1. What is a flow transmitter and how does it work?
A flow transmitter measures fluid flow and converts it into a standardized signal (4–20 mA).
Works on principles like differential pressure, magnetic induction, vibration or ultrasonic waves.
Provides accurate, continuous measurement for process control and monitoring.
2. What are the types of flow meters used in industries?
Differential Pressure (DP) type: Orifice, Venturi, Pitot tube.
Electromagnetic flow meters for conductive liquids.
Vortex flow meters for steam and gases.
Coriolis and thermal mass flow meters for mass flow.
Ultrasonic flow meters for clamp-on and inline measurements.
3. What is the principle of a differential pressure (DP) flow transmitter?
Based on Bernoulli’s principle: flow creates a pressure drop across a restriction.
Differential pressure is proportional to flow rate using formula:
Flow ∝ √(ΔP).
DP transmitter measures ΔP and outputs a flow signal.
4. What is an orifice plate and why is it used?
A thin plate with a precise hole installed in a pipeline.
Creates differential pressure proportional to flow rate.
Simple, low-cost and widely used for gas, liquid and steam applications.
5. What is the working principle of a magnetic flow meter?
Works on Faraday’s Law of Electromagnetic induction.
Conductive fluid passing through a magnetic field generates a voltage.
Voltage is proportional to flow velocity, ensuring accurate volumetric flow.
6. What are the limitations of a magnetic flow meter?
Only works with conductive fluids (typically > 5 µS/cm).
Not suitable for non-conductive oils, gases or steam.
Performance affected by air bubbles and empty pipe conditions.
7. How does a vortex flow meter work?
Uses the Kármán vortex street principle.
Fluid passing a bluff body creates vortices at a frequency proportional to flow velocity.
Suitable for steam, gases and clean liquids.
8. What is the principle of a Coriolis mass flow meter?
Uses vibrating tubes; mass flow causes Coriolis force, twisting the tubes.
Tube deflection is proportional to mass flow rate.
Measures mass flow, density and temperature simultaneously.
9. What is Reynolds number and why is it important in flow measurement?
Dimensionless number indicating flow regime:
Re = (ρ × V × D) ÷ μ
Determines whether flow is laminar or turbulent.
Affects flow profile, meter accuracy and sizing.
10. What causes zero drift in flow transmitters?
Temperature changes, sensor aging or mechanical stress.
Blocked impulse lines or unstable power supply.
Requires periodic calibration and impulse line maintenance.
11. What is flow coefficient (Cv) in flow systems?
Cv indicates how much fluid a valve or component can pass at a given pressure drop.
Higher Cv means higher flow capacity.
Used for valve sizing and flow calculations in liquid systems.
12. Why straight pipe length is required before and after flow meters?
Ensures a stable flow profile without turbulence.
Improves accuracy of DP, vortex and ultrasonic meters.
Typically 5D–10D upstream and 3D–5D downstream depending on meter type.
13. What is turndown ratio in flow transmitters?
Ratio of maximum measurable flow to minimum measurable flow.
Higher turndown means better range and flexibility.
Important for processes with variable flow conditions.
14. What is the difference between mass flow and volumetric flow?
Volumetric flow measures volume per time (m³/hr).
Mass flow measures mass per time (kg/hr).
Mass flow is independent of temperature and pressure changes.
15. What is the purpose of a flow averaging pitot tube?
Measures flow by averaging velocity across multiple points.
Suitable for large ducts or uneven flow profiles.
Lower installation cost compared to full-bore meters.
16. What is K-factor in flow meters?
Number of pulses generated per unit volume of fluid.
Used in turbine and positive displacement meters.
Essential for accurate totalization and calibration.
17. Why calibration is required for flow transmitters?
Ensures measurement accuracy and compensates for drift.
Detects sensor wear, clogging or electronics deviation
Maintains compliance with process and regulatory standards.
18. What are common calibration methods for flow transmitters?
Master meter comparison for liquids.
Gravimetric or volumetric calibration in test rigs.
Dry calibration for DP transmitters using pressure standards.
19. What causes flow transmitter signal fluctuation?
Air bubbles, vibration, pulsating flow or noisy power supply.
Partially blocked impulse lines or unstable process conditions.
Requires damping adjustment and sensor troubleshooting.
