Sensors

Process sensors measure the four key variables — temperature, pressure, flow and level — plus composition. Learn thermocouples and RTDs, pressure transmitters, differential-pressure, magnetic, vortex and Coriolis flowmeters, level measurement methods, analysers, and accuracy, range and installation.

You can't control what you can't measure. A refinery has thousands of instruments measuring temperatures, pressures, flows and levels, plus analysers for composition. Each choice depends on the fluid, range, accuracy, cost and hazards.

Good installation matters as much as the sensor itself — a perfect flowmeter in the wrong place reads wrong.

An analogy

Sensors are the plant's senses: thermometers are its skin, pressure gauges its pulse, flowmeters its blood-flow monitors, analysers its taste buds.

Temperature and pressure

Thermocouples (two dissimilar metals) are rugged and cover wide ranges up to 1200 °C+; RTDs (platinum resistance) are more accurate below ≈ 600 °C. Thermowells protect sensors. Pressure transmitters use strain-gauge or capacitive diaphragms; gauge, absolute and differential types exist.

Flow measurement

Differential-pressure (orifice, venturi) — cheap, square-root, limited turndown (≈ 4:1). Magnetic — conductive liquids, no obstruction. Vortex — steam and gases. Turbine — clean liquids. Ultrasonic — clamp-on options. Coriolis — direct mass flow and density, highest accuracy (≈ 0.1 %) but costly.

Level measurement

DP (hydrostatic) transmitters, displacers, radar (non-contact, most popular), guided-wave radar, ultrasonic, nuclear (difficult services), float switches and sight glasses. Density changes and foam affect some methods.

Accuracy, selection and analysers

Specify accuracy, repeatability, turndown, response time, materials, hazardous-area certification (ATEX/IECEx) and SIL capability. Analysers (gas chromatographs, pH, conductivity, O₂, NIR) give composition for quality control. Smart transmitters (HART, Foundation Fieldbus, PROFIBUS PA) provide diagnostics.

Formulas

• Pt100 RTD: R ≈ 100 (1 + 0.00385 T) Ω
• DP flowmeter: Q = K √ΔP (square-root relationship)
• Hydrostatic level: h = ΔP / (ρ g)
• 4–20 mA scaling: PV = LRV + (I − 4)/16 × (URV − LRV)

Worked example: Signal conversions

Given: Type K thermocouple at 400 °C (cold junction 0 °C); Pt100 at 150 °C

Find: Signals.

• T/C ≈ 41 µV/°C × 400 = 16.4 mV
• RTD R = 100 × (1 + 0.00385 × 150) = 157.8 Ω

Answer: ≈ 16.4 mV and ≈ 157.8 Ω

Worked example: Level from DP

Given: DP transmitter reads 30 kPa; Water ρ = 1000 kg/m³

Find: Level.

• h = 30 000/(1000 × 9.81) = 3.06 m

Answer: 3.06 m

Common mistakes

• Wrong: Installing flowmeters right after bends or valves. — Right: Provide straight-run lengths or flow conditioners.
• Wrong: Using DP level without density compensation. — Right: Level = ΔP/ρg — changing density changes reading.

Applications

• Custody transfer: Coriolis and ultrasonic meters for billing oil and gas.
• Reactor & column control: Temperature profiles, pressures and levels drive control loops.
• Safety systems: Independent SIL-rated sensors for trips.

Key takeaways

• Temperature, pressure, flow, level + analysers.
• DP flow ∝ √ΔP; level = ΔP/ρg.
• 4–20 mA: PV = LRV + (I − 4)/16 × span.
• Select for fluid, range, accuracy and hazards.

Going further

• Smart diagnostics: HART/fieldbus devices report health, plugged impulse lines and drift.
• Soft sensors: Models infer hard-to-measure qualities from routine measurements.
• Wireless instrumentation: WirelessHART/ISA100 for monitoring points.