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.
When looking at the raw input value of a single channel on an analog input card in a PLC, you see that the raw input is expressed in “counts” and is an integer value that varies between 0 and 8092. You 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 you see for a sensed pressure of 25psig?
a) 2048 counts
b) 4096 counts
c) 3277 counts
d) 6144 counts
e) none of the above
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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