Dynasonics PFP-600 - Manual (Page 6)

or entrained gas. The system will also sense velocity from turbulence caused by
flowing liquids. It should be noted that in un-pressurized partially filled pipes, most
liquids will contain a sufficient amount of entrained gas for proper operation even though
the liquid may be virtually clean. For sensing velocity, the transmitted signal from the
velocity transducer is passed through the pipe wall material and comes in contact with a
discontinuity in the liquid. The transmitted signal is reflected off of the particle or gas
bubble: This reflected signal, according to Doppler theory, has change in frequency
with respect to the transmitted frequency: This shift in frequency is proportional to the
velocity of the liquid.
The level sensor is a pulsed ultrasonic transit time type. A carefully selected
transmission frequency and beam spread have been incorporated into this non-invasive
transducer. This design allows for bottom-up sensing of liquid level without coming in
contact with the liquid. The transmission frequency has been selected for its unique
ability to penetrate pipe wall material, pass freely through the contained liquid, yet
effectively reflect off the liquid/air/vapor interface within the pipe.
The electronics is designed to emit a short duration ultrasonic pulse. during the
transmitter “off” period the transducer then “listens” for a reflected signal. At the
moment of transmission a timer determines the beginning of the transmit cycle to the
end of the received cycle. The circuit then divides this time by two and then relates this
time to distance or level in the case of the PFP-600.
The micro-processor acts as a controller for the timing of the periods (1 second) in
which the Doppler circuit detects velocity and the level monitor senses level. This circuit
is essential to eliminate the possibility of flow and level data from becoming entangled
with each other thus producing erroneous information. The micro-processor essentially
turns on the Doppler circuit while turning off the level circuits and visa versa on a
continual basis to keep the micro-processor updated as to the latest occurrences in
velocity and level changes. The micro-processor section of the PFP-600 is the heart of
the system. This is the portion of the electronics that evaluates the flow and level data
and then, converts the data to volumetric flow information based on the equation:
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