Teledyne 4250, Инструкция по установке и эксплуатации, Страница: 15 / 204

4250 Flow Meter
Section 1 Introduction
1-3
1.3 Flow Meter Operation
When measuring flow rate, the 4250 is normally used with an
open channel and calculates flow rate from a combination of mea-
sured level, stream velocity, and channel cross-sectional area.
This is the only method of flow measurement that can measure
submerged, full pipe, surcharged, and reverse flows, and it
renders a primary measuring device unnecessary. The flow meter
provides standard or optional flow-related output signals to be
used for:
• Flow-proportional sampler pacing and enabling
• Transmitting level and flow data to an external device
on a serial communications loop
• Data transfer to a remote location through a modem
• Control of an external 4-20 mA device
• Data transfer by a laptop computer
The 4250 contains microprocessor-controlled circuitry to cal-
culate level and flow rates from the output produced by the
area-velocity sensor, store user programming instructions,
operate the display, and drive the internal plotter. An alphanu-
meric liquid crystal display (LCD) shows current total flow, level,
and flow rate information. It also prompts you during initial
setup or subsequent program changes. An internal plotter pro-
vides a hard copy printout of the information computed, plots
level or flow rate, and generates reports. Connectors for other
equipment used with the 4250 are on the right side of the flow
meter’s case.
1.4 Area Velocity Sensor
Operation
The AV sensor is mounted beneath the surface of the flow stream
and measures liquid level by responding to changes in hydro-
static pressure against a solid state pressure transducer. It mea-
sures average velocity ultrasonically by using the
Doppler
effect . This principle states that the frequency of a sound (or
other wave) passed from one body to another is relative to both
their motions. As the two approach each other, the frequency
increases; as they recede, the frequency decreases. The motion of
the bodies relative to each other is added to or subtracted from
the frequency of the wave. A familiar example is this: You are in
a car at a railroad crossing waiting for a train. As the train
approaches, it sounds its whistle in warning. The whistle sounds
a certain pitch to you. As the train passes through the crossing,
the pitch drops noticeably. The transmitted frequency, of course,
is always the same, but the Doppler effect will make it seem
higher or lower to you because of the movement of the train
toward or away from you.
1.4.1 Velocity Measurement The probe’s ultrasonic transducer transmits a high-frequency
pulse into the flow stream. Bubbles and particles carried by the
stream reflect the pulse back to the receiving transducer. The
reflected pulse will have a different frequency, depending on
whether the bubbles or particles are moving away from or toward
the sensor.