Clarifying the transmitter self-diagnosis function in the instrument selection specifications

When formulating specifications for instrument selection and transmitter self-diagnosis functions, it is necessary to take into account process safety, equipment reliability, and the impact on production. The self-diagnosis function on the transmitter can detect abnormal conditions of the device and handle fault signals according to pre-set logic. Depending on the different application scenarios and safety requirements, there are various ways of handling the state of the output signal in the event of a failure: 1. Fail-Safe operation: In situations where process safety is of utmost importance, the transmitter can be set to put the output signal into a safe state once a failure is detected. This usually means that the output will be set to 4 mA or 20 mA (for 4-20 mA signal systems), representing the minimum or maximum safe operating limit of the device. This ensures that the system can shut down safely or remain in a safe state when it fails to receive signals properly. 2. Maintain existing state (Fail-Freeze): For some applications, it may be preferable to maintain the current output state upon detecting a fault, until manual intervention or other external conditions are met. This setup is particularly suitable for situations where production must continue even in the event of a failure, but it may pose safety risks, so careful consideration is required. 3. Alarm signal: In some cases, the transmitter may send an additional alarm signal to indicate a fault condition; this signal can be a diagnostic warning within digital communication protocols such as HART, Fieldbus, or PROFIBUS. When specifying the self-diagnosis functions of the transmitter, the following points should be detailed: - Fault detection types: The types of faults that need to be monitored, such as sensor disconnection, circuit failures, communication errors, etc. - Fault response: The expected behavior of the transmitter’s output signal in the event of a fault, including safety shutdown, maintaining the current status, or sending an alarm signal as mentioned above. - Technical parameters: include the sensitivity of fault detection, response time, recovery strategies, etc. - Operation and maintenance requirements: Specify the procedures to be followed and the maintenance actions to take in the event of a failure, so as to resolve it promptly or carry out necessary maintenance. When formulating standards, it is necessary to fully consult the opinions of control engineers, safety engineers, and production operators to ensure that the self-diagnosis function of transmitters meets actual needs while taking into account both safety and production efficiency. Furthermore, in existing engineering projects, standard practices for such settings may already exist; more detailed information can be obtained by reviewing the technical documents of similar projects or by consulting experienced engineers. .

This is a regulatory requirement, and most manufacturers of SIS and GDS systems include this function in their designs. In the channel, it is possible to set the maximum or minimum value for the fault output, or other parameters, to ensure that the fault output does not cause interlocks or false alarms. This requires the process to indicate whether the fault output is large or small, but it is used very rarely.

The self-diagnosis feature of transmitters is nothing new; the image below shows product samples from EJA and Rosemount from 30 years ago.

They all answered the questions earlier; I’ll add more details later. As required, this fault mode should be specified in the process documentation or in the SRS. For circuits with a SIL1 rating, the fault handling should be such that the system does not shut down; for SIL2/3 circuits, it is recommended that the system shut down in case of a fault. The specific approach depends on the impact and losses caused by faults in those circuits on the process