Measurement Accuracy

What is Measurement Accuracy?

Measurement accuracy represents how closely an instrument's reading matches the true value. The higher the accuracy, the closer the flow meter reading is to the actual flow rate.

Accuracy is expressed as a percentage, but there are two different calculation methods — and the practical difference between them is significant.

---

%FS and %OR — Two Ways to Express Accuracy

Full-Scale Accuracy (%FS)

%FS calculates the error based on the maximum measurable value (full scale), regardless of the actual flow rate at the time. This means the error value remains fixed and constant across all flow rates.

Example: A flow meter with a full scale of 30 L/min and accuracy of ±1% FS:

Flow	Error Value	% of Reading
30 L/min (full flow)	±0.3 L/min	±1%
3 L/min (low flow)	±0.3 L/min	±10%

At low flow rates, %FS accuracy appears relatively poor compared to the actual reading — this is especially important to note when operating near the lower end of the measurement range.

Reading Accuracy (%OR / %Rd )

%OR (Of Reading), also referred to as %Rd, calculates the error based on the actual measured value at the time, so the error value scales proportionally with the flow rate.

Example: A flow meter with accuracy of ±2.5% OR:

Flow	Error Value	% of Reading
30 L/min	±0.75 L/min	±2.5%
15 L/min	±0.375 L/min	±2.5%
3 L/min	±0.075 L/min	±2.5%

%OR accuracy maintains proportional consistency across the entire measurement range, making it especially meaningful at low flow rates.

---

Full Scale (%FS) vs Reading Value (%OR) — Illustration

---

FP-AS310 Measurement Accuracy Specifications

FP-AS310 uses a dual-range accuracy specification — applying a different accuracy expression depending on where the actual flow falls within the measurement range. This design ensures the specification is meaningful across the entire operating range.

Flow Range	Accuracy	Method
20% – 100% of Full Scale	±2.5% OR	% of Reading — error scales proportionally with actual flow rate
0% – 20% of Full Scale	±0.5% FS	% of Full Scale — fixed error value floor

Why use two methods?

At higher flow rates (above 20% of full scale), expressing accuracy as a percentage of reading (±2.5% OR) is both precise and meaningful — the error naturally scales proportionally with flow.

However, at very low flow rates (below 20% of full scale), applying a percentage error to an extremely small value would produce an error value that is too small to reflect actual measurement performance. Using a fixed %FS error floor (±0.5% FS) provides a more realistic accuracy reference at low flow.

Practical implication

The key is knowing the 20% FS switchover point for your model — this is the boundary between the two accuracy methods.

To find your model's switchover point:

1. Locate your model in the reference table below.

2. Take the upper end of the recommended flow range as your full-scale reference value.

3. Multiply by 20% — the result is your switchover point.

Example using FP-P025 (Full Scale: 294.5 L/min): Switchover point = 20% × 294.5= 58.9 L/min

Flow	Range	Applicable Method	Error Value
294.5 L/min（100% FS）	Above 20% FS	±2.5% OR	±7.4L/min
50 L/min（50% FS）	Above 20% FS	±2.5% OR	±3.7 L/min
10 L/min（20% FS）	Switchover point	±2.5% OR = ±0.5% FS	±1.47 L/min
30 L/min（10% FS）	Below 10% FS	±0.5% FS	±1.47 L/min
9 L/min（min flow）	Below 10% FS	±0.5% FS	±1.47 L/min

Note that at the switchover point (58.9 L/min), both methods produce exactly the same error value — this is by design. The dual-range specification is calibrated to ensure there is no jump or gap in accuracy values at the switchover point, making the transition seamless.

---

Applying to your model

Refer to the table below to find your model's recommended flow range, then calculate the switchover point using the same method.

Model	Recommended Flow Range	20% FS Switchover Point
FP-P015	4 – 120.6 L/min	24.1 L/min
FP-P020	7 – 188.5 L/min	37.7 L/min
FP-P025	9 – 294.5 L/min	58.9 L/min
FP-P032	19 – 452.9 L/min	90.6 L/min
FP-P040	28 – 754.0 L/min	150.8 L/min
FP-P050	50 – 1,225.7 L/min	245.1 L/min
FP-P065	95 – 2,179.0 L/min	435.8 L/min

Measurement accuracy figures are obtained under controlled laboratory conditions, with fixed pipe and fluid conditions, instruments zeroed prior to testing, and tests conducted at 25°C. Although statistical results were obtained under these conditions, accuracy may vary in customer field environments due to differences in fluid type, pipe condition, installation method, and operating temperature.