May/June 2013
Certification Review

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

Which of the following features is unique to a pneumatic differential pressure sensor/transmitter?

a.  high pressure inlet

b.  d/p cell diaphragm

c.  low pressure inlet

d.  feedback bellows

CCST answer

The correct answer is D, feedback bellows. In a pneumatic transmitter, the nozzle faces a baffle, so, as the measured pressure increases, the baffle moves toward the nozzle. This causes the backpressure within the nozzle to rise. The rising pressure is amplified by a pneumatic relay, with the output pressure applied to both the feedback bellows and to the sensor output port.

As the bellows expands, it draws the nozzle away from the baffle. There is a balance that is achieved by matching the motion of the baffle with the motion of the nozzle. This balance keeps the distance between the baffle and nozzle, and, hence, the output pressure, constant for a constant input pressure. Only pneumatic differential pressure transmitters have this feature. There are other classes of devices, such as electro-pneumatic relays (I/P transducers) that also use this principle.


Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.