Flowmeter Troubleshooting: Installation, Process, and Calibration Effects

1. Problem description & scope of application

This guide is intended to systematically diagnose flowmeters that deliver inaccurate readings or fail completely. It is aimed at maintenance technicians, maintenance engineers and plant managers in the manufacturing industry, particularly in the automotive, aerospace, food, chemical and energy sectors. The symptoms and diagnostic procedures described here are applicable to the most common flow measurement technologies, including electromagnetic flowmeters (EMF), ultrasonic flowmeters (USF), Coriolis flowmeters (CMF), differential pressure flowmeters (DPF), and turbine flowmeters.

Affected device types and symptoms

• Electromagnetic flow meters (EMF): Commonly affected by electrode coating, grounding faults, magnetic interference.
• Ultrasonic flow meters (USF): Susceptible to air bubbles, deposits on the transducers, temperature gradients, insufficient coupling.
• Coriolis flow meters (CMF): Vibrations, erosion damage, seal leaks, mechanical overload, air/gas in the medium.
• Differential pressure flow meters (DPF) (orifices, venturi, dynamic pressure): Blockage of the impulse lines, sealing failure, incorrect pressure transmitter calibration, deposits on the primary element edge.
• Turbine flowmeter: Bearing damage, erosion of the turbine wheel, cavitation, contamination in the medium.

Typical symptoms include:

• Deviating or fluctuating flow display
• Complete failure of flow measurement
• Alarm messages from the measuring device or the control technology
• Discrepancies between measured and expected flow
• Sudden changes in indicated flow without process change

Severity classification

• Critical: Production downtime, quality defects, safety risk. Immediate fix required.
• Major: Increased energy consumption, inefficient process control, slight fluctuations in quality. Fix within 24 hours.
• Minor: Minor deviations that do not directly affect production but can lead to inefficiencies in the long term. Fix at the next scheduled downtime.

2. Safety instructions

ATTENTION! Danger to life due to electrical voltage, rotating parts, hot media and high pressures!

• Before starting any work on the flowmeter or the associated pipelines, the system must be ensured in accordance with DIN EN ISO 14118 (Safety of machines - avoiding unforeseen start-ups) and the local operating guidelines. This includes complete energy decoupling (electrical, pneumatic, hydraulic) according to the LOTO principle (Lockout/Tagout).
• Ensure that there is no residual energy (pressure, temperature, mechanical stress) in the systems before dismantling components. Ventilation and drainage valves must be used.
• Always wear the prescribed personal protective equipment (PPE) in accordance with DIN EN ISO 20345 (safety shoes), DIN EN 388 (protective gloves), DIN EN 166 (eye protection) and, if necessary, hearing protection (DIN EN 352) or respiratory protection.
• Work on electrical systems or components may only be carried out by authorized specialist personnel in accordance with VDE 0100 (installation of low-voltage systems).
• When handling process media, observe the safety data sheets (SDS) and take appropriate protective measures. Special precautions must be taken when using aggressive or toxic media.
• Hot process fluid can cause severe burns. Before opening pipes and equipment, ensure that the temperature has fallen to a safe level.

3. Required diagnostic tools

Correct selection and application of diagnostic tools is crucial for efficient troubleshooting.

4. Checklist for initial assessment

Before starting the detailed diagnosis, a careful initial assessment of the situation is essential. This helps to avoid unnecessary steps and focus troubleshooting.

5. Systematic diagnostic flowchart

This flowchart provides a structured approach to troubleshooting flow measurement errors.

