Top 7 Flow Transmitter Failures (and How to Prevent Them)
Process Control & Solutions
Technical Guide ยท Process Instrumentation
Top 7 Flow Transmitter Failures (and How to Prevent Them)
A practical field guide to the most common causes of inaccurate or failed flow measurement โ covering differential pressure, electromagnetic, ultrasonic, and Coriolis technologies.
Flow transmitters are among the most relied-upon instruments in process plants, water treatment facilities, and industrial operations across Australia. When they fail โ or more commonly, when they quietly drift into inaccuracy โ the consequences range from substandard product quality and off-spec chemical dosing through to unplanned shutdowns and serious process upsets.
The good news is that the vast majority of flow transmitter problems are preventable. Most failures trace back to a handful of recurring root causes: poor installation, fouled sensors, unmanaged process conditions, or neglected calibration routines. This guide walks through the seven most common failure modes, explains what causes them, and gives you practical steps to prevent each one โ regardless of whether you’re running differential pressure, electromagnetic, ultrasonic, vortex, or Coriolis technology.
PCSPL has been supplying and supporting flow measurement instrumentation to Australian industry since 1991. The failure modes described here are drawn directly from the enquiries and service calls our applications team receives regularly from sites across the country.
At a Glance
The 7 Failures at a Glance
Use this table to quickly identify which failure mode matches the symptoms you’re seeing in the field. Full detail on each failure โ including prevention steps โ follows below.
| # | Failure Mode | Primary Symptom | Technologies Affected |
|---|---|---|---|
| 1 | Impulse line blockage or plugging | Frozen or drifting reading; zero output | DP transmitters |
| 2 | Air or gas entrainment | Erratic spikes; over-reading; unstable signal | DP, electromagnetic, ultrasonic |
| 3 | Electrode or sensor fouling | Drifting zero; sluggish response; under-reading | Electromagnetic, ultrasonic |
| 4 | Incorrect installation | Systematic error; non-repeatable readings | All types |
| 5 | Calibration drift | Gradual offset vs. reference; meter vs. totals discrepancy | All types |
| 6 | Moisture and condensation ingress | Intermittent faults; corrupted output; permanent failure | All types |
| 7 | Signal loop and power supply faults | Flat-line output; noise on signal; out-of-range reading | All types (4โ20 mA loop) |
Impulse Line Blockage or Plugging
Differential pressure (DP) flow transmitters โ used with orifice plates, venturi tubes, averaging pitot tubes, and annubars โ rely on clear impulse lines to accurately transmit the high- and low-side pressure signals to the transmitter body. When those lines block, the transmitter reads the last known pressure and produces a frozen or heavily distorted flow signal.
Blockages are caused by process fluid solids settling in the impulse lines, process fluid polymerising or crystallising in the line, condensate or ice forming in steam or gas applications, and wax or viscous material solidifying at ambient temperature. In dirty-service applications, partial blockages are often more dangerous than full blockages โ a partial block creates a sustained false reading that can go undetected for days or weeks.
How to Spot It
The flow reading freezes at a fixed value regardless of process changes. Comparing the DP transmitter output against an independent measurement (another meter, pump curve, or level change) quickly reveals the discrepancy. On smart transmitters, diagnostic alerts for frozen output or implausible DP ratio can be configured.
How to Prevent It
Specify impulse lines with a continuous slope (no sags in liquid service, no humps in gas service) so gas or condensate cannot accumulate. For slurry and particulate service, use remote seals or diaphragm seals to eliminate impulse lines entirely. Schedule regular impulse line blowdown on scheduled maintenance cycles. For wax-bearing fluids, trace-heat the impulse lines.
Air or Gas Entrainment in the Process Line
Air and gas bubbles in a liquid process line are one of the most common โ and most under-diagnosed โ causes of flow meter error. Every major flow measurement technology is affected to some degree. The result is typically an over-reading or a highly erratic, spiking signal that makes process control unreliable.
Gas entrainment occurs when the process fluid is partially de-aerated upstream of the meter, when there is a low-pressure zone at the meter inlet that causes dissolved gas to come out of solution, when pump cavitation introduces air upstream, or when a suction-side air leak exists. In water treatment plants, aeration systems and chemical dosing of outgassing chemicals (such as sodium hypochlorite) are common culprits upstream of flow meters.
