RRB ALP Instrument Mechanic Trade — Solved Practice Question Bank

Q: Why does the standard industrial current signal use a range of 4-20mA rather than 0-20mA? Sol: The "live zero" of 4mA allows a broken wire or power failure (which would read 0mA) to be distinguished from a genuine zero-value process reading — improving fault detection. Q: Which temperature sensor generates its own small voltage without external power, using two dissimilar metals? Sol: A thermocouple. Q: What does a Bourdon tube pressure gauge use to indicate pressure? Sol: A curved tube that straightens (deflects) proportionally under internal pressure, mechanically driving a pointer via a linkage.

Q: What device converts a standard 4-20mA electronic signal into a 3-15 psi pneumatic signal? Sol: An I/P (Current-to-Pneumatic) converter. Q: Why is pneumatic instrumentation still used in hazardous, explosive-atmosphere areas? Sol: Pneumatic devices use compressed air rather than electricity, carrying no electrical spark risk that could ignite a flammable atmosphere. Q: What are the two basic adjustments made during instrument calibration? Sol: Zero adjustment (shifts the reading uniformly) and Span adjustment (corrects the output's rate of change per unit input) — both are needed to calibrate accurately across the full measurement range.