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Model PT295 is a microprocessor-based sensor for toxic gas detection in parts per million (ppm) level. The unit features easy one-man calibration initiated by simply activating a magnetic switch and applying the gas. The complete sensor consists of a stainless steel sensor head assembly, a user connection board and a transmitter board assembly. All of the sensor electronics are enclosed in an explosion proof instrument box. The unit's modular design allows easy maintenance, troubleshooting, and high reliability. The unit provides a fully programmable alarm relay and a linear 4-20mA signal proportinoal to 0 to 100% of the detection range of the sensor. The sensor transmitter board includes a three (3) digit LED display for continuous indication of the gas concentration. The calibration mode, alarm and fault status of the sensor are also indicated on the digital display. In the normal operating mode the digital display and 4-20mA reading is proportional to the gas concentration detected by the sensor. In the calibration mode the digital display indicates the calibration status, alarm relay output is inhibited and analog signal output is set to 2 mA to avoid any false alarm by the control monitor. In the sensor fault mode the transmitter drops the output signal to < 1 mA.