INF-TN1FSC-E 173
IV. Q & A about accuracy
1. What is the approximate accuracy of measurement ?
Specifi cations:
Formerly, the expression % of full scale was often used. But, in the recent age of digital
system, it is more frequently expressed in % of the displayed value. Under the condition of
low fl ow velocity, the absolute value of error is used as a standard of accuracy in consideration
of the threshold of device performance.
2. What about error factors ?
On PORTAFLOW C, ultrasonic waves are emitted from the outside of the piping and the time
is measured while the waves are passing through the piping material - fl uid - piping material.
The following points become the error factors to be considered when evaluating the measured
values.
(1) Piping size
When the value set for piping size is different from the actual size of piping, and if the
difference from the inside diameter is about 1% in size, the error is about 3% of deviation
obtained by fl ow conversion.
(The following shows an example of 1mm deviation in inside diameter)
100
6
−6
−2
0
−4
4
2
400 500
[%]
300 mmφ
+1mm
−1mm
200
Inside diameter
Flow error
(2) Difference in sensor mounting length
As a general standard, when the error in mounting length is ±1mm, the error of fl ow is
within 1%.
L
100
0
−1
1
400 500
[%]
300 mmφ
+1mm
−1mm
200
Inside diameter
Flow error
Flow velocity
Inside diameter
*1: Example of calculation
Error at 2m/s? → ± 0.03 × 100/2 = ± 1.5%
Error at 1m/s? → ± 0.03 × 100/1 = ± 3.0%
φ15 to φ25 or less
φ25 to φ50 or less
φ50 to φ300 or less
φ300 to φ6000
± 2.5% of measured flow
± 0.05m/s
± 1.5% of measured flow
± 0.03m/s*
1
± 1.0% of measured flow
± 0.02m/s
± 1.0% of measured flow
± 0.01m/s
2 to 32m/s
0 to 2m/s
2 to 32m/s
0 to 2m/s
2 to 32m/s
0 to 2m/s
1 to 32m/s
0 to 1m/s
Accuracy
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