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This paper describes a microprocessor-based temperature monitoring instrumentation system based on phase modulation principle using interferometric optical sensors system, where the phase of a beam through a temperature sensing fiber (placed in a hot chamber) is compared with that of a beam from a reference fiber placed out side the hot chamber. The sensor system consists of a He-Ne laser source, two beam splitters, two single-mode optical fibers, and two light dependent resistor (LDR)-based timer circuits as detectors. The frequencies of the timer circuits change according to the change in temperature of the hot chamber. A 8085 microprocessor-based system is used to sample the frequencies of the timer circuits.