Water Hydraulics - Wastewater Operators - page 96

With regard to minimum piping straight-run requirements, mag-
netic flow meters are quite forgiving of piping configuration. The down-
stream side of the magnetic flow meter is much less critical than the
upstream side. Essentially, all that is required of the downstream side is
that sufficient backpressure is provided to keep the magnetic flow meter
full of liquid during flow measurement. Two diameters downstream
should be acceptable (Mills, 1991).

Note: Magnetic flow meters are designed to measure conductive liq-
uids only. If air or gas is mixed with the liquid, the output becomes
unpredictable.

2.13.6 ultrasonic flow Meters

Ultrasonic flow meters use an electronic transducer to send a beam
of ultrasonic sound waves through the water to another transducer on
the opposite side of the unit. The velocity of the sound beam varies
with the liquid flow rate, so the beam can be electronically translated
to indicate flow volume. The accuracy is ±1% for a flow velocity ranging
from 1 to 25 ft/s, but the meter reading is greatly affected by a change
in the fluid composition. Two types of ultrasonic flow meters are in gen-
eral use for closed pipe flow measurements. Time-of-flight or transit
time flow meters usually use pulse transmission and are intended for
use with clean liquids. The Doppler type usually uses continuous wave
transmission and is intended for use with dirty liquids.

2.13.6.1 Time-of-Flight Ultrasonic Flow Meters *

Time-of-flight flow meters make use of the difference in the time
required for a sonic pulse to travel a fixed distance, first in the direc-
tion of flow and then against the flow. This is accomplished by position-
ing opposing transceivers on a diagonal path across a meter spool, as
shown in Figure 2.30. Each transmits and receives ultrasonic pulses
with the flow and against the flow. The fluid velocity is directly propor-
tional to the time difference in pulse travel. The time-of-flight ultrasonic
flow meter operates with minimal head loss and has an accuracy range
of 1 to 2.5% full scale. They can be mounted as integral spool piece
transducers or as externally mountable clamp-ons. They can measure
flow accurately when properly installed and applied.

The advantages of the time-of-flight ultrasonic flow meter include:

• No obstruction to flow•

• Minimal head loss

• Can be clamped on

• Can be portable

* This section is adapted from Brown, A.E., in flow measurement , Spitzer, D.W., Ed.,
Instrument Society of America, Research Triangle Park, NC, 1991, pp. 415-432.