How to Spot It
The flow signal shows intermittent sharp spikes, over-reads consistently against a reference meter, or becomes wildly unstable under certain process conditions. Electromagnetic flow meters will report an “empty pipe” alarm if gas void fraction becomes high enough. On ultrasonic meters, signal strength (gain) drops and “no signal” faults trigger.
How to Prevent It
Install the flow meter in a pipe section that is guaranteed to remain full โ a rising pipe section is ideal. Ensure the meter is always downstream of the highest point in the line. For applications with known aeration risk, specify a Coriolis mass flow meter, which is inherently less affected by gas entrainment than volumetric technologies. Fit an air release valve upstream of the meter where persistent aeration is unavoidable.
Electrode or Sensor Fouling
In electromagnetic flow meters, the measurement depends on a clean electrical contact between the process fluid and the electrodes embedded in the meter tube. Any coating on the electrodes โ scale, biological growth, chemical deposition, or suspended solids build-up โ degrades that contact and causes the meter to drift, under-read, or output a noisy signal. In ultrasonic clamp-on and wetted transducer meters, fouling of the pipe wall or transducer faces similarly impairs the acoustic signal path.
Fouling is progressive and insidious. A meter in a heavily fouled service can lose accuracy slowly over months before the problem becomes apparent. By the time a significant error is detected, large volumes of product, chemical, or water may have been incorrectly measured or invoiced.
Electromagnetic Meters
Scale and biological fouling on electrodes increases electrode impedance. Smart transmitters with electrode impedance monitoring (available on modern instruments such as the Yokogawa ADMAG and ABB AquaMaster) will alarm when impedance rises above a threshold, giving advance warning before accuracy is compromised.
Ultrasonic Meters
Internal scale or biological growth in the pipe changes the acoustic path length and velocity used in the flow calculation. Clamp-on meters are additionally affected by coupling gel degradation between the transducer and the pipe. Signal strength (AGC gain) trending upward over time is a reliable early indicator of internal fouling.
Prevention: Specify the Right Liner and Electrode Material
For biologically active or scaling duty, select a hard-rubber or polyurethane liner rather than soft rubber, and choose Hastelloy C or platinum electrodes rather than 316 SS. Harder materials resist adhesion and are easier to clean. For severe fouling duty, scraper electrode designs are available.
Prevention: Plan for In-Service Cleaning
High-frequency (AC) excitation transmitters help prevent polarisation fouling. For chemical dosing and water treatment applications, specify meters with cleaning-in-place (CIP) compatibility and position isolation valves to allow the meter to be isolated and chemically cleaned without line shutdown.
Incorrect Installation
A flow transmitter can be perfectly calibrated and in excellent condition yet produce systematically wrong measurements because of where and how it was installed. Installation errors are responsible for a disproportionate share of the “the meter is wrong” calls that instrument teams investigate โ and they are almost always avoidable with good upfront planning.
The most common installation mistakes are insufficient straight pipe runs upstream and downstream of the meter, installation at a high point in a liquid line (leading to gas accumulation), incorrect orientation for the meter type, inadequate grounding and earthing of electromagnetic meters, and failure to align flanges concentrically with the meter bore.
Straight Pipe Runs
Most flow technologies require undisturbed, fully developed flow profiles to achieve rated accuracy. DP devices with orifice plates typically require 10โ20D upstream and 5D downstream (where D is the pipe inside diameter). Vortex meters are particularly sensitive, requiring up to 40D upstream of elbows in the same plane. Always check the manufacturer’s minimum requirements โ and add margin where possible.
Orientation Requirements
Electromagnetic flow meters can generally be installed in any orientation, but the electrodes must be in the horizontal plane (not at the top or bottom of the pipe) to avoid gas or sediment contact. Vortex meters must not be installed vertically-downward in liquid service. Coriolis meters have specific orientation requirements depending on the tube geometry โ always follow the manufacturer’s datasheet.
Grounding (Electromagnetic Meters)
Electromagnetic flow meters measure the tiny voltages induced in the flowing fluid. Any stray electrical noise picked up through an inadequate earth connection will corrupt that signal. Always install earth rings or grounding electrodes in plastic or lined piping, verify earth continuity with a milliohm meter, and connect both transmitter and meter body to a dedicated instrument earth โ not a power earth.
Flange Bore and Gasket Alignment
A gasket protruding into the bore, or a pipe bore that is larger than the meter bore, creates a flow disturbance immediately upstream of the sensing element. For orifice plates, a misaligned gasket is equivalent to changing the orifice diameter โ the impact on accuracy is significant. Always use the correct gasket inner diameter and verify bore alignment before closing up flanges.
