Diagnosis and Resolution of Measurement Errors in Flowmeters

1. Problem Description and Scope

This manual provides a structured approach for the diagnosis and correction of measurement errors in industrial flow meters. Inaccurate flow measurements can lead to significant operational inefficiencies, loss of product quality, increased energy costs and in critical cases even safety risks. These errors often manifest themselves as deviating process values, absent signals, sporadic or fluctuating measurements, and persistent measurement drift. Diagnosis focuses on a wide range of common flow meter technologies, including electromagnetic, Coriolis, vortex, differential pressure (such as orifice plates and Venturi tubes), and ultrasonic flow meters.

The severity of a measurement error is classified as follows:

• Critical: Direct impact on safety, environmental compliance, or primary product quality. Requires immediate action.
• Major: Leads to significant loss of efficiency, increased energy consumption, or batch consistency deviations. Requires rapid planning and implementation of corrective actions.
• Minor: Slight deviations that primarily affect process optimization without direct critical consequences. Requires planning for routine maintenance.

The diagnosis in this guide includes effects related to installation, changes in process conditions, calibration drift, and the formation of scale or contamination.

2. Safety measures

CAUTION REQUIRED! Prior to any diagnostic or maintenance work on flow meters or related piping, it is essential to strictly follow all applicable safety procedures.

• Lockout/Tagout (LOTO): Ensure that all energy sources (electrical, pneumatic, hydraulic, mechanical) are isolated and locked according to NEN-EN-ISO 14118 and company standards. Check for absence of voltage with a suitable multimeter (e.g. Fluke 1587 FC) before working with electrical components.
• Personal Protective Equipment (PPE): Always wear the required PPE for the specific process intervention. This includes at least safety glasses (EN 166), chemical-resistant gloves (EN 374), safety shoes (EN ISO 20345) and, if necessary, hearing protection (EN 352) and respiratory protection (EN 140/143).
• Stored Energy and Hazardous Materials: Use extreme caution with pressurized pipes, hot fluids, corrosive chemicals, or cryogenic media. Completely depressurize and bleed piping systems before loosening flanges or fittings. Follow the procedures for the safe handling of hazardous substances in accordance with the safety data sheets (SDS).
• ATEX directive: Work in potentially explosive areas only with intrinsically safe equipment (in accordance with ATEX directive 2014/34/EU) and follow the specific work permit procedures for 'hot work' or 'cold work'.
• Access and Fall Hazard: Ensure safe access to the flow meter. Use approved ladders or scaffolding and take measures against the risk of falls.

3. Required Diagnostic Tools

The effective diagnosis of flowmeter errors requires specific instrumentation. The table below describes the essential tools:

4. Initial Assessment Checklist

Before beginning detailed diagnostics, a thorough initial assessment is essential to save time and determine the right direction. Record all observations:

5. Systematic Diagnosis Flowchart

Follow this structured flowchart to isolate the root cause of flowmeter measurement errors. Always start at 'Start' and follow the paths based on observed symptoms and test results.

