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Today's 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 information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.
A 4-20 mA electronic transmitter has an input range of 50-330 degrees F. If the output is 13.77 mA, what is the indicated temperature in degrees F?
a) 173
b) 221
c) 266
d) 302
e) none of the above
4-20 mA is an analog electrical transmission standard for industrial instrumentation and communication.
mA is a milliampere, or 1/1000 of an ampere. The signal is a current loop where four mA represents zero percent signal and 20 mA represents the 100 percent signal.
The reason zero is at four milliamps and not zero mA is this allows the receiving instrumentation to differentiate between a zero signal and a broken wire or a dead instrument.
This standard evolved in the 1950s and is still widely used in industry today, even though there were many attempts to replace it with digital forms of communication like fieldbus technology.
There is a straightforward linear relationship between the units of milliamps between four and 20 and the degrees between 50 and 330. One milliamp equals 17.5 degrees ((330-50) / (20-4)).
So, (13.77-4) (17.5) + 50 = 220.975. The best answer is B, 221.