Measurement and Instrumentation
What to master
Measurement begins with defining the process variable, then selecting a sensor and transmitter whose range, span, and zero match the application. Know accuracy versus precision, repeatability, hysteresis, drift, turndown, and rangeability. Understand the 4–20 mA current loop with its live zero, and how HART adds digital diagnostics without disturbing the analog signal. Match the sensor to the job: RTDs are stable and nearly linear but need lead-wire compensation; thermocouples need cold-junction compensation; DP flow is square-root related to flow; DP level in closed vessels needs zero elevation or suppression; radar and ultrasonic level need vapor and foam considerations. Installation effects matter: impulse-line slope and fill, condensing pots, straight run, grounding and shielding, and static-pressure effects on DP transmitters.
Traps
Accuracy is often stated as ±% of span, not % of reading, so low readings carry larger relative error. Forgetting square-root extraction is common: 25% DP is 50% flow, not 25%. Three-wire RTDs compensate for lead resistance; two-wire installations do not. Thermocouple readings without cold-junction compensation are wrong. Closed-tank level transmitters may be calibrated with elevation or suppression, so a zero-inch reading does not mean an empty tank.
Worked example
A DP flow transmitter is ranged 0–100 inH2O for 0–200 gpm and reads 25 inH2O. DP fraction is 25/100 = 0.25; flow fraction is sqrt(0.25) = 0.50; flow is 0.50 × 200 = 100 gpm.