1. Symptom: Inaccurate or fluctuating flow display
   1. Initial check of the signal
      • IF 4-20mA output fluctuates greatly:
        1. Check cable connections (fitting, corrosion).
        2. Check ground connections (EMF particularly critical, < 1 ohm).
        3. Measure loop resistance (should be in the range of the transmitter, e.g. 250-500 ohms).
        4. Check the power supply for stable voltage (e.g. 24V DC ± 5%).
        5. IF external interference is suspected (e.g. powerful motors, frequency converters):
           • Check shielded cables (VDE 0800 – cables and lines for information technology).
           • Check the grounding concept (DIN EN 50310 – application of potential equalization and grounding).
      • IF Display on the device itself is stable, but the control technology shows fluctuations:
        • Problem in the transmission path (line, IO card, software).
   2. Evaluate process conditions
      • IF Process parameters (pressure, temperature, viscosity) outside the specified range of the flow meter:
        • Adjust process to setpoints.
        • Alternatively: Check whether the flow meter is suitable for current conditions (manufacturer data sheet).
      • IF Medium contains air/gas bubbles (especially when measuring liquids):
        • Check ventilation of the system.
        • Consider using a bubble detector or flow straightener.
      • IF Medium highly viscous or non-Newtonian:
        • Check whether flow meter (e.g. EMF) is suitable for such media. Coriolis flow meters are often more reliable here.
   3. Check measuring device integrity
      • IF EMF:
        1. Measure electrode resistance (with multimeter, separated from measuring device). A sharp increase indicates coating.
        2. Check the grounding of the pipeline and the measuring device.
        3. Activate the device's mirror or test function (if equipped) to check electronics.
      • IF USF:
        1. Check transducers for deposits (endoscope or dismantling).
        2. Check coupling fluid/gel (if available).
        3. Check signal quality (if supported by the device, e.g. signal strength display).
      • IF CMF:
        1. Check for excessive vibration of the sensor or pipeline (vibration analyzer, threshold > 5 mm/s at 100 Hz).
        2. Check external damage to the measuring tube (erosion, dents).
        3. Check the fit of the fasteners (torque wrench, according to manufacturer's instructions).
      • IF DPF:
        1. Check impulse lines for blockages (measure differential pressure at the taps, pressure peaks, VDI/VDE 3511 – process control technology; pressure measurement).
        2. Check primary element (aperture, venturi) for damage or deposits (endoscope).
        3. Check the tightness of the impulse lines and screw connections.
2. Symptom: Complete failure of the flow measurement / 0 value
   1. Power supply
      • IF no display on the device or error LED lights up:
        1. Check the voltage supply on the device (multimeter, 24V DC ± 5%).
        2. Check fuse in control box.
   2. Signal output
      • IF Device itself works (display, status LEDs OK), but no signal to control technology:
        1. Check cable break or interruption in the 4-20mA loop.
        2. Check IO card of the PLC/DCS (input area, configuration).
        3. Use a process calibrator to simulate a 4-20mA signal and check whether the control system responds.
   3. Sensor defect
      • IF all external tests negative:
        • Device is probably defective. Replacement or manufacturer inspection required.

6. Error-cause matrix

This matrix presents the most common symptoms, likely causes, diagnostic methods, and expected results to expedite troubleshooting.

7. Root cause analysis for each error

A thorough understanding of the causes of errors is crucial to avoid repeat errors.

7.1 Installation errors

Why it happens: Flowmeters are sensitive to the flow profile of the medium. Insufficient straight inlet and outlet sections after elbows, reducers or valves create turbulence and turbulent flows. This leads to an inconsistent measurement because the sensor does not record a stable, homogeneous flow profile. Incorrect sensor orientation (e.g. vertical instead of horizontal on certain types of devices) can also cause problems.

How to confirm: A visual inspection of the installation and a comparison with the manufacturer's information and relevant standards (e.g. DIN EN ISO 5167 for differential pressure measuring devices) reveals deviations. An endoscope can check internal turbulence or the correct alignment of internals.

Damage if not corrected: Continuously inaccurate measurements lead to inefficient processes, incorrect dosing of materials, increased operating costs and potential quality problems. In the long term, this can lead to premature wear due to uneven flow loading.

7.2 Changes to process conditions

Why it happens: Many flow measurement technologies are calibrated and designed for specific process conditions. Changes in the temperature, pressure, viscosity, density or physical state of the medium can significantly affect the measurement accuracy. For example, temperature changes lead to density changes, which lead to mass flow errors in volume flow meters (e.g. EMF) without density compensation. Gas/air inclusions in liquids or particles in gases can disrupt USF and CMF.

How to confirm: Comparing process data (pressure, temperature, density, viscosity) with the flowmeter specifications and flow measurement history shows correlations. A laboratory analysis of the medium can reveal changes in the composition. Observation with an endoscope may indicate bubble formation or particles.

Damage if not rectified: Incorrect process management due to inaccurate flow values, over- or under-dosing of reagents, incorrect balancing and potential damage to downstream systems due to media that does not conform to specifications.

7.3 Deposits and coatings

Why it happens: Process media may contain solids, suspended solids or dissolved substances that accumulate on the internal surfaces of the meter or electrodes (in the case of EMF). These deposits can change the cross-section of the measuring tube, hinder acoustic transmission in USF or change the electrical properties of the electrodes in EMF, resulting in incorrect or no readings.

How to confirm: A visual inspection (endoscope or disassembly) of the sensor internal surfaces and electrodes. In the case of EMF, a resistance measurement on the electrodes (after separation from the device) can show increased values ​​when coated. Ultrasonic thickness measurement from the outside can reveal a narrowing of the cross-section of non-metallic deposits.

Damage if not rectified: The measurement accuracy continuously decreases, which leads to gradual process deviations. In extreme cases, the measuring device can fail completely or the deposits can come loose and block or damage downstream components.

7.4 Calibration drift

Why it happens: Over time, electronic components can age or change due to thermal/mechanical stress. This leads to a gradual deviation of the device display from the actual value. This is a natural process and requires regular calibration according to VDI/VDE 2645.

How to confirm: An on-site reference calibration using a certified process calibrator (e.g. Fluke 754) or in the calibration laboratory. The output signal of the flow meter is compared with a known reference value.

Damage if not repaired: Although the display appears to be stable, the measuring device systematically delivers incorrect values. This can lead to long-term quality problems, increased raw material consumption or inefficient energy balancing without an acute error being detected.

