COMMON QUESTIONS AND ANSWERS
The pipe vibrates. Will it affect the flow meter?
Common vibration frequencies are far lower than the sonic frequencies used by the Greyline flow meter,
and will not normally affect accuracy or performance. However, applications where very weak Doppler
signal is present (when sensitivity is adjusted to maximum and signal strength is low), accuracy may be
affected by pipe vibration, or the flow meter may show readings under no-flow conditions. Attempt to
relocate the sensor on a pipe section where vibration is reduced, or arrange pipe mounting brackets to
reduce vibration at the sensor mounting location.
The flow meter must be installed in a high noise environment. Will this affect operation?
Greyline flow meters are designed to discriminate between environmental noise and the Doppler signal.
High noise environments may affect the flow meter’s performance where low signal strength and/or low
flow velocities are being measured. Relocate the sensor in a more quiet environment if possible.
Will pipe corrosion affect accuracy of the flow meter?
Yes. Rust, loose paint etc. must be removed from the outside of the pipe to provide a clean mounting
position when installing a Doppler sensor. Severe corrosion/oxidation on the inside of the pipe may
prevent the Doppler signal from penetrating into the flow. If the pipe cannot be cleaned, a spool piece
(PVC recommended) should be installed for sensor mounting.
What effect do pipe liners have on the flow meter?
The air gap between loose insertion liners and the pipe wall prevent the Doppler signal from entering the
flow. Better results can be expected with bonded liners such as cement, epoxy or tar, however an on site
test is recommended to determine if the application is suitable for a Doppler flow meter.
Why is Doppler only recommended for liquids containing suspended solids or gases?
The Doppler sensor transmits sound into the flow stream which must be reflected back to the sensor to
indicate flow velocity. Gas bubbles or suspended solids act as reflectors for the Doppler signal. As a
guideline, Greyline Doppler flow meters are recommended for liquids containing solids or bubbles with
a minimum size of 100 microns and a minimum concentration of 75 ppm. Most applications (except
potable, distilled or deionized water) will meet this minimum requirement.
Can the sensor be submerged in water?
Yes, for short periods of time or by accident, but it is not recommended for continuous operation. The
sensor is constructed to withstand submersion to 10 psi without damage, but external liquid moving in
contact with the sensor can be interpreted as flow and cause false readings.
Page 20
PDFM 5.0 Portable Doppler Flow Meter