Instrument Technician Basic Interview Notes
🔧 INSTRUMENT TECHNICIAN COMPLETE INTERVIEW GUIDE
---
1. INSTRUMENT TECHNICIAN BASICS
1.1 What is an Instrument Technician?
An Instrument Technician installs, calibrates, maintains, troubleshoots, tests, and repairs field instruments.
Instruments include: Transmitters, valves, switches, analyzers, sensors, control systems.
Easy Memory: Install + Calibrate + Troubleshoot + Maintain
---
1.2 Main Responsibilities
Installation of instruments
Loop checking
Calibration
Maintenance
Troubleshooting
Commissioning support
---
1.3 What is a Transmitter?
A transmitter converts process variable into electrical signal (4–20 mA).
Example: Pressure → 4–20 mA
---
1.4 Types of Transmitters
Pressure transmitter
Differential pressure transmitter
Temperature transmitter
Level transmitter
Flow transmitter
---
1.5 Why 4–20 mA Signal?
4 mA = Live zero
20 mA = Full scale
0 mA = wire break indication
---
1.6 Basic Formula
Span = URV − LRV
---
2. LOOP CHECKING
2.1 What is Loop Checking?
Loop checking is verification of complete signal path from field to control room.
Easy Flow: Field → JB → Marshalling → DCS/PLC → Field
---
2.2 Loop Checking Steps
1. Check tag number
2. Check wiring
3. Apply signal
4. Verify DCS reading
5. Check alarms
6. Check interlocks
7. Record results
---
2.3 Loop Check Documents
P&ID
Loop diagram
Hook-up drawing
Instrument index
Cable schedule
Data sheet
Cause & Effect
Logic diagram
I/O list
Calibration certificate
---
2.4 Loop Check Tools
Multimeter
Loop calibrator
HART communicator
Megger
Pressure pump
24V DC supply
---
2.5 Loop Check Standard Values
Pressure % Output mA
0% 4 mA 25% 8 mA 50% 12 mA 75% 16 mA 100% 20 mA
---
2.6 What is Marshalling Panel?
It is a termination panel where field cables are organized before going to DCS.
---
2.7 What is Junction Box?
A junction box connects field cables and distributes signals to control room.
---
3. CONTINUITY TEST
3.1 What is Continuity Test?
Checks whether electrical path is complete.
---
3.2 Why Continuity Test is Done?
To verify wiring
To detect open circuit
To confirm cable cores
---
3.3 Tools Used
Multimeter
Cable tester
Buzzer tester
---
3.4 Continuity Reading
Condition Value
Good 0–2 Ω Open OL
---
3.5 What does OL mean?
OL = Open Line → No continuity
---
4. MEGGER TEST
4.1 What is Meggering?
Testing insulation resistance using Megger.
---
4.2 Why Megger Test is Done?
Check insulation health
Detect leakage
Identify cable damage
---
4.3 Megger Voltage
Instrument cable: 500V DC
Power cable: 1000V+
---
4.4 Megger Reading
Value Condition
<1 MΩ Bad 5–50 MΩ OK >100 MΩ Excellent
---
4.5 Important Safety Step
✔ LOTO required ✔ Remove sensitive devices
---
4.6 Difference: Continuity vs Megger
Test Checks Unit
Continuity Wire path Ω Megger Insulation MΩ
---
4.7 Can Megger be done on live circuit?
❌ No, never
---
5. CONTROL VALVES
5.1 What is Control Valve?
A valve that controls flow, pressure, level, or temperature.
---
5.2 Parts of Control Valve
Body
Plug
Seat
Stem
Actuator
Positioner
I/P converter
---
5.3 Control Valve Signal
mA Position
4 0% 8 25% 12 50% 16 75% 20 100%
---
5.4 Valve Fail Actions
FO = Fail Open
FC = Fail Close
FL = Fail Last
---
5.5 What is Valve Stroking?
Movement of valve from 0% to 100% to verify smooth operation.
---
6. ON/OFF VALVES
6.1 What is On/Off Valve?
Only two positions: Open or Close
---
6.2 Types
Ball valve
Gate valve
Butterfly valve
Solenoid valve
---
6.3 Solenoid Valve Voltage
24 VDC
110 VAC
220 VAC
---
6.4 Limit Switch Function
Shows valve OPEN or CLOSE status to DCS/PLC.
---
6.5 What is ESD Valve?
Emergency Shutdown Valve used for plant safety shutdown.
---
7. PRESSURE & TEMPERATURE LOOP CHECK
7.1 Pressure Transmitter Example
0 bar → 4 mA 2.5 bar → 8 mA 5 bar → 12 mA 7.5 bar → 16 mA 10 bar → 20 mA
---
7.2 Temperature Loop Example
0°C → 4 mA 25°C → 8 mA 50°C → 12 mA 75°C → 16 mA 100°C → 20 mA
---
8. TROUBLESHOOTING
8.1 If No Output in DCS
Check:
Power supply
Wiring
Fuse
AI card
Transmitter
---
8.2 Common Faults
Wrong polarity
Loose connection
Wrong scaling
Sensor failure
---
8.3 What is 22 mA output?
It indicates transmitter fault or over-range condition.
---
9. PRE-COMMISSIONING & COMMISSIONING
9.1 Pre-Commissioning
Continuity
Megger
Calibration
Loop check
---
9.2 Commissioning
Live testing of system under operating conditions.
---
10. MEASURING & CALIBRATION
10.1 What is Calibration?
Comparison of instrument with standard reference.
---
10.2 Steps
Isolate instrument
Apply standard signal
Check output
Adjust if needed
---
10.3 HART Communication
Used for:
Configuration
Calibration
Diagnostics
---
11. TUBE FITTING
11.1 What is Tube Fitting?
Connects tubing without leakage.
---
11.2 Types
Double ferrule (most common)
Union
Elbow
Tee
Reducer
---
11.3 Tube Sizes
1/4 inch
3/8 inch
6 mm
8 mm
10 mm
---
11.4 Missing Question
What is Impulse Line?
Small tube connecting process line to transmitter.
---
12. BASIC FIELD TOOLS
Multimeter
Megger
Loop calibrator
Pressure pump
HART communicator
Tube cutter
Tube bender
---
13. FINAL SHORT MEMORY FORMULAS
✔ Continuity = LOW Ω = GOOD ✔ Megger = HIGH MΩ = GOOD ✔ Loop = 4–20 mA SIGNAL ✔ Valve = 0–100% POSITION ✔ Transmitter = PROCESS → SIGNAL