The post AutoQuiz: How to Perform PLC Raw Count Calculations for Pressure Transmitters first appeared on the ISA Interchange blog site.
This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
a) 2048 counts
b) 4096 counts
c) 3277 counts
d) 6144 counts
e) none of the above
So now all you need 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.)
The correct answer is C, 3277 counts. First, you 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 you divide (8192-0) counts (full range of counts for the analog channel) by the range of corresponding current in milliamps (20-0), you will get 409.6 counts/mA.
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition
Source: ISA News