From Wiring and Calibration to Control Systems and Explosion Protection
Anyone who works in field instrumentation knows the pain points:
Wrong RTD wiring causing crazy temperature readings…
Pressure transmitters damaged because someone forgot to depressurize…
Control valves selected in the wrong fail-safe action…
Or the constant pressure of working in hazardous zones…
No worries—this guide summarizes five core knowledge blocks every instrumentation technician should master.
Whether you’re new to the field or a seasoned engineer, these are worth bookmarking.
1️⃣ Fundamentals – Build the Right Knowledge Foundation
Before troubleshooting or calibration, a solid grounding in instrumentation basics is essential.
1. Measurement & Measurement Error
Every instrument compares the measured variable to a standard.
The difference between the reading and the actual value = measurement error.
No instrument is absolutely accurate—only more or less precise.
2. Direct vs. Indirect Measurement
✔ Direct measurement: reading temperature with a thermometer
✔ Indirect measurement: using differential pressure to calculate liquid level
Most industrial parameters are measured indirectly.
3. Understanding Accuracy
Lower accuracy numbers mean better performance.
e.g., 0.06% is more accurate than 0.075%.
Accuracy grade defines the allowable maximum error and is critical for selection.
4. Pressure Units Simplified
Most plants work with:
Gauge pressure (barg/psig) – pressure above atmospheric
Absolute pressure (bara/psia) – includes atmospheric pressure
Knowing which one is required avoids costly mistakes.
5. The Three Instrumentation Effects
Most pressure/force sensors use one of these principles:
Piezoelectric
Magnetostrictive
Piezoresistive
In simple terms: under pressure, something generates electricity, changes magnetism, or changes resistance.
2️⃣ Field Skills – Get the Job Done Right and Safe
Correct wiring, commissioning and shutdown procedures prevent failures and protect equipment.
1. RTDs (Resistance Temperature Detectors)
Use three-wire connection to compensate for lead resistance.
Two-wire RTDs introduce significant reading error.
Always identify the common lead correctly.
2. Thermocouples
Two-wire connections are fine.
Check wire resistance—high resistance = oxidation or loose joints, fix immediately.
3. Pressure Transmitters – Three Critical Do’s & Don’ts
Do wire polarity correctly
Do open instrument valves slowly—pressure surges rupture diaphragms
Do isolate and depressurize fully before removal
Residual pressure can damage equipment—or injure people.
4. Differential Pressure Level Measurement
Correct start-up sequence matters:
Open equalizing valve
Close high/low pressure block valves
Open impulse lines
Slowly open block valves
Close equalizing valve
One wrong step produces inaccurate levels or damages the transmitter.
3️⃣ Control Systems – Beyond Field Devices
Modern plants rely on automation; instrumentation connects directly to control logic.
1. Four Building Blocks of a Control Loop
Transmitter → Controller → Actuator → Control Valve
2. Feedback Matters
Negative feedback = stable control (used in industry)
Positive feedback = runaway behavior (generally avoided)
3. PID Control Explained
P – Proportional: fast response but leaves steady-state error
I – Integral: removes residual error but reacts slowly
D – Derivative: anticipates trends, prevents overshoot
Most loops run PI or PID, rarely D-only.
4️⃣ Actuators & Control Valves – Make the System React Correctly
1. Pneumatic vs Electric Actuators
Pneumatic
Safe in hazardous areas
Strong torque
Cost-effective
(Use electro-pneumatic positioner to reduce lag)
Electric
Precise, fast response
Long-distance control
Requires explosion-proof rating when applicable
2. Fail Action: Air-to-Open vs Air-to-Close
Always choose based on process safety:
Fuel supply → Fail Closed (air-to-open)
Feed or inlet valves → Fail Closed to avoid overflow
The rule: when control signal is lost, the plant must remain safe
3. Valve Positioners
Improve performance by:
Enhancing accuracy
Reducing hysteresis
Speeding actuator response
5️⃣ Hazardous Areas – Zero Tolerance for Mistakes
1. Two Common Protection Concepts
Explosion-proof (Ex d)
Contains internal ignition without igniting the external atmosphereIntrinsic Safety (Ex ia)
Limits energy so ignition cannot occur
Requires barrier/safety isolator between control room and field device
2. Safety Barriers
Act as a firewall:
Transmit signals while preventing excessive current or voltage from reaching the hazardous area.
3. Golden Rules for Hazardous Area Instrumentation
✔ Confirm equipment Ex rating
✔ Intrinsically safe devices must pair with barriers
✔ Ensure protective grounding
✔ Never open enclosures energized
✔ Follow cable specifications strictly
🔚 Final Thought
Instrumentation isn’t just wiring and numbers—it is:
Measurement science
Safety discipline
Equipment protection
Control theory
Real-world troubleshooting
Master these five pillars and you don’t just “install instruments”—
you keep processes safe, stable and running at peak reliability.