Two commonly used terms in flow measurement are accuracy and repeatability are often used to describe a meter’s performance. While they’re sometimes used interchangeably in casual conversation, they have very distinct technical meanings. Understanding the difference between these two specifications is critical when selecting the right flow meter for your application—especially in industries where precision and reliability impact product quality, cost, or compliance.
We breakdown what flow meter accuracy and repeatability mean, how they’re measured, and why each matters in different operating conditions.
Defining Flow Meter Accuracy
Accuracy is a measure of how close a flow meter’s reading is to the actual flow rate. It’s usually expressed as a percentage of the actual reading, full scale, or calibrated span. For example, a flow meter with ±1% accuracy of reading indicates that the meter will report flow values within 1% of the true flow rate under specified conditions.
How Accuracy Is Measured
Accuracy testing typically involves comparing the flow meter’s readings against a trusted reference or calibration standard under controlled conditions. These reference standards are often traceable to national metrology institutes such as NIST (National Institute of Standards and Technology).
There are several ways accuracy might be expressed:
-
% of Reading: More precise, especially at low flow rates.
-
% of Full Scale: Accuracy gets worse as flow rate decreases.
-
% of Calibrated Span: Useful for meters that are only used within a portion of their full range.
Each flow meter type: turbine, ultrasonic, Coriolis, etc. has its own accuracy profile depending on the physics of how it measures flow. For example, turbine flow meters tend to offer high accuracy in steady, non-pulsating, clean flow conditions, and are often calibrated to within ±0.5% of reading.
Defining Repeatability
Repeatability refers to a flow meter’s ability to produce the same output reading when measuring the same flow under the same conditions, over multiple trials. It’s a measure of consistency, not correctness.
For instance, if a flow meter reads 100 GPM today and also reads 100 GPM tomorrow when the actual flow is unchanged, the meter is repeatable, even if it’s consistently reading 3 GPM too high.
Repeatability is typically expressed as a percentage of the reading and is often much tighter than the meter’s stated accuracy. A meter might have an accuracy of ±1.0% but a repeatability of ±0.1%.
The Key Differences
| Feature | Accuracy | Repeatability |
|---|---|---|
| What it Measures | Closeness to actual/true value | Consistency of repeated measurements |
| Expressed As | % of reading/full scale/span | % of reading |
| Affected By | Calibration, flow conditions, temperature | Mechanical/electronic stability |
| Use Case | Regulatory compliance, custody transfer | Process control, trending |
A Practical Analogy
Think of accuracy and repeatability like shooting arrows at a target:
-
High accuracy, high repeatability: All arrows hit the bullseye, grouped tightly.
-
High repeatability, low accuracy: Arrows cluster closely, but off to one side of the bullseye.
-
Low repeatability, high accuracy: Arrows are scattered, but average around the bullseye.
-
Low accuracy, low repeatability: Arrows are scattered all over, missing the bullseye inconsistently.
Why Accuracy Matters
In applications such as custody transfer (e.g., buying/selling fuel, chemicals, or DEF), regulatory compliance, or billing, accuracy is paramount. A small error can lead to significant financial discrepancies over time.
For example, in custody transfer of hydrogen or DEF, a ±0.5% error could translate to thousands of dollars in annual losses or overcharges depending on the volume being metered.
Accuracy also matters in batching applications, where incorrect readings could result in over- or under-filling vessels, leading to costly waste or rework.
Why Repeatability Matters
Repeatability is crucial for process control. If a system is tuned to react to changes in flow rate, and your meter isn’t repeatable, the system can behave unpredictably, even if the readings are accurate on average.
In pharmaceutical manufacturing, for example, where precision dosing of fluids is critical, consistent measurement is key to maintaining quality and ensuring each batch is within spec. Even small variances can compromise product integrity.
Likewise, in R&D environments, you might care more about how a system behaves under varying conditions than the precise value at any given moment. A repeatable meter lets you detect changes and trends reliably, even if it’s slightly off from a true value.
Accuracy Without Repeatability?
It’s possible, but not ideal, for a meter to be accurate but not repeatable. In such cases, the average reading over time may match the true flow, but individual readings fluctuate. This creates problems in control systems, alarms, and automated processes.
More often, though, users encounter meters that are repeatable but not accurate, especially if the meter has drifted from its calibration or hasn’t been recalibrated for new fluid conditions.
What Impacts Accuracy and Repeatability?
Both accuracy and repeatability are influenced by several factors:
1. Fluid Properties
-
Viscosity, density, temperature, and composition can all affect meter performance. For example, turbine meters are sensitive to changes in viscosity and density, making calibration under actual operating conditions ideal.
2. Installation
-
Straight-run piping, orientation, and absence of upstream disturbances are important. Poor installation can introduce turbulence or cavitation, reducing both accuracy and repeatability.
3. Calibration
-
Over time, wear or contamination can cause a meter to drift from its factory calibration. Routine calibration checks are essential to maintaining accuracy.
4. Meter Type
-
Some technologies inherently offer better accuracy and repeatability. For instance:
- Turbine meters: Excellent repeatability, very good accuracy in clean, steady flows.
-
Coriolis meters: Excellent accuracy and repeatability for mass flow.
-
Ultrasonic meters: High repeatability, good accuracy in clean fluids.
How to Specify the Right Meter
When selecting a flow meter, consider:
-
What’s more critical, absolute precision or consistency?
If you’re billing based on flow, prioritize accuracy. If you’re adjusting a process loop, repeatability is key. -
How will the meter be installed and used?
Ensure that the real-world conditions match the assumptions behind the manufacturer’s accuracy claims. -
What’s your calibration plan?
Even the most accurate meter will drift. If you can’t recalibrate frequently, a highly repeatable meter may give you better long-term performance for control.
Final Thoughts
Understanding the difference between accuracy and repeatability is vital to choosing the right flow meter and interpreting its data. While accuracy tells you how close your readings are to reality, repeatability tells you whether your readings can be trusted over time.
In many applications, especially those in regulated, commercial, or scientific environments, you need both. But knowing which matters more for your specific process can help you make smarter, more cost-effective decisions.
Whether you’re measuring hydrogen gas in an aerospace application or batching DEF into transport containers, your choice of flow meter. and how well you understand its specs, can have lasting implications on performance, compliance, and cost.
Need help choosing a flow meter that balances accuracy and repeatability for your specific application?
Contact the experts at Turbines, Inc. today to find the right solution for your process.