1. Start: Anomalous Flow Measurement (None, Inaccurate, Erratic, Drift)
   1. Check Basic Function and Signal Integrity
      1. Is there any output from the flow meter (display, 4-20 mA, pulse)?
         • No: No output.
           1. Check supply voltage: Measure with multimeter. Expected: 24 VDC (±10%).
              • Result OK: Go to 1.1.1.2.
              • Result NOT OK: Solve power supply problem (fuse, broken cable, defective power supply unit).
           2. Check signal wiring (4-20 mA, pulse): Measure resistance, check for opens or shorts.
              • Result OK: Go to 1.1.1.3.
              • Result NOT OK: Repair cabling.
           3. Check internal diagnosis of the meter: Read error codes via display or HART communicator.
              • Error code present: Consult manual for specific error code solution.
              • No error code / Problem persists: The problem probably lies with the sensor itself or primary measuring elements. Go to 1.2.
         • Yes: Output present, but different. Go to 1.2.
   2. Evaluate Installation Effects
      1. Does the installation meet the minimum straights (upstream/downstream)? (e.g. 5D upstream, 2D downstream for DP meters; 10D/5D for vortex/ultrasonic; consult OEM manual and NEN-EN-ISO 5167 for specific requirements).
         • No:
           1. Probable cause: Insufficient straight section, flow profile disturbance (swirl, turbulence).
           2. Action: Consider relocating the meter or installing flow conditioners/rectifiers.
         • Yes: Go to 1.2.2.
      2. Is there excessive vibration at the meter? Use vibration analyzer (reference ISO 10816, > 4.5 mm/s RMS is problematic).
         • Yes:
           1. Probable cause: External vibrations affect sensor (especially vortex, ultrasonic, Coriolis).
           2. Action: Isolate meter from vibration sources, check pump alignment/bearings.
         • No: Go to 1.2.3.
      3. Is grounding correct and effective for electromagnetic flow meters? Measure resistance between meter housing/electrode and ground (< 10 Ohm).
         • No:
           1. Probable cause: Improper grounding leads to noise and unstable measurements.
           2. Action: Improve grounding, check grounding cables and points.
         • Yes: Go to 1.3.
   3. Evaluate Process Conditions
      1. Are the process temperature and pressure within the specified range of the flow meter? Measure with pressure and temperature meter. Check that T < max T meter, P < max P meter.
         • No:
           1. Probable cause: Over/under range process conditions, possible phase change, density/viscosity deviations.
           2. Action: Optimize process, adjust compensation or consider other meter technology.
         • Yes: Go to 1.3.2.
      2. Are there any changes in fluid composition (gas bubbles, solids, viscosity, density)? Check process logs, take samples.
         • Yes:
           1. Probable cause: Gas inclusions (ultrasonic, magmeter), abrasive solids (erosion), change in density/viscosity (DP, vortex).
           2. Action: Remove gas via breathers, filter solids, reconfigure meter with new density/viscosity.
         • No: Go to 1.4.
   4. Check for deposits / contamination
      1. Is there deposits or erosion in the pipe or on the sensor? Use an endoscope for internal inspection. Measure pipe wall thickness with ultrasonic thickness gauge to detect erosion or extreme scale.
         • Yes:
           1. Probable cause: Deposits on electrodes (magmeter), on sensors (ultrasonic, Coriolis), or narrowing in orifice/Venturi. Erosion of measuring elements.
           2. Action: Clean meter and pipe, consider material adjustment, preventative cleaning cycles.
         • No: Go to 1.5.
   5. Check Calibration Status
      1. Has the flow meter been recently calibrated? What was the result? Check calibration certificates and dates. Recommended interval: annually (NEN-EN-ISO 10012).
         • No / Last calibration deviation:
           1. Probable cause: Calibration drift due to aging, wear, mechanical stress.
           2. Action: Perform in-situ verification with a portable ultrasonic flow meter or send the meter to an accredited calibration laboratory.
         • Yes / Calibration OK: If all of the above checks have failed to identify a cause, consider a deeper analysis of the signal processing or a defective transmitter. End of diagnosis for this flow meter; consider replacement.

6. Error Cause Matrix

This matrix provides an overview of common symptoms, their likely root causes, the diagnostic tests and the expected results.

7. Root Cause Analysis for Each Error

7.1. Installation effects

Why it happens: Flow meters require a fully developed and stable flow profile to measure accurately. Insufficiently straight inlets and outlets, bends, valves, pumps, or other piping components directly before or after the meter can disrupt the flow profile by creating turbulence, swirl, or asymmetrical velocity distributions. This mainly affects differential pressure meters (whose measurement depends on a stable velocity profile), vortex meters (which form vortices based on a stable profile), and ultrasonic meters (which send the sound signal through the fluid). Vibrations, from pumps or mechanical resonance, can directly affect the sensitive sensors of vortex and Coriolis meters, leading to noise on the signal or completely unreliable measurements. Improper grounding of electromagnetic flowmeters causes ground loops and introduces electrical noise into the low-level measurement signal, which seriously affects accuracy.

How to confirm: Check the installation drawings and compare them with the actual situation and the OEM installation requirements (e.g. NEN-EN-ISO 5167 for orifice plates). Use a vibration analyzer to measure vibration levels on and around the meter; values ​​above 4.5 mm/s RMS (according to ISO 10816, good condition) indicate a problem. Check the grounding of electromagnetic flow meters with a multimeter; the resistance between the meter body and ground should not exceed 10 Ω.

