At normal operating, the instrument sends a constant signal of, e.g., 5 ma (or 7 Hz). If the temperature increases above the normal operating temperature, e.g., a 19 ma (or 14 Hz) signal is sent. The recognition of the danger occurs in the instrument, not in the control system. Furthermore, a warning temperature (e.g., 40 °C) with e.g., 12 ma (or 10 Hz) and a maximum temperature (e.g., 60 °C) with e.g., 17 mA (or 13 Hz) will be transmitted. These values are variable and can be set beforehand.
To ensure the safety switch, the instrument occurs a self-test which occurs discretely by restarting the instrument, by a set time interval for an alternating signal of e.g., 9 ma (or 9Hz) and 15 ma (or 12 Hz) transmitted. This self-test occurs at every restart and can be triggered manually on site or externally.
Moreover, a further variant of monitoring and transmitting is possible. In normal operating the instrument sends a “1” signal. If the dangerous temperature rise for the system is detected, the instrument sends a “0” signal.
On demand, the self-test can be done out from outside by a magnetic switch or the measuring instrument can be acknowledged if the limit value has been exceeded.