In Situ Instrumental Verification of Flow Meters: Electromagnetic, Ultrasonic and Vortex Methods

1. Purpose and Objective

This technical guide describes in situ instrumental verification methods for electromagnetic, ultrasonic (Doppler and Transit Time) and vortex (Vortex) flow meters, commonly used in the mechanical manufacturing industry. The objective is to provide maintenance technicians with clear and replicable procedures to evaluate the accuracy and reliability of these instruments without removing them from the process line, optimizing plant availability and ensuring compliance with production and quality standards (UNI EN ISO 9001).

In situ verification is indicated in the following circumstances:

• Scheduled preventative maintenance (according to the maintenance plan).
• Diagnosis of anomalies or deviations in process readings.
• After maintenance interventions that may have affected the meter (e.g. replacement of upstream/downstream components, hydraulic modifications).
• For performance validation prior to full laboratory calibration (where required).

2. Safety Precautions

ATTENTION: Before starting any maintenance or verification operation, it is MANDATORY to implement the LOTO (LockOut/TagOut) procedures to completely isolate the system from any source of energy (electrical, pneumatic, hydraulic) and residual pressure. Verify the absence of power before proceeding. Always use Personal Protective Equipment (PPE) specific to the work environment and the process fluids involved. Comply with the relevant CEI regulations for electrical safety and the UNI standards for the handling of industrial fluids.
ELECTRICAL DANGER: Electromagnetic and ultrasonic meters require electrical power. Disconnect power before manipulating electrical connections. Use a CEI certified multimeter to verify the absence of voltage.
PRESSURE/FLUIDS HAZARD: Process systems may contain fluids at high pressure and/or high temperature, or corrosive/toxic fluids. Make sure that the line section of the meter is depressurized and drained (if necessary) before any intervention involving opening the circuit. Use gloves, safety glasses and appropriate protective clothing.

3. Tools and Materials Required

4. Pre-Maintenance Inspection Checklist

5. Step-by-Step Procedure for In Situ Verification

5.1. Common Procedures (Applicable to all meter types)

1. System Isolation and Securing:
   • Implement the LOTO (LockOut/TagOut) procedure for the flowmeter and adjacent line sections. Check all energy sources (electrical, pneumatic, hydraulic) are deactivated.
   • Common mistake: Not properly isolating all energy sources. Consequence: Serious injuries or damage to the instrument.
2. Check Power Supply (no load):
   • With the power supply restored (only for the meter, if necessary and safely), measure the supply voltage at the converter/transmitter terminals.
   • Acceptable voltage: Within ±10% of nominal value (e.g. 24 VDC ±2.4 V).
   • Common error: Measure voltage under load or with faulty wiring. Consequence: Incorrect readings or invalid diagnostics.
3. Check Output Signal (no load/zero):
   • Disconnect the output signal (e.g. 4-20 mA) from the control system and connect a multimeter in ammeter mode in series.
   • At zero or simulated flow, the reading should be 4.0 mA ±0.05 mA. For pulsed outputs, check for unwanted pulses.
   • Common error: Do not disconnect the signal before measurement. Consequence: Incorrect load on the transmitter or unwanted feedback to the PLC.
4. Check Configuration Parameters:
   • Access the transmitter configuration menu (locally or via software).
   • Check that parameters such as internal diameter of the pipe, density/viscosity of the fluid, units of measurement and flow rate range comply with the system documentation.
   • Common mistake: Assuming parameters are correct. Consequence: Systematic measurement error.

5.2. Electromagnetic Flow Meters (Magmeters)

Electromagnetic meters measure the flow rate of conductive fluids. The test focuses on the integrity of the magnetic fields, electrodes and liner insulation.

5. Check Electrode Integrity:
   • Disconnect the electrode cables from the converter (with power off and LOTO active).
   • Measure the resistance between each electrode and the meter ground.
   • Acceptable reading: Resistance > 1 MOhm (indicative of good insulation). If the reading is low, it may indicate fouling or damage to the liner.
   • Measure the resistance between the two electrodes.
   • Acceptable reading: Resistance > 1 MOhm (indicative of good insulation between the electrodes).
   • Common error: Not cleaning the electrodes before the measurement or not checking the cleanliness of the liner. Consequence: Falsely low readings.
6. Liner Visual Inspection:
   • If possible (depressurized and drained line), visually inspect the internal liner through the flange openings.
   • Criteria: Absence of encrustations, bubbles, cracks or abrasions.
   • Common mistake: Ignoring minor damage to the liner. Consequence: Decreased accuracy or premature failure.
7. Harmonic Check (if available):
   • Some advanced magmeters offer signal diagnostics that detect fouling. Please refer to your OEM manual for this feature.
   • Criteria: Cleanliness status 'Good' or 'Acceptable'.
8. Flow Simulation (If supported by transmitter):
   • Use the transmitter's internal simulation function to generate a current output (e.g. 12 mA for 50% flow).
   • Check the output with the multimeter.
   • Acceptable reading: 12.0 mA ±0.05 mA.
   • Common error: Not recording the multimeter reading and comparing it to the transmitter simulation. Consequence: Inaccurate diagnosis.