Damage if unresolved: Persistent inaccuracy, loss of production, and in extreme cases wear of measuring elements due to incorrect flow conditions.

7.2. Process Condition Changes

Why it happens: Many flowmeter principles are sensitive to changes in the properties of the measured fluid. Temperature and pressure changes affect the density and viscosity of liquids and gases. This has direct consequences for differential pressure meters (where the measurement is dependent on density) and vortex meters (where the vortex frequency is somewhat dependent on the Re number, i.e. viscosity). Gas bubbles or trapped solid particles in liquids can scatter ultrasonic signals, disrupt electromagnetic fields, or clog differential pressure gauges, leading to unstable or incorrect readings. Phase change (e.g. cavitation due to too low pressure) also causes measurement errors, because the meter is designed for one specific phase.

How to confirm: Analyze the historical process data (SCADA/DCS) for correlations between measurement errors and fluctuations in temperature, pressure, or product mix. Take samples of the medium to verify the presence of gas or solids. A visual inspection through a sight glass may reveal gas bubbles or particles.

Damage if unresolved: Continuous incorrect batching, incorrect dosages, and potential damage to the meter from abrasive particles.

7.3. Calibration drift

Why it happens: Even the most rugged flowmeters experience calibration drift over time. This can be caused by the natural aging of electronic components, mechanical wear of moving parts (e.g. bearings in turbine meters, but also slight deformation of orifice plates or Coriolis measuring tubes), or exposure to extreme process conditions that lead to mechanical stress. Particularly in demanding environments with high temperatures, pressure cycles, or abrasive media, this drift can occur at an accelerated rate.

How to confirm: The most direct method is a recalibration. This can be done in-situ with a portable reference (such as a clamp-on ultrasonic meter) or, for the highest accuracy, by removing the meter and testing it in an accredited calibration laboratory (in accordance with NEN-EN-ISO/IEC 17025). Compare the new calibration results to the original factory calibration or the last known good calibration. An out-of-specification deviation confirms calibration drift.

Damage if unresolved: Long-term inaccuracy in process control, financial losses due to incorrect billing (if custody transfer), or non-compliance with regulatory requirements.

7.4. Scale / Contamination

Why it happens: Many process fluids, especially in the food, chemical, and wastewater industries, contain particles that can deposit on the internal surfaces of the flowmeter or measuring line. This can vary from hard mineral deposits (e.g. lime), organic biofilms, to polymerization products. In electromagnetic flowmeters, deposits on the electrodes can isolate them from the process fluid, weakening or blocking the inductive signal. In ultrasonic flow meters, deposits can dampen or deflect the ultrasonic pulses. With differential pressure meters (orifice plates, Venturi tubes), deposits lead to a narrowing of the flow opening and thus a change in the pressure difference at the same flow, which results in overestimation of the flow. Erosion caused by abrasive particles can in turn damage measuring elements and lead to incorrectly increased or decreased output.

How to confirm: Use an endoscope for visual internal inspection. For magmeters, check the resistance between the electrodes; an unexpectedly high resistance may indicate insulating deposits. Use an ultrasonic thickness gauge to check for abnormal deviations in pipe wall thickness that indicate significant internal scaling or erosion.

Damage if unresolved: Complete failure of the meter, increased pressure drop due to restriction, structural damage to the meter, and uncontrollable process operation.

8. Step-by-Step Troubleshooting Procedures

Once the root cause has been identified, the correction follows:

