Ultraflux Uf 841, User Manual

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26
NT UF 841 CP GB (d) / June 2015
4.1 Composition of a measuring point
Apart from the electronic converter (the unit), a measuring point includes the following components:
•
1 to 4 pairs of probes
•
1 to 8 special cables for the probes (provided by Ultraflux)
4.2 Choosing the measurement location
It is essential to follow the instructions given in our document “Ultrasonic
transit time flowmeter
". As a
reminder, the main precautions to be taken are as follows:
4.2.1 Straight lengths to be used
1) Monochord measurement
Using a monochord measurement implies that the speed profile at the measuring point is symmetrical
relative to the axis of the pipe, and is fully developed and stable.
(a) Symmetrical laminar flow (b) Unsymmetrical laminar flow
The following three conditions are not fulfilled in an area of disturbed flow, for example, at the outlet from
an elbow or downstream from a regulating valve:
The speed profile is stable but not symmetric relative to the axis of the pipe.
The symmetry of the speed profile varies relative to the pipe axis but is nether stable nor
fully developed.
This is why probes need to be placed on a straight section of pipe sufficiently far upstream or downstream
of a hydraulic disturbance (narrowing or widening of the pipe, elbow, pump, etc.).
The following values show, for liquids, the minimum distances (L) to comply with before and after a
disturbance depending on the internal diameter of the pipe (D) for the errors induced by these disturbances
to be less than ± 1 %.