Process control instrumentation is the hardware that lets a control system see and act on a process: sensors that measure temperature, pressure, flow, level and composition; transmitters that convert those measurements into standard signals, most often 4–20 mA; and final control elements, mainly control valves, that change the process. A control loop can only be as good as its instruments, so selecting, installing and calibrating them is half of process control engineering.
This article supports our guide to industrial process control, which shows where instrumentation fits in a complete control system.
Instrument signals reach the control software through a data acquisition system, which sets the accuracy and sample rate the loop can actually use.
Sensors by process variable
| Variable | Common sensors | Notes |
|---|---|---|
| Temperature | Thermocouples, RTDs (Pt100), thermistors | RTDs are more accurate at moderate temperatures; thermocouples cover higher ranges and respond faster |
| Pressure | Gauge, absolute and differential pressure transmitters | Differential pressure also measures flow and level |
| Flow | Magnetic, Coriolis, vortex, differential pressure, thermal mass, ultrasonic | Coriolis measures mass flow and density directly; magnetic needs a conductive liquid |
| Level | Differential pressure, radar, ultrasonic, guided-wave radar, load cells | Load cells give mass directly, common on reactors and pilot plant vessels |
| Composition | pH, conductivity, dissolved oxygen, gas analyzers, online spectroscopy | Analyzers often add sampling dead time that limits control speed |
Temperature measurement has its own pitfalls; see thermocouple data acquisition and signal conditioning.
Transmitters and the 4–20 mA signal
A 4–20 mA loop represents the measurement range as a current: 4 mA is the bottom of the range and 20 mA the top. Current signals ignore wire resistance over long runs and resist electrical noise, which is why they are still the most common signal in process plants.
Worked example: scaling a 4–20 mA signal
Range. A pressure transmitter is ranged 0–10 bar, so 16 mA of span covers 10 bar.
Reading. The input reads 12.8 mA. Pressure = (12.8 − 4) ÷ 16 × 10 = 8.8 ÷ 16 × 10 = 5.5 bar.
Fault detection. Because the live zero is 4 mA, a broken wire reads 0 mA, which cannot be a real value. NAMUR NE 43 recommends treating signals below 3.6 mA or above 21 mA as a fault, so the control system can alarm and take safe action instead of controlling on a bad reading.
HART and digital fieldbuses
HART adds a digital signal on top of the 4–20 mA current, carrying diagnostics, range and device information without new wiring. Fully digital networks such as PROFIBUS PA, FOUNDATION Fieldbus, and Ethernet-based protocols carry many devices on one cable with richer diagnostics, at the cost of more complex engineering.
Control valves and other final elements
Control valves do most of the work in process plants. Getting them right matters more than most tuning:
- Sizing: a valve should work mostly between about 20% and 80% open at normal flows. An oversized valve controls poorly near its seat.
- Characteristic: equal-percentage trims suit most pressure-drop-limited flow loops; linear trims suit applications where the valve takes most of the pressure drop.
- Fail position: decide whether the valve should fail open, closed or in place on loss of air or signal, based on what is safe for the process.
- Positioner: a positioner makes the valve follow the signal accurately and overcomes friction.
- Stiction: sticky valves cause loops to cycle no matter how they are tuned.
Other final elements include SCR power controllers for electric heaters, variable frequency drives for pumps and fans, dosing pumps, and mass flow controllers on gas feeds in pilot plants.
Instrument tags: reading a P&ID
ISA-5.1 tags identify each instrument by what it measures and what it does. The first letter is the measured variable; the following letters are functions.
| Tag | Meaning |
|---|---|
| TT-101 | Temperature transmitter, loop 101 |
| TIC-101 | Temperature indicating controller for that loop |
| FT / FIC | Flow transmitter / flow indicating controller |
| PT / PIC | Pressure transmitter / pressure indicating controller |
| LT / LIC | Level transmitter / level indicating controller |
| FV-203 | Flow valve, the final element of loop 203 |
| TSHH / PSHH | Temperature or pressure switch high-high, usually a trip |
Accuracy, calibration and installation
- Specify accuracy for the whole loop: sensor, transmitter, input card and any wiring errors add up.
- Calibrate on a schedule based on criticality and drift history, and record as-found and as-left values.
- Install sensors where they see the real process: a thermowell in a dead leg, or a pressure tap in turbulent flow, will mislead the controller.
- Keep the safety instruments for trips separate from the control instruments where the hazard requires it.
Specifying instruments for a new system?
Sciotex specifies instrumentation and integrates sensors, transmitters, analyzers and valves into complete process control systems for pilot and production plants, supported in the US since 1997.
Frequently asked questions
It is the set of sensors, transmitters and final control elements, such as control valves, that measure process variables and act on the process under the control system’s direction.
The 4 mA live zero lets the system tell the difference between a real zero reading and a broken wire or failed transmitter, which reads near 0 mA.
A sensor responds to the process variable, for example an RTD whose resistance changes with temperature. A transmitter converts the sensor output into a standard signal such as 4–20 mA for the control system.
TIC stands for temperature indicating controller: a controller that displays and controls a temperature loop.
A control valve should normally operate between about 20% and 80% open across its expected flows. Oversized valves are a common cause of poor control.