Calibration Drift
All flow transmitters drift over time. The question is not whether drift will occur, but whether it will remain within the acceptable error band for your application before the next scheduled calibration check. Most flow meter specifications state an annual drift figure; exceeding the recommended calibration interval allows that drift to compound unchecked.
DP transmitters drift due to static pressure effects on the sensing capsule, temperature cycling, and gradual changes in the reference zero. Electronic transmitters drift due to ageing of electronic components, particularly at elevated process temperatures. For fiscal metering, custody transfer, and chemical dosing applications, calibration drift directly translates to financial loss โ either through inaccurate billing or suboptimal chemical consumption.
Establish a Calibration Schedule
Base your calibration interval on the manufacturer’s stated drift specification, the criticality of the measurement, and your process conditions. High-accuracy fiscal meters may require six-monthly verification; general-purpose process meters are often acceptable on an annual cycle. Document the calibration interval in your site’s instrument maintenance plan and treat it as mandatory โ not aspirational.
Trend Historical Calibration Data
Each calibration check produces data on the actual drift since the last check. Trending this data over several calibration cycles allows you to identify meters that are drifting faster than expected โ a sign of deteriorating conditions, fouling, or a process change โ and adjust the calibration interval accordingly. Smart transmitters can log this data on-board for easy download.
Use In-Situ Verification Where Possible
For critical flow loops, consider installing a clamp-on ultrasonic meter as a periodic check meter. A portable clamp-on unit can verify the installed meter’s output against an independent reference without breaking into the process โ a fast, low-cost verification method between scheduled calibrations. PCSPL supplies portable ultrasonic flow meters for exactly this application.
Account for Process Condition Changes
A flow meter calibrated at 20ยฐC on water will produce errors when the process temperature changes significantly โ because fluid density and viscosity change, affecting the meter’s indicated flow. For DP meters, changes in the fluid’s specific gravity directly affect the inferred flow rate. Always re-verify calibration when process fluid properties change significantly.
Moisture and Condensation Ingress
Water inside an electronics enclosure is one of the most destructive and common causes of premature flow transmitter failure in industrial environments. It causes corrosion of circuit boards and terminals, leakage currents that corrupt the 4โ20 mA signal, insulation breakdown in the signal cable, and ultimately permanent failure of the transmitter electronics.
The source of moisture is almost never a single dramatic water ingress event โ it is typically slow, cumulative condensation caused by daily thermal cycling of the enclosure. As the enclosure heats during the day and cools at night, moist air is drawn in through cable glands, conduit entries, or improperly sealed cover gaskets. Over weeks and months, enough moisture accumulates to cause damage. Outdoor installations, steam environments, and locations subject to washdown are particularly at risk.
How to Spot It
Moisture ingress often first presents as intermittent signal noise, a slowly drifting output, or a span/zero that is unstable and cannot be corrected by recalibration. Removing the transmitter cover may reveal visible corrosion on terminals, condensation on the electronics, or rust staining inside the enclosure. If caught early, cleaning and drying the electronics often restores function โ if caught late, board replacement is necessary.
How to Prevent It
Specify IP66 or IP67-rated enclosures as a minimum for outdoor or washdown duty. Ensure all cable glands are correctly sized, tightened, and rated for the cable diameter used โ oversized or loose glands are a primary entry point for moisture. Use conduit sealing compound (Duxseal or equivalent) at conduit entries. Install breather/drain plugs where specified by the manufacturer to allow pressure equalisation without moisture ingress.
Conduit and Cable Entry Best Practice
Where conduit is used, install a conduit drain fitting at the lowest point below the enclosure to prevent condensate pooling inside the conduit and wicking up to the transmitter. Never run conduit that rises toward the transmitter without a drain โ it acts as a collection reservoir for condensate. Metal conduit in corrosive environments should be PVC-coated and terminated with a watertight fitting.
Steam and High-Temperature Environments
In steam service or near hot process equipment, transmitter enclosures experience severe thermal cycling. Consider specifying a transmitter with a remote electronics head separated from the process connection by a capillary โ this moves the electronics away from the heat source and reduces the thermal gradient that drives condensation. Ensure the electronics head is still shielded from direct steam impingement or condensate drip.
Signal Loop and Power Supply Faults
A perfectly functioning flow transmitter will appear to fail if the 4โ20 mA signal loop that carries its output to the control system is compromised. Signal loop faults are commonly misdiagnosed as transmitter failures โ instruments are unnecessarily removed and sent for repair when the actual fault lies in the wiring, the power supply, or the receiving instrument.
