Open Channel Flow Meter Principle

Measuring principle: Time-of-flight method

The open channel flow meter probe is mounted on the top of the flume, and ultrasonic pulses is transmitted by the probe to the surface of the monitored material. There, they are reflected back and received by the pro be. The host measures the time t between pulse transmission and reception. The host uses the time t (and the velocity of sound c) to calculate the distance d between the sensor bottom and the monitored liquid surface: d = c •t/2. As the host knows the installation height H from parameters setting, it can calculate the level as follows: h = H – d . Since speed of sound through air is affected by changes in temperature, the ULFM O.C.M. has integrated a temperature senor to improve accuracy.

For determined flumes, there is a fixed functional relationship between the instantaneous flow and liquid level. The formula is Q=h (x). Q means instantaneous flow, h means liquid level in flumes. So the host can calculate the flow rate though determined flumes and the level value. It is very important to understanding the working principle for further installation and operation.


Blind zone: Level echo from the blind zone cannot be evaluated due to the transient characteristics of the sensor. Span F may not extend into the blanking distance B.

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