20. What is compensated flow measurement?
Uses temperature, pressure and density correction to calculate actual flow.
Essential for gas and steam flow accuracy.
Achieved using multivariable transmitters or external compensation.
21. What is the function of a transmitter manifold?
Provides isolation, equalization and venting for DP transmitters.
Allows safe removal or calibration without process interruption.
Typically includes 3-valve or 5-valve configurations.
22. Why impulse lines are used in DP flow transmitters?
Carry pressure signals from the process to the transmitter.
Provide safe distance from hot or hazardous fluids.
Prevent direct exposure of transmitter to high temperature or vibration.
23. What are the types of orifice plates?
Concentric orifice for clean liquids and gases.
Eccentric orifice for fluids with solids.
Segmental orifice for slurry or dirty fluids.
Each type ensures stable DP generation based on fluid characteristics.
24. What is vena contracta in flow measurement?
The narrowest point where fluid jet contracts after passing the orifice.
DP is measured here for accurate flow calculation.
Affects sizing and installation of restriction devices.
25. What is the purpose of a flow straightener?
Corrects swirl, turbulence and asymmetric flow profile.
Ensures accurate measurement in DP, turbine and ultrasonic meters.
Used when straight pipe lengths are insufficient.
26. What is density compensation in mass flow measurement?
Adjusts flow calculation for changes in fluid density.
Essential for gases and steam affected by temperature and pressure.
Improves accuracy of volumetric and DP-based meters.
27. What is pulse output in flow transmitters?
Provides pulses proportional to flow volume for totalizers.
Common in turbine, PD and electromagnetic meters.
Used for batching, dosing and flow accumulation.
28. What is 4–20 mA output and why is it used?
Standard analog signal representing process variable.
Immune to noise, supports long-distance transmission.
“4 mA” indicates zero value; “20 mA” indicates full scale.
29. What is HART communication in flow transmitters?
Hybrid signal combining analog 4–20 mA with digital data.
Allows configuration, diagnostics and calibration through handheld devices.
Widely used for smart transmitters in process industries.
30. What is the difference between inline and insertion flow meters?
Inline meters are installed by cutting the pipeline; provide high accuracy.
Insertion meters are inserted through a fitting; lower cost and easier installation.
Insertion type suitable for large pipelines and retrofit applications.
31. What causes noise in flow measurement signals?
Mechanical vibration from pumps or compressors.
Turbulent or pulsating flow due to control valves.
Electrical noise from improper grounding or cable shielding.
32. What is viscosity and how does it affect flow measurement?
Viscosity is the fluid’s resistance to flow.
High viscosity slows velocity and affects turbine and vortex meters.
Requires correction or meter selection suitable for viscous fluids.
33. What is cavitation in flow systems?
Formation and collapse of vapor bubbles when pressure drops below vapor pressure.
Damages flow meters, valves and impellers.
Prevented by maintaining adequate upstream pressure.
34. Why grounding is important in magnetic flow meters?
Ensures accurate measurement by eliminating electrical noise.
Prevents electrode polarization and signal instability.
Essential for plastic or lined pipelines using grounding rings.
35. What is bidirectional flow measurement?
Measurement of flow in both forward and reverse directions.
Required in systems with reverse circulation or batch operations.
Available in Coriolis, magnetic and ultrasonic meters.
36. What are the advantages of ultrasonic flow meters?
No moving parts, low maintenance and high reliability.
Suitable for large pipes and corrosive fluids.
Clamp-on type allows installation without process shutdown.
37. What is a clamp-on ultrasonic flow meter?
Mounted externally on pipeline without cutting or drilling.
Uses transit-time or Doppler principle to measure flow.
Ideal for temporary measurements, audits and non-intrusive applications
38. What is a positive displacement flow meter?
Measures flow by trapping fixed fluid volumes and counting cycles.
Highly accurate for viscous and low-flow applications.
Used in chemical dosing, oil and polymer flow.
39. Why purge systems are used in flow measurement?
Prevents impulse line blockage due to solids or condensation.
Protects DP transmitters from corrosive or dirty fluids.
Ensures reliable and stable DP measurement.
40. What is rangeability in flow meters?
Ratio of maximum flow to minimum measurable flow.
Indicates how well a meter performs across varying flow rates.
Higher rangeability improves accuracy in fluctuating processes.