November/December 2010

ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

dec20

A pressure measurement has a range of 0-100 psig, a normal operating pressure of 50 psig, and a high alarm setpoint of 60 psig. There is some noise on the measurement as shown in the figure.

Which of the following would be the best choice for the alarm deadband setting on the high alarm?

         A.        10%

         B.         5%

         C.        2%

         D.        1%

CAP answer

The correct answer is C, 2%, as the noise is under 2%. 

Answer A, 10% is not the best choice as it is the difference between normal operation and the high alarm condition, so the high alarm will not clear until the pressure has dropped below 50 psig.

Answer B, 5% is not the best choice as it is more than twice the signal noise and a significant part of the range between normal operation and the high alarm condition.

Answer D, 1% is not the best choice as it is less than the signal noise and will likely create a chattering alarm.
Reference: Hollifield, Alarm Management: A Comprehensive Guide, ISA Press, 2010.