Common loop faults include insufficient loop supply voltage (the loop needs at least 12โ24 VDC at the transmitter terminals depending on the device), excessive loop resistance from corroded connections or undersized cable, ground loops caused by multiple earth connections in the signal circuit, signal noise from parallel power cables run in the same conduit as signal cables, and open circuits from broken wires or failed terminal blocks.
Insufficient Loop Voltage
Two-wire (loop-powered) transmitters draw their operating power from the signal loop itself. The loop supply voltage minus the voltage drops across all loop resistances (cable resistance, barriers, isolators, input cards) must leave at least the transmitter’s minimum terminal voltage โ typically 10.5โ12 V. Under-voltage causes the transmitter to behave erratically or saturate at full-scale. Measure the voltage directly at the transmitter terminals under full-load (20 mA) conditions to confirm.
Ground Loops
A ground loop occurs when the signal circuit has two or more earth connections at different electrical potentials โ typically at the transmitter end and the control system end. The resulting circulating current superimposes itself on the 4โ20 mA signal as noise or a DC offset. The solution is to earth the signal loop at one point only โ usually at the control system end โ and float the transmitter end. Galvanic isolators break ground loops without interrupting the signal.
Electromagnetic Interference (EMI)
Signal cables run parallel to power cables, variable-speed drives (VSDs), or contactors pick up induced noise. VSDs are a particularly aggressive EMI source and are responsible for a significant proportion of the “noisy flow signal” calls PCSPL’s team receives. Run signal and power cables in separate conduits, maintain physical separation of at least 300 mm, and use screened (shielded) cable for all analogue signal wiring. Terminate the screen at one end only.
Corroded or High-Resistance Connections
Terminal block corrosion, loose wire terminations, and oxidised connections in outdoor junction boxes add resistance to the loop and can cause a permanent signal offset or intermittent faults. A systematic loop resistance check with a calibrator (inject 4 mA and 20 mA, measure voltage at transmitter terminals) will quickly identify high-resistance connections. Clean and re-terminate suspect connections, and use tinned-copper cable in corrosive environments.
Summary
Flow Transmitter Reliability Checklist
Use the items below as a starting point for your site’s flow meter maintenance and inspection programme. Addressing each point systematically will eliminate the majority of unplanned flow transmitter failures before they occur.
Installation and commissioning:
- Minimum straight pipe runs verified against datasheet
- Meter orientation correct for fluid type and technology
- Electromagnetic meters earthed with earth rings or grounding electrodes
- Impulse lines sloped correctly with no traps or high points
- All cable glands correctly sized and tightened
- Signal cables separated from power cables and VSDs
- Loop voltage verified at transmitter terminals at 20 mA
- Loop earthed at one point only โ ground loop continuity checked
Routine maintenance and inspection:
- Calibration interval documented and scheduled in CMMS
- Zero verified with process at steady state and correct static pressure
- Electrode impedance checked (electromagnetic meters)
- Impulse line blowdown performed on schedule (DP meters)
- Cable gland tightness and cover seal condition inspected
- Calibration drift trended across successive service intervals
- Process condition changes (temperature, fluid) re-verified against calibration
- HART diagnostics reviewed for active alerts or degraded parameters
A Note From Our Team
Persistent Flow Meter Problem? We Can Help.
The seven failures described in this guide account for the overwhelming majority of the flow transmitter problems our team investigates. In our experience, most are resolved quickly once the root cause is correctly identified โ and in many cases the meter itself is perfectly fine and the fault lies elsewhere in the installation or loop.
PCSPL supplies flow measurement instrumentation across all major technologies โ electromagnetic, differential pressure, ultrasonic (clamp-on and wetted), vortex, Coriolis, and open channel โ from leading manufacturers. We also supply portable ultrasonic flow meters for in-situ verification, calibration equipment, and field service support for difficult-to-diagnose installations.
If you have a persistent or unexplained flow meter problem, contact our team with a description of the symptoms, the meter type and model, your process fluid and conditions, and anything that changed prior to the fault appearing. That information allows our applications engineers to give you a targeted answer quickly โ rather than a generic response that wastes your time.
Need Help with a Flow Transmitter Problem?
Send us your symptoms and process details โ our instrumentation team will help you diagnose the fault and recommend the right solution, with local Australian support.<