Fieldbus Field Instruments (Part 2 of 2)

E-Zine May 2012

Click here to review Part 1

Some field transmitters can check alarms, such as

• Hi Span
• Hi-Hi Limit
• Hi Limit
• Low Limit
• Low-Low Limit
• Low Span
• Rate of Change Limit
• Setpoint Deviation (setpoint supplied by the loop controller)

Digital field instruments usually have the ability to perform diagnostic tests to determine if they are operating properly. A most common diagnostic is the determination of the internal temperature of the field instrument itself. If the instrument is operating outside its own temperature specifications, its accuracy cannot be assured and the instrument can be damaged. To perform this diagnostic, a temperature sensor is included inside the instrument with appropriate temperature measurement circuitry. Vibration can also be measured using an accelerometer within the instrument case. In these cases, when out-of-limits values are detected, the digital instrument can send a digital signal to the controller indicating that the quality of the measurement signal may be compromised. It is then a function of the control system to determine if the process variable is used for control purposes. FOUNDATION™ Fieldbus calls such values obtained from instruments with active diagnostic violations as "dubious," and allows the user to change control loop status based on this information.

Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control

ISSN 1538-5280