Process Measurement and Instrumentation
Process Measurement and Instrumentation tests how variables are sensed, transmitted, and calibrated. Expect pressure, temperature, level, flow, density, and position measurement; zero and span; accuracy; repeatability; installation effects; and transmitter calibration.
Core rules
- Define lower range value (LRV), upper range value (URV), span = URV − LRV, and error as a percentage of span or reading as specified.
- For a linear 4–20 mA transmitter, output = 4 mA + 16 mA × (PV − LRV)/span.
- A DP flow measurement is square-root related to flow: flow ratio = √(DP ratio). Apply square root in the transmitter or control system, not both.
- A 100 Ω platinum RTD is about 100 Ω at 0°C and rises with temperature; use three- or four-wire compensation. Thermocouples need correct extension wire and cold-junction compensation.
- Install pressure impulse lines with slope, vents, and drains; keep DP legs equal and free of gas or liquid as appropriate.
Worked example
A DP transmitter is calibrated 0–100 inH₂O for 0–500 gpm. At 25 inH₂O, the DP is 25% of span but flow is not 25%. Flow = 500 × √(25/100) = 500 × 0.5 = 250 gpm. The transmitter's linear mA at 25 inH₂O is 4 + 16 × 0.25 = 8 mA. If the display shows 125 gpm, someone has incorrectly treated the linear mA as flow. The correct display applies the square root: at 8 mA it should show 250 gpm. This distinction is a classic exam calculation.
Traps
Confusing % span with % flow, calibrating a DP flow transmitter with the square root turned on, ignoring static-pressure effects, using copper extension wire on a thermocouple, and failing to equalize DP legs before zeroing are common. Also check whether accuracy is ±% of span or ±% of reading.
Key points
- Know LRV, URV, span, and output formula.
- DP flow follows a square-root relationship.
- Use correct RTD/thermocouple wiring.
- Installation and impulse lines affect accuracy.
- Zero DP transmitters with legs equalized.