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32
SECTION 3 OPERATION
Perform any purging of piping with spool piece in place. Once
completed, install the flowmeter and connect cabling to pickup
coil. If false counting action occurs turn sensitivity control
clockwise.
The pulse output and analog output commence with flow
through the flowmeter.
For the analog output, the span is that established by either the
factory calibration or field calibration. The range is 4-20 mA DC
into a maximum of 325 ohms of loop resistance.
3.1 PRINCIPLE OF OPERATION
A simplified block diagram of the FLSC-34 Frequency/Current
Flow Converter Subsystem is shown in Figure 3-1. Key
functional blocks as well as information flow are designated.
The basic operation of the system is as follows.
The frequency signal f rom the turbine flowmeter is connected
to the FLSC-34 with a twisted pair shielded cable. The signal
enters through 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 amplified and shaped into a train of
digital pulses whose frequency is related to the volume f low rate.
The signal entering the frequency to analog converter is passed
through a combination of a divide by N and a DIP switch
matrix. The ON output is chosen whose pulse rate is between 75
and 150 Hz at the maximum flow rate to be measured. This
scaled pulse rate is fed to a precision monostable circuit. The
output of the monostable circuit is then filtered into an analog
voltage that is proportional to volumetric flow rate.
The output amplifier is a voltage to current amplifier. It offers
zero and span available in a standard process range of 4-20 mA.
The Power Supply provides for operating bias voltage for all
internal circuitry. The pulse output amplifier provides a
TTL/CMOS compatible square pulse of 5V amplitude. The
output amplifier is buffered from the signal driving the analog
output.
3.2 CONTROLS AND ADJUSTMENTS (See Figure 3-2.)
FUSE A circuit protection device located inside of case (see
paragraph 3.2.1)
SENS. A multiple turn control used to set the threshold
sensitivity level above the ambient noise pickup.
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