March/April 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

When looking at the raw input value of a single channel on an analog input card in the PLC, Karen sees that the raw input is expressed in "counts" and is an integer value that varies between 0 and 8092. She learned from the PLC manual that 0 counts occur when the input signal is 0 mA, and 8192 counts is obtained for an input signal of 20mA. If a 4-20mA pressure transmitter is connected to this analog input channel, and if the pressure sensor has a range of 0-100psig, approximately how many counts should Karen see for a sensed pressure of 25psig?

A. 2048 counts    

B. 4096 counts

C. 3277 counts        

D. 6144 counts

CCST answer

The correct answer is C, 3277 counts. First, Karen must realize that the counts at the analog input card are based on 0-20mA range and the transmitter is based on a 4-20mA range. If she divides (8192-0) counts (full range of counts for the analog channel) by the range of corresponding current in milliamps (20-0), she will get 409.6 counts/mA.

So now all Karen needs to do is determine how many milliamps are output by the transmitter. This will be [(25psig/100psig)*(20mA-4mA)] + 4mA = 8mA.
8mA x (409.6 counts/mA) = 3277 counts (approx.)

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