Fluctuating flow readings are a common issue across all flow meter technologies. In most cases, the meter is not malfunctioning; rather, it is responding to actual hydraulic or process instability within the system. Any flow meter—ultrasonic, magnetic, Coriolis, turbine, vortex, positive displacement, differential pressure, or thermal mass—will reflect changing flow conditions if the process itself is unsteady.
What Causes Flow Readings to Fluctuate?
The most common cause is transient flow, meaning the flow rate or velocity profile is changing with time. Under these conditions, the meter output may appear unstable because the instrument is capturing real-time variations in the process.
Common causes of fluctuating readings include:
Pump cycling or pulsation
Control valve movement
Rapid process demand changes
Turbulence or swirl
Entrained air or gas bubbles
Two-phase flow conditions
Inadequate straight pipe runs
Poor sensor placement
Flow profile distortion caused by elbows, reducers, tees, or partially open valves near the meter
Low-flow operation near the meter’s measurement threshold
Electrical noise or grounding issues
Why This Happens Across All Meter Types
All flow meters rely on a measurement principle that assumes some degree of flow stability. When the velocity profile is distorted, pulsating, asymmetric, or rapidly changing, the meter output can fluctuate even when the instrument is operating correctly. In many cases, the displayed value is a direct reflection of actual process variability rather than sensor error.
For example:
Ultrasonic meters can respond to changing transit-time conditions caused by unstable velocity distribution or entrained air.
Magnetic meters can show variation when conductivity changes or when flow is not fully developed.
Turbine and vortex meters may react strongly to pulsation and turbulence.
Differential pressure meters can amplify apparent instability because flow is derived from pressure signals that vary with velocity.
Coriolis meters may show variation when gas entrainment or severe process instability is present.
How to Reduce Displayed Fluctuation
1. Increase the Time Constant or Filter Setting
Most flow meters include a damping, filter, or time constant setting that averages the signal over a selected period. Increasing this value helps reduce short-term variation in the displayed reading and makes the output easier to interpret.
For SoundWater Technologies ultrasonic flow meters, this setting is available in the Orcas Mobile App, Flow Computer, and Thermal Energy Flowmeter by navigating to:
Setup → Advanced → Time Constant
2. Use Averaging Functions
SoundWater Technologies ultrasonic flow meters also include an Infinite Averaging Mode, which calculates a cumulative average over time. While the meter remains in this mode, the displayed value reflects the statistical average over the full averaging period. As the averaging interval increases, the displayed measurement will gradually stabilize and show less short-term fluctuation.
Watch a short video here.
Additional Troubleshooting Considerations
If readings are fluctuating more than expected, review the following:
Confirm the meter is installed in accordance with the manufacturer’s straight-run recommendations.
Check for nearby sources of flow disturbance such as pumps, elbows, valves, or reducers.
Verify that the pipe remains full during operation.
Inspect for air entrainment or cavitation.
Confirm the meter is operating within its specified flow range.
Check grounding and electrical noise conditions where applicable. As a general rule, avoid sharing power or ground with high-power electrical equipment such as pumps or variable frequency drives (VFDs).
Summary
Fluctuating flow readings are not unique to any one measurement technology. They are typically the result of unstable flow conditions, changing velocity profiles, pulsation, installation effects, or electrical interference. In many cases, the most effective solution is to improve flow conditions or apply appropriate filtering and averaging so the displayed value better reflects the information needed for the application.
If you have questions about your application or need help optimizing meter performance, please contact us.