8. Step-by-step fix procedure

The following steps must be carried out to eliminate the identified causes of the error. Always follow the safety instructions in Section 2!

8.1 Fixing installation errors

1. Ensure safety: Switch off the system in accordance with LOTO, depressurize it and de-energize it.
2. Visual inspection and documentation: Exactly measure existing inlet and outlet sections and compare them with the manufacturer's information. Document photos.
3. Planning of corrective measures: If the routes are too short, a new installation or the installation of flow straighteners must be considered. If necessary, the flow meter must be moved to another, more suitable position.
4. Dismantling and conversion: Dismantle existing pipeline sections (according to DIN 86000 - pipeline components, welded fittings), insert new sections with the correct lengths and, if necessary, flow straighteners. Tighten flange connections according to DIN EN 1591-1 (flanges and their connections) with the correct torque (e.g. M16 screws, class 8.8, 170-190 Nm).
5. Leak test: After assembly, carry out a leak test of the system (e.g. pressure test with medium or inert gas).
6. Recommissioning and verification: Put the system into operation step by step. Verify flow measurement with reference (e.g. volume measurement over time) or comparison measurement.

8.2 Adaptation to process conditions

1. Ensure safety: If direct work on the device is required, use LOTO.
2. Process data analysis: Analyze historical process data (pressure, temperature, density) and identify correlations to the measurement deviations.
3. Identification of causes for process deviations: Work closely with process control technology to find and eliminate the causes of process parameter changes (e.g. defective control valves, pump problems, heating failure).
4. Device replacement/upgrade: If the existing flowmeter is unsuitable for the new/varying process conditions, a device with adapted specifications (e.g. Coriolis flowmeter for strong density fluctuations, heated EMF for media susceptible to crystallization) must be evaluated and installed.
5. System adaptation: If necessary, implement density compensation in the control technology or install an additional density sensor.
6. Verification: After process stabilization or device adjustment, monitor and validate the flow measurement over a longer period of time.

8.3 Cleaning and stripping

1. Ensure safety: Switch off the system in accordance with LOTO, depressurize it and de-energize it. WARNING: Wear suitable PPE (full protective suit, respiratory protection) when dealing with aggressive media!
2. Dismantling: Dismantle the flow meter from the pipeline.
3. Inspection: Check internal surfaces and electrodes (for EMF) for deposits. Document photos.
4. Cleaning:
   • Mechanical: Careful removal of soft deposits with plastic brushes. Do not use abrasive tools to avoid damaging the sensor surface.
   • Chemical: For stubborn deposits, use chemical cleaning agents as recommended by the manufacturer. The compatibility of the cleaning agent with the device materials must be checked critically (e.g. V4A for aggressive acids). Observe exposure times.
   • Ultrasonic bath: For smaller components, an ultrasonic bath can be effective.
5. Rinsing and drying: Thorough rinsing with demineralized water to remove residues. drying.
6. Recommissioning: Reinstalling the device, leak test, recommissioning and checking the function.

8.4 Recalibration

1. Provide safety: Switch off in accordance with LOTO, de-energize.
2. Preparation: Remove the measuring device (if laboratory calibration is required) or prepare for on-site calibration. Provide calibration documentation (e.g. according to ISO/IEC 17025 - General requirements for the competence of testing and calibration laboratories).
3. Performing calibration:
   • On-site calibration: Using a certified process calibrator (e.g. Fluke 754), the flow meter output signal (e.g. 4-20mA) at multiple points over the measurement range (e.g. 0%, 25%, 50%, 75%, 100%) check. Deviations > 0.5% from the final value (> 0.08mA) are alarming. If necessary, readjust the zero point and span on the device or via software.
   • Laboratory calibration: Sending the device to an accredited laboratory (e.g. DAkkS) for a precise reference calibration.
4. Documentation: Create a calibration certificate or calibration report, stating the “as found” and “as left” values.
5. Recommissioning: Reinstallation and checking of function.

9. Preventive measures

Preventative maintenance is the key to avoiding flow measurement errors.

10. Spare Parts & Components

UNITEC-D offers an extensive range of spare parts and components for flow meters.

For detailed information and orders, please visit our e-catalogue: www.unitecd.com/e-catalog/

11. References

• DIN EN ISO 5167: Measuring the fluid flow with throttle devices in full-flow pipes with a circular cross-section.
• DIN EN ISO 14118: Safety of machines – avoiding unforeseen startups.
• VDE 0100: Setting up low-voltage systems.
• VDE 0800: Cables and wires for information technology.
• DIN EN 50310: Application of potential equalization and grounding in buildings with information technology facilities.
• VDI/VDE 2640: Measurement technology for quality assurance - guidelines for the selection and use of measuring devices.
• VDI/VDE 2645: Calibration of measuring equipment.
• VDI/VDE 3511: Process control technology; Pressure measurement.
• ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories.
• Manufacturer-specific documentation and maintenance manuals for the respective flowmeters.