8.1. Solution Installation effects

1. Disruption Flow profile:
   1. Solution: If possible, move the flow meter to a section of the pipe that meets the minimum straights (e.g. 5-20 x pipe diameter upstream, 2-5 x pipe diameter downstream, depending on meter type and pipe components).
   2. Alternative: Install flow conditioners or rectifiers directly before the meter. These effectively reduce swirls and turbulence. Ensure that these are installed in accordance with NEN-EN-ISO 5167 for differential pressure gauges.
   3. Verification: Monitor the stability of the flow measurement and compare with a reference measurement.
2. Vibrations:
   1. Solution: Mechanically isolate the flow meter from vibration sources. Use flexible connections, vibration dampers, and check the alignment of nearby rotating equipment (e.g. pumps).
   2. Verification: Measure vibration levels again with a vibration analyzer; aim for values ​​< 2.8 mm/s RMS (good condition).
3. Improper Grounding (Magmeters):
   1. Solution: Check and repair ground connections. Provide a low-impedance ground path (< 10 Ω) between the flow meter, the pipe (if conductive) and the central ground point of the installation. Use proper grounding rings if the pipe is non-conductive or coated.
   2. Verification: Measure the ground resistance again. Monitor the stability of the 4-20 mA signal with a process calibrator.

8.2. Solution Process Condition Changes

1. Abnormal Temperature/Pressure:
   1. Solution: Optimize process conditions to remain within the operational range of the meter. If this is not possible, the flow meter should be reconfigured with the new density and viscosity values, or consider installing a meter less sensitive to these variations (e.g. Coriolis meter for density independent mass flow).
   2. Verification: Monitoring temperature and pressure and comparing the flow measurement with known values.
2. Gas bubbles / Solids:
   1. Solution: Install gas separators or air vents upstream of the flow meter. Implement filters to remove solid particles. Adjust flow meter settings for media with higher solids concentrations, if supported.
   2. Verification: Visual inspection after installation of breathers/filters. The measurement should become more stable.

8.3. Solution Calibration Drift

1. Solution: Perform a recalibration. This can be done in the following ways:
   • In-situ verification: Use a portable ultrasonic clamp-on flow meter to compare the existing meter. This method is fast, but accuracy is limited by the clamp-on meter and process conditions.
   • Laboratory calibration: Remove the flow meter and send it to an accredited calibration laboratory (according to NEN-EN-ISO/IEC 17025) for a primary calibration against a reference standard. This provides the highest accuracy and a new calibration certificate.
2. Verification: The meter must be within the specified accuracy limits after calibration. Keep the calibration certificate for compliance.

8.4. Solution Scale / Contamination

1. Solution:
   1. Cleaning: Clean the flow meter and adjacent pipe sections. This can be done mechanically (brushing, scraping after disassembly) or chemically (by rinsing with suitable cleaning agents). For magmeters, carefully clean the electrodes.
   2. Material selection: If scale is a recurring problem, consider installing a flowmeter with internal materials that are less susceptible to adhesion (e.g. specific coatings or liners).
   3. Preventive Cycles: Implement periodic cleaning cycles as part of the preventive maintenance plan.
2. Verification: Visual inspection after cleaning. The flow measurement should return to expected values ​​and show stability.

9. Preventive Measures

Prevention is essential to ensure the longevity and reliability of flow meters.

10. Spare Parts & Components

The timely availability of critical spare parts is crucial for the rapid resolution of flow meter errors. Consult the UNITEC-D e-catalogue for a complete range.

For a complete overview and direct ordering options, visit our UNITEC-D e-catalogue.

11. References

• NEN-EN-ISO 5167-1:2003, Measuring liquid flow with differential pressure devices in closed pipes – Part 1: General principles and requirements.
• NEN-EN-ISO 10012:2003, Measurement management systems – Requirements for measuring processes and measuring equipment.
• NEN-EN-ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories.
• NEN-EN-ISO 10816-3:2009, Mechanical vibrations - Measurement and assessment of vibrations of machines - Part 3: Industrial machines with a rated power above 15 kW and rated speeds between 120 rpm and 15 000 rpm when measured on site.
• NEN-EN 166:2001, Personal eye protection – Specifications.
• NEN-EN 374-1:2016+A1:2018, Protective gloves against hazardous chemicals and micro-organisms – Part 1: Terminology and performance requirements for risk.
• NEN-EN-ISO 20345:2022, Personal protective equipment – ​​Safety footwear.
• NEN-EN-ISO 14118:2018, Safety of machines - Preventing unexpected start-up.
• Directive 2014/34/EU (ATEX), Equipment and protective systems intended for use in places where there may be a risk of explosion.
• Manufacturer-specific flow meter installation and maintenance manuals.