the admittance of materials between a probe electrode and a grounded support me

An intrinsically safe system for monitoring the condition of materials includes a low power, stable frequency RF oscillator of the class C type comprising a resonant circuit which is coupled to a bridge network including the admittance of materials between a probe electrode and a grounded support member juxtaposed to the materials. The output of the network generates an AC error signal which is applied to a phase sensitive detector including a chopper and a low power chopper drive for generating a DC signal representing the magnitude of the AC signal at a predetermined phase angle. The bridge network which may be linearly calibrated is isolated from the oscillator and the output error signal circuitry so as to allow the oscillator and the output error signal circuitry to float with respect to the grounded support member and the power supply associated therewith. The rms voltage across the admittance representing the condition of materials is limited so as to permit the system to comprise a two-wire transmitter wherein the sole source of power for the transmitter is derived from a 4-20 milliamp current drawn by the transmitter.

대표청구항 ▼

In a two-wire transmitter system comprising a power supply and a load at one location and a two-wire transmitter at another location interconnected by a pair of transmission lines carrying a variable signaling current in a signaling range having a minimum current of substantially 4 milliamps and pow

In a two-wire transmitter system comprising a power supply and a load at one location and a two-wire transmitter at another location interconnected by a pair of transmission lines carrying a variable signaling current in a signaling range having a minimum current of substantially 4 milliamps and powered solely by said power supply, the improvement residing in said two-wire transmitter comprising: an admittance sensing probe including a probe electrode adapted to detect the admittance of materials for monitoring the condition of the materials; an RF signal generator; a bridge network coupled to said RF signal generator, said bridge network including the admittance detected by said probe such that the unbalance of the network corresponds to the condition of the materials being monitored, said RF signal generator applying a voltage of less than the √2V rms across the admittance detected where V is the voltage across the two-wire transmitter; and output means coupled to said bridge network for changing the current flow through the transmission lines in said signaling range in response to the unbalance of the bridge network so as to represent the condition of the materials.