Valeport BFM001, Operating Manual, Page: 31 / 32

Appendices
Page | 28
©
2021
– Valeport Ltd
Group Calibration of the 1178 Series Impeller
As fitted to the Valeport "Braystoke" BFM002 flowmeter
Summary
It is customary to calibrate current meters individually, which is expensive and time consuming. For
a family of physically identical meters a less costly and generally satisfactory alternative is a group
calibration. This is synthesised from the performance of a sample batch or group of meters and
indicates the average performance of the entire family.
The following Group Calibration B2/2 has been derived for the Braystoke type BFM002 current
meter with 50 mm impeller, which became available from early 1978. Subsequently, the impellers
carried the reference number 1178. The calibration is based on tests of 20 meters, all on 20 mm
diameter rod suspension, in the meter rating tank at the Hydraulics Research Station over the
period June 1976 to February 1982.
Speed of Rotation, n Speed of Flow, V
(rev/s) (m/s)
Min Max
0.26 0.97 0.034 + 0.0991 n Equation 1
0.97 4.71 0.023 + 0.1105 n Equation 2
4.71 27.86 0.039 + 0.1071 n Equation 3
The limiting values of rotation speed correspond to flow speeds of 0.060, 0.130, 0.543 and 3.023
m/s respectively. The equations are evaluated as a ready reference rating table, see following
pages.
The tolerance, or uncertainty, of computed flow speed using the above group calibration compared
with the measured flow speed at the same rotation speed is approximately constant in percentage
terms above 0.5 m/s. The value then does not exceed 2.2% at the 95% confidence level.
However, at lower speeds the percentage uncertainty is much greater. In this range it is more
realistic to consider the uncertainty in terms of absolute speed; the value is near constant around
0.01 m/s at speeds between 0.06 and 0.50 m/s.
The current meter should not be used at speeds below about 0.15 m/s if avoidable, due to the wide
uncertainty at low speed.
A Preliminary Group Calibration for the Braystoke BFM002 Current Meter was printed by the
former Water Data Unit of the Department of the Environment as an un-numbered research paper
in October 1980. It was derived from tests of 11 meters. The present report includes 10 of these
results supplemented by others from a further 10 meters; the rating equations given above replace
the preliminary group calibration which should not be used.