5.3. Ultrasonic Flow Meters (Doppler and Transit Time)

Ultrasonic meters are versatile and often non-intrusive. The test focuses on the coupling of the transducers and the integrity of the acoustic signal.

9. Transducer Inspection and Positioning:
   • Thoroughly clean the external surfaces of the tube and the face of the transducers.
   • Apply a generous layer of thermally conductive paste (coupling gel) to the faces of the transducers.
   • Position transducers according to OEM specifications (distance, angular alignment).
   • Common error: Trapped air or not enough gel. Consequence: Loss of signal or erratic readings.
10. Check Acoustic Signal:
    • Access the transmitter diagnostics to check the signal intensity, quality and signal-to-noise ratio.
    • Criteria: Strong and stable signal, no excessive noise. Consult the manual for specific values ​​(e.g. signal > 60%).
    • Common mistake: Ignoring a weak signal. Consequence: Reduced accuracy, measurement instability.
11. Zero Flow check:
    • Make sure there is no flow in the piping (LOTO on upstream/downstream valves).
    • The meter should indicate zero or very close to zero flow rate (e.g. <0.5% of full scale).
    • Common mistake: Not ensuring true zero flow. Consequence: Zero offset.
12. Portable Comparison Test (Optional but Recommended):
    • Use a portable ultrasonic flow meter (clamp-on) to simultaneously measure flow in the same section of pipe.
    • Compare the reading of the installed meter with that of the portable meter.
    • Criteria: Difference < ±2% of full scale for in situ verification.
    • Common error: Installing the portable meter in an unrepresentative area or not configuring it correctly. Consequence: Invalid comparison.

5.4. Vortex Flow Meters (Vortex)

Vortex meters are sensitive to fluid conditions and vibrations. The test focuses on the release element and the sensor electronics.

13. Bluff Body Inspection:
    • If the line can be safely opened (full LOTO, depressurization and drain), visually inspect the bluff body inside the meter.
    • Criteria: No damage, excessive wear, fouling or build-up. The item must be firm and intact.
    • Common error: Ignore fouling or wear. Consequence: Measurement error, vortex instability.
14. Test Vortex Sensor Integrity:
    • With the power off and LOTO active, disconnect the vortex sensor wires (piezoelectric or capacitive) from the transmitter.
    • Measure the resistance and/or capacitance of the sensor according to OEM specifications.
    • Acceptable reading: Within manufacturer ratings (e.g. resistance 1 kOhm ±10%, capacitance 100 pF ±20%).
    • Common error: Not checking sensor specifications before measurement. Consequence: Wrong diagnosis.
15. Zero Flow and Stability check:
    • Make sure there is no flow in the pipeline (LOTO on upstream/downstream valves).
    • The meter should indicate zero or very close to zero flow (e.g. <0.5% of full scale) and the output should not exhibit significant fluctuations.
    • Common mistake: Not considering external vibrations. Consequence: Noise signal at zero flow.
16. Verify Pressure/Temperature Compensation (if present):
    • If the meter compensates for pressure and/or temperature, check the readings of the integrated or external pressure and temperature sensors.
    • Use a digital pressure gauge and probe thermometer to compare readings.
    • Criteria: Difference < ±1% of nominal values ​​for pressure and temperature.
    • Common error: Ignoring compensation sensors. Consequence: Mass or volume errors compensated.
17. Reconnection and Functional Test:
    • Reconnect all wiring and reset the system.
    • Reinstate the meter into the process and observe its response under normal operating conditions.
    • Criteria: Stable reading and compliant with expectations, absence of alarms.
    • Common error: Not fully testing the system after reconnecting. Consequence: New faults or hidden problems.

6. Post-Maintenance Verification Checklist

7. Troubleshooting Guide

8. Recommended Maintenance Schedule

9. Spare parts reference

To purchase original and compatible spare parts, consult our electronic catalogue: www.unitecd.com/e-catalog/

10. References

• UNI EN ISO 9001: Quality management systems.
• CEI EN 60529: Degrees of protection of the enclosures (IP Code).
• CEI EN 61010-1: Safety requirements for electrical equipment for measurement, control and laboratory use.
• Manufacturer-specific Operation and Maintenance Manuals (OEM) (e.g. Endress+Hauser, Siemens, Krohne, Emerson).
• Internal Safety Regulations and Standard Operating Procedures (SOP) of the plant.