Model 001 & 002 Flowmeter Operating Manual
MANUAL-1550859975-1 | issue: 1.1
©
2021
– Valeport Ltd
Page | 27
Reliability
Over most of the flow speed range, above about 0.15 m/s, any particular BFM001 meter with 8011
type Impeller is expected (at 95 per cent confidence level) to perform within ±1.5 per cent of the
indication of the group calibration. At very low flow speeds, although the uncertainty remains small
in absolute terms, it increases rapidly if considered as a percentage of the flow speed being
measured. The following table illustrates this.
Flow Speed
(m/s)
3.00
2.00
1.00
0.50
0.25
0.15
0.10
0.07
0.03
Uncertainty
(m/s)
0.045
0.023
0.011
0.005
0.004
0.002
0.002
0.003
0.004
%
1.5
1.2
1.1
1.1
1.5
1.4
2.3
4.4
14.0
It is particularly important therefore, not to extrapolate the first equation of the group calibration
below the stated lower limit.
Individual Calibration
If the uncertainty levels shown above are higher than desirable for a particular application of the
meter, then it should be calibrated individually. However, there will still be the unavoidable
tolerance which exists with any calibration process. If the meter is to be used mainly with cable
suspension, individual calibration may also be justified.
Further Details
The method of deriving the group calibration, together with detailed results, is given in Report No.
DE53 Calibration B1/2 'Group Calibration for the Braystoke Type BFM001 Current Meter with 8011
Series Impeller June 1981' of the Hydraulics Research Station, Wallingford, Oxon OX10 8BA, UK.