KROHNE MFC 010 C Converter - Manual (Page 81)

MFC010 Interface Manual 79
8.10 Pressure Suppression
The Pressure Suppression feature has been included to permit the user to eliminate any influences on the
measurement result of sudden termination of flow, such as when a valve is shut. W hen this occurs the
propagation of pressure waves along the pipe work and through the meter may produce an “Over-shoot”
or “ringing” effect, where the flow rate will oscillate backwards and forwards until it settles to a stable zero
flow condition, as is indicated in the diagram below.
The amplitude and duration of the “Ringing” in proportion to the flow rate in the diagram above has been
greatly exaggerated to provide a c lear indication of what may occur. I t appears at first glance that the
positive and negat ive variations would cancel each other in any totaliser result, however, as the result is
decaying t he negat ive peak s w ill al ways be s lightly l arger t han t he pos itive peaks and will therefore
accumulate a negative error in any totaliser result. This will of course become a positive error when the
flow is in the opposite (negative) direction.
In most cases the amplitude of the ringing will be bel ow the “Low Flow Threshold” (see Holding Register
No 3047) and will therefore not influence the result. However, in some cases the amplitude of the ringing
will be above the Low Flow Threshold and will still contribute to an error in the totaliser values (see figure
below).