Ringing: - the inherent nature of the transducer to con-
tinue vibrating after the transmit pulse has
ceased.
Secondary measuring device: - any instrument for measuring the head or
flow related to the primary measuring device.
Stilling well: - a well separate from but adjacent to the pri-
mary measuring device and interconnected
by a small duct to provide an ideal point of
measurement.
Subcritical flow: - same as submerged flow.
Submerged flow: - when the downstream level rises or the dis-
charge flow is so slow that it impedes the
free flow of liquid through the primary meas-
uring device.
Ullage: - the remaining spatial volume of a vessel or
the volume required to fill a vessel.
Weir: - a dam with or without flow notch across an
open channel to produce a crest in the liquid
upstream. The head of the crest is propor-
tional to the flow.
PL-399 10 – 3
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