488 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
by a pulsed electromagnet (coil) gives rise to an electric field perpendicular
to the flow and magnetic vectors. This electric field is sensed by measuring the voltage difference between two electrodes. The voltage difference is
directly proportional to the flow velocity.
The EM current meter has an analog voltage output that is typically
converted to digital form for storage on computer disk. Two electrode pairs
can be situated to give orthogonal velocity components, and several designs
of two-component EM current meters are available, including sphere and
disk-shaped designs.
Electromagnetic currents meters must be calibrated; but once calibrated, they only need to be checked periodically or after suffering a mishap.
Calibration can be performed by the manufacturer or in a special facility,
such as a calibrated flow tank or tow tank.
Care must be taken in the laboratory to minimize any source of electrical
fields that might induce a voltage in the EM current meter’s electrodes.
Other nearby electrical instruments or another EM current meter placed
too close may disrupt the measurement or bias the results. Also, fouling
of the electrodes on an EM current meter will degrade the instrument’s
output, so it is wise to keep the electrodes clean. Fouling is more of a
problem in the field than in the laboratory.
The EM current meter is intrusjve, and it will disturb the flow in the immediate vicinity of the sensor. The resultant velocities represent an average
over a volume having a diameter approximately three times the diameter
of the sensor sphere or cylinder (Hydraulic Laboratory Techniques 1980).
Acoustic Current Meters
The propagation of sound waves in a fluid can be used in two different
ways to measure flow velocity. Acoustic transit time current meters
are based on the principle that sound waves travel faster with the flow than
against the flow. The current meter transmits a sound wave that is picked
up by a receiver located a known distance from the transmitter. The transit
time is calculated, and compared to the theoretical transit time for sound
waves propagating in a still fluid. The difference between the calculated
time and theoretical time is used to determine the fluid velocity component
on the axis between the transmitter and receiver. The resolved velocity
is the average over the “flight path” of the sound wave, so the sampling
volume is large compared to the laser Doppler velocimeter or the hot-film
anemometer.
Various arrangements have been used for acoustic transit time current
meters. The transmitter and receiver can be located along a straight line, or
the transmitted wave can be reflected by a clean surface back toward a re
by a pulsed electromagnet (coil) gives rise to an electric field perpendicular
to the flow and magnetic vectors. This electric field is sensed by measuring the voltage difference between two electrodes. The voltage difference is
directly proportional to the flow velocity.
The EM current meter has an analog voltage output that is typically
converted to digital form for storage on computer disk. Two electrode pairs
can be situated to give orthogonal velocity components, and several designs
of two-component EM current meters are available, including sphere and
disk-shaped designs.
Electromagnetic currents meters must be calibrated; but once calibrated, they only need to be checked periodically or after suffering a mishap.
Calibration can be performed by the manufacturer or in a special facility,
such as a calibrated flow tank or tow tank.
Care must be taken in the laboratory to minimize any source of electrical
fields that might induce a voltage in the EM current meter’s electrodes.
Other nearby electrical instruments or another EM current meter placed
too close may disrupt the measurement or bias the results. Also, fouling
of the electrodes on an EM current meter will degrade the instrument’s
output, so it is wise to keep the electrodes clean. Fouling is more of a
problem in the field than in the laboratory.
The EM current meter is intrusjve, and it will disturb the flow in the immediate vicinity of the sensor. The resultant velocities represent an average
over a volume having a diameter approximately three times the diameter
of the sensor sphere or cylinder (Hydraulic Laboratory Techniques 1980).
Acoustic Current Meters
The propagation of sound waves in a fluid can be used in two different
ways to measure flow velocity. Acoustic transit time current meters
are based on the principle that sound waves travel faster with the flow than
against the flow. The current meter transmits a sound wave that is picked
up by a receiver located a known distance from the transmitter. The transit
time is calculated, and compared to the theoretical transit time for sound
waves propagating in a still fluid. The difference between the calculated
time and theoretical time is used to determine the fluid velocity component
on the axis between the transmitter and receiver. The resolved velocity
is the average over the “flight path” of the sound wave, so the sampling
volume is large compared to the laser Doppler velocimeter or the hot-film
anemometer.
Various arrangements have been used for acoustic transit time current
meters. The transmitter and receiver can be located along a straight line, or
the transmitted wave can be reflected by a clean surface back toward a re