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7
SECTION 3 OPERATION
Perform any purging of piping with spool piece in place, Once
completed, install the flowmeter and connect cabling to pickup
coil and power to the unit. If false indication occurs turn
SENSITIVITY control counter clockwise until indication stops.
3.1 PRINCIPLE OF OPERATION
A simplified block diagram of the FLSC-18B Frequency to
Current Converter is shown in Figure 3-1. Key functional blocks
as well as flow information are designated. The basic operation
of the system is as follows.
The frequency signal from the flowmeter is connected to the
FLSC-18B with a twisted shielded pair cable. The signal enters
the SENSITIVITY control which is used to reject unwanted noise
by raising the trigger threshold above the background noise
present.
The low level flowmeter signal is then passed through a signal
conditioning chain where it is filtered, amplified and shaped
into a train of digital pulses whose frequency is related to the
volume flow rate and where each pulse represents a discrete
volume of fluid.
The signal entering the frequency to voltage converter is passed
through a combination of a divide by N and DIP switch matrix.
The ON output is chosen whose pulse rate is between 75 and
150 Hz at the maximum flow rate measured. This scaled pulse
rate is then filtered into an analog voltage. This voltage is
proportional to volumetric flow rate.
The resulting output voltage related to flow is then fed into the
output amplifier. The output amplifier is a voltage to current
amplifier and offers ZERO and SPAN adjustments in the
process range of 4-20 mA.
3.2 CONTROLS AND ADJUSTMENTS (See Figure 3-2.)
SENS A single turn control used to set the threshold
sensitivity level above the ambient noise pickup.
RANGE A dual in line (DIP) switch (see Figure 3-3 and Table 3-
1.) located within the enclosure which is used to
program the module to accept an input frequency range.
SPAN A multiple turn adjustment which is used to set the
voltage output signal to the desired span
corresponding to the equivalent flow range li.e., 4 to
20mA corresponding to 0-100 GPM).
ZERO A multiple turn adjustment which is used to set output
signal with no flow to the desired “zero” value (ie., 